|Oracle9i Database Concepts
Release 2 (9.2)
Part Number A96524-01
This chapter provides an overview of the Oracle server. The topics include:
This chapter contains information relating to both Oracle9i Standard Edition and Oracle9i Enterprise Edition. Some of the features and options documented in this chapter are available only if you have purchased the Oracle9i Enterprise Edition. See Oracle9i Database New Features for information about the differences between Oracle9i Standard Edition and Oracle9i Enterprise Edition.
An Oracle database is a collection of data treated as a unit. The purpose of a database is to store and retrieve related information. A database server is the key to solving the problems of information management. In general, a server reliably manages a large amount of data in a multiuser environment so that many users can concurrently access the same data. All this is accomplished while delivering high performance. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.
The database has logical structures and physical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.
A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are the logical structures that directly refer to the database's data. Schema objects include structures like tables, views, and indexes. (There is no relationship between a tablespace and a schema. Objects in the same schema can be in different tablespaces, and a tablespace can hold objects from different schemas.)
Some of the most common schema objects are defined in the following section.
Chapter 10, "Schema Objects" for detailed information on these schema objects, and for information on other schema objects, such as dimensions, the sequence generator, synonyms, index-organized tables, domain indexes, clusters, and hash clusters
Tables are the basic unit of data storage in an Oracle database. Database tables hold all user-accessible data. Each table has columns and rows. Oracle stores each row of a database table containing data for less than 256 columns as one or more row pieces. A table that has an employee database, for example, can have a column called employee number, and each row in that column is an employee's number.
Views are customized presentations of data in one or more tables or other views. A view can also be considered a stored query. Views do not actually contain data. Rather, they derive their data from the tables on which they are based, referred to as the base tables of the views.
Like tables, views can be queried, updated, inserted into, and deleted from, with some restrictions. All operations performed on a view actually affect the base tables of the view.
Views provide an additional level of table security by restricting access to a predetermined set of rows and columns of a table. They also hide data complexity and store complex queries.
Indexes are optional structures associated with tables. Indexes can be created to increase the performance of data retrieval. Just as the index in this manual helps you quickly locate specific information, an Oracle index provides an access path to table data.
When processing a request, Oracle can use some or all of the available indexes to locate the requested rows efficiently. Indexes are useful when applications frequently query a table for a range of rows (for example, all employees with a salary greater than 1000 dollars) or a specific row.
Indexes are created on one or more columns of a table. After it is created, an index is automatically maintained and used by Oracle. Changes to table data (such as adding new rows, updating rows, or deleting rows) are automatically incorporated into all relevant indexes with complete transparency to the users.
You can partition indexes.
Clusters are groups of one or more tables physically stored together because they share common columns and are often used together. Because related rows are physically stored together, disk access time improves.
Like indexes, clusters do not affect application design. Whether or not a table is part of a cluster is transparent to users and to applications. Data stored in a clustered table is accessed by SQL in the same way as data stored in a nonclustered table.
The logical storage structures, including data blocks, extents, and segments, enable Oracle to have fine-grained control of disk space use.
At the finest level of granularity, Oracle database data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. The standard block size is specified by the initialization parameter
DB_BLOCK_SIZE. In addition, you can specify of up to five other block sizes. A database uses and allocates free database space in Oracle data blocks.
The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
Above extents, the level of logical database storage is a segment. A segment is a set of extents allocated for a certain logical structure. The following table describes the different types of segments.
Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.
For a partitioned index, each partition has an index segment.
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the extents in the temporary segment are returned to the system for future use.
If you are operating in automatic undo management mode, then the database server manages undo space using tablespaces. Oracle Corporation recommends that you use "Automatic Undo Management" management.
The information in a rollback segment is used during database recovery:
Oracle dynamically allocates space when the existing extents of a segment become full. In other words, when the extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.
A database is divided into logical storage units called tablespaces, which group related logical structures together. For example, tablespaces commonly group together all application objects to simplify some administrative operations.
The relationship between databases, tablespaces, and datafiles (datafiles are described in the next section) is illustrated in Figure 1-1.
This figure illustrates the following:
SYSTEMtablespace has 2 megabit (Mb) storage capacity, and
USERStablespace has 4 Mb).
A tablespace can be online (accessible) or offline (not accessible). A tablespace is generally online, so that users can access the information in the tablespace. However, sometimes a tablespace is taken offline to make a portion of the database unavailable while allowing normal access to the remainder of the database. This makes many administrative tasks easier to perform.
The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.
Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database.
The characteristics of datafiles are:
Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. For example, assume that a user wants to access some data in a table of a database. If the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory.
Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once, as determined by the database writer process (DBWn) background process.
Every Oracle database has a set of two or more redo log files. The set of redo log files is collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records).
The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.
To protect against a failure involving the redo log itself, Oracle allows a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks.
The information in a redo log file is used only to recover the database from a system or media failure that prevents database data from being written to the datafiles. For example, if an unexpected power outage terminates database operation, then data in memory cannot be written to the datafiles, and the data is lost. However, lost data can be recovered when the database is opened, after power is restored. By applying the information in the most recent redo log files to the database datafiles, Oracle restores the database to the time at which the power failure occurred.
The process of applying the redo log during a recovery operation is called rolling forward.
Every Oracle database has a control file. A control file contains entries that specify the physical structure of the database. For example, it contains the following information:
Like the redo log, Oracle lets the control file be multiplexed for protection of the control file.
Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery.
The three utilities for moving a subset of an Oracle database from one database to another are Export, Import, and SQL*Loader.
The Export utility transfers data objects between Oracle databases, even if they reside on platforms with different hardware and software configurations. Export extracts the object definitions and table data from an Oracle database and stores them in an Oracle binary-format Export dump file typically located on disk or tape.
Such files can then be copied using file transfer protocol (FTP) or physically transported (in the case of tape) to a different site. They can be used with the Import utility to transfer data between databases that are on machines not connected through a network or as backups in addition to normal backup procedures.
When you run Export against an Oracle database, it extracts objects, such as tables, followed by their related objects, and then writes them to the Export dump file.
The Import utility inserts the data objects extracted from one Oracle database by the Export utility into another Oracle database. Export dump files can be read only by Import. Import reads the object definitions and table data that the Export utility extracted from an Oracle database.
The Export and Import utilities can also facilitate certain aspects of Oracle Advanced Replication functionality, such as offline instantiation.
Export dump files can be read only by the Oracle Import utility. If you need to read load data from ASCII fixed-format or delimited files, you can use the SQL*Loader utility. SQL*Loader loads data from external files into tables in an Oracle database. SQL*Loader accepts input data in a variety of formats, performs filtering (selectively loading records based on their data values), and loads data into multiple Oracle database tables during the same load session.
Oracle9i Database Utilities for detailed information about Export, Import, and SQL*Loader
Each Oracle database has a data dictionary. An Oracle data dictionary is a set of tables and views that are used as a read-only reference about the database. For example, a data dictionary stores information about both the logical and physical structure of the database. A data dictionary also stores the following information:
A data dictionary is created when a database is created. To accurately reflect the status of the database at all times, the data dictionary is automatically updated by Oracle in response to specific actions, such as when the structure of the database is altered. The database relies on the data dictionary to record, verify, and conduct ongoing work. For example, during database operation, Oracle reads the data dictionary to verify that schema objects exist and that users have proper access to them.
This section explains how Oracle adheres to industry accepted standards for data access languages, and how Oracle controls data consistency and data integrity. This section includes the following topics:
SQL (pronounced SEQUEL) is the programming language that defines and manipulates the database. SQL databases are relational databases, which means that data is stored in a set of simple relations.
All operations on the information in an Oracle database are performed using SQL statements. A SQL statement is a string of SQL text. A statement must be the equivalent of a complete SQL sentence, as in:
Only a complete SQL statement can run successfully. A sentence fragment, such as the following, generates an error indicating that more text is required:
A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. SQL statements are divided into the following categories:
These statements create, alter, maintain, and drop schema objects. DDL statements also include statements that permit a user to grant other users the privileges to access the database and specific objects within the database.
These statements manipulate data. For example, querying, inserting, updating, and deleting rows of a table are all DML operations. The most common SQL statement is the
SELECT statement, which retrieves data from the database. Locking a table or view and examining the execution plan of an SQL statement are also DML operations.
These statements manage the changes made by DML statements. They enable a user to group changes into logical transactions. Examples include
These statements let a user control the properties of the current session, including enabling and disabling roles and changing language settings. The two session control statements are
These statements change the properties of the Oracle server instance. The only system control statement is
SYSTEM. It lets users change settings, such as the minimum number of shared servers, kill a session, and perform other tasks.
These statements incorporate DDL, DML, and transaction control statements in a procedural language program, such as those used with the Oracle precompilers. Examples include
Oracle object technology is a layer of abstraction built on Oracle's relational technology. New object types can be created from any built-in database types or any previously created object types, object references, and collection types. Metadata for user-defined types is stored in a schema available to SQL, PL/SQL, Java, and other published interfaces.
An object type differs from native SQL datatypes in that it is user-defined, and it specifies both the underlying persistent data (attributes) and the related behaviors (methods). Object types are abstractions of the real-world entities, for example, purchase orders.
Object types and related object-oriented features, such as variable-length arrays and nested tables, provide higher-level ways to organize and access data in the database. Underneath the object layer, data is still stored in columns and tables, but you can work with the data in terms of the real-world entities--customers and purchase orders, for example--that make the data meaningful. Instead of thinking in terms of columns and tables when you query the database, you can simply select a customer.
Internally, statements about objects are still basically statements about relational tables and columns, and you can continue to work with relational data types and store data in relational tables. But you have the option to take advantage of object-oriented features too. You can use object-oriented features while continuing to work with most of your relational data, or you can go over to an object-oriented approach entirely. For instance, you can define some object data types and store the objects in columns in relational tables. You can also create object views of existing relational data to represent and access this data according to an object model. Or you can store object data in object tables, where each row is an object.
In general, the object-type model is similar to the class mechanism found in C++ and Java. Like classes, objects make it easier to model complex, real-world business entities and logic, and the reusability of objects makes it possible to develop database applications faster and more efficiently. By natively supporting object types in the database, Oracle enables application developers to directly access the data structures used by their applications. No mapping layer is required between client-side objects and the relational database columns and tables that contain the data. Object abstraction and the encapsulation of object behaviors also make applications easier to understand and maintain.
PL/SQL is Oracle's procedural language extension to SQL. PL/SQL combines the ease and flexibility of SQL with the procedural functionality of a structured programming language, such as
When designing a database application, consider the following advantages of using stored PL/SQL:
The following sections describe the PL/SQL program units that can be defined and stored centrally in a database.
Program units are stored procedures, functions, packages, triggers, and anonymous transactions.
Procedures and functions are sets of SQL and PL/SQL statements grouped together as a unit to solve a specific problem or to perform a set of related tasks. They are created and stored in compiled form in the database and can be run by a user or a database application.
Procedures and functions are identical, except that functions always return a single value to the user. Procedures do not return values.
Packages encapsulate and store related procedures, functions, variables, and other constructs together as a unit in the database. They offer increased functionality (for example, global package variables can be declared and used by any procedure in the package). They also improve performance (for example, all objects of the package are parsed, compiled, and loaded into memory once).
Database triggers are PL/SQL, Java, or C procedures that run implicitly whenever a table or view is modified or when some user actions or database system actions occur. Database triggers can be used in a variety of ways for managing your database. For example, they can automate data generation, audit data modifications, enforce complex integrity constraints, and customize complex security authorizations.
You can call autonomous transactions from within a PL/SQL block. When an autonomous PL/SQL block is entered, the transaction context of the caller is suspended. This operation ensures that SQL operations performed in this block (or other blocks called from it) have no dependence or effect on the state of the caller's transaction context.
Java is an object-oriented programming efficient for application-level programs. Java has key features that make it ideal for developing server applications. These features include the following:
XML, eXtensible Markup Language, is the standard way to identify and describe data on the Web. It is a human-readable, machine-understandable, general syntax for describing hierarchical data, applicable to a wide range of applications, databases, e-commerce, Java, web development, searching, and so on.
The Oracle server includes the Oracle XML DB, a set of built-in high-performance XML storage and retrieval technologies. The XML DB fully absorbs the W3C XML data model into the Oracle server and provides new standard access methods for navigating and querying XML. You get all the advantages of relational database technology and XML technology at the same time. Key aspects of the XML database include the following:
XMLType-- to store and manipulate XML. Multiple storage options (
CLOB, decomposed object-relational) are available with
XMLType, and DBAs can choose a storage that meets their requirements for fidelity to original, ease of query, ease of regeneration, and so on. With
XMLType, you can perform SQL operations, such as queries and OLAP functions on XML data, as well as XML operations, such as XPath searches and XSL transformations, on SQL data. You can build regular SQL indexes or Oracle Text indexes on
XMLTypefor high performance for a broad spectrum of applications.
Complementing the XML Database is the Oracle XML Developer Kit, or XDK. XDK is a set of commonly used building blocks or utilities for development and runtime support. The Oracle XDK contains the basic building blocks for reading, manipulating, transforming, and viewing XML documents. To provide a broad variety of deployment options, the Oracle XDKs are available for Java, JavaBeans, C, C++, and PL/SQL. Oracle XDKs consist of XML Parsers, an XSLT Processor, XML Schema Processor, XML Class Generator, XML Transviewer Java Beans, XML SQL Utility, and XSQL Servlet.
Advanced Queuing (AQ) is the message queuing functionality of the Oracle database. With this functionality, message queuing operations can be performed similar to that of SQL operations from the Oracle database. Message queuing functionality enables asynchronous communication between applications and users on Oracle databases using queues. AQ offers enqueue, dequeue, propagation, and guaranteed delivery of messages, along with exception handling in case messages cannot be delivered. Message queuing takes advantage of
XMLType for XML message payloads.
A transaction is a logical unit of work that comprises one or more SQL statements run by a single user. According to the ANSI/ISO SQL standard, with which Oracle is compatible, a transaction begins with the user's first executable SQL statement. A transaction ends when it is explicitly committed or rolled back by that user.
Consider a banking database. When a bank customer transfers money from a savings account to a checking account, the transaction can consist of three separate operations: decrease the savings account, increase the checking account, and record the transaction in the transaction journal.
Oracle must guarantee that all three SQL statements are performed to maintain the accounts in proper balance. When something prevents one of the statements in the transaction from running (such as a hardware failure), then the other statements of the transaction must be undone. This is called rolling back. If an error occurs in making any of the updates, then no updates are made.
Figure 1-2 illustrates the banking transaction example.
Oracle9i SQL Reference for information about Oracle's compliance with ANSI/ISO standards
The changes made by the SQL statements that constitute a transaction can be either committed or rolled back. After a transaction is committed or rolled back, the next transaction begins with the next SQL statement.
Committing a transaction makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions' transactions that start only after the transaction is committed.
Rolling back a transaction retracts any of the changes resulting from the SQL statements in the transaction. After a transaction is rolled back, the affected data is left unchanged, as if the SQL statements in the transaction were never run.
Savepoints divide a long transaction with many SQL statements into smaller parts. With savepoints, you can arbitrarily mark your work at any point within a long transaction. This gives you the option of later rolling back all work performed from the current point in the transaction to a declared savepoint within the transaction. For example, you can use savepoints throughout a long complex series of updates, so if you make an error, you do not need to resubmit every statement.
Transactions let users guarantee consistent changes to data, as long as the SQL statements within a transaction are grouped logically. A transaction should consist of all of the necessary parts for one logical unit of work--no more and no less. Data in all referenced tables are in a consistent state before the transaction begins and after it ends. Transactions should consist of only the SQL statements that make one consistent change to the data.
For example, recall the banking example. A transfer of funds between two accounts (the transaction) should include increasing one account (one SQL statement), decreasing another account (one SQL statement), and recording the transaction in the journal (one SQL statement). All actions should either fail or succeed together; the credit should not be committed without the debit. Other nonrelated actions, such as a new deposit to one account, should not be included in the transfer of funds transaction. Such statements should be in other transactions.
Data must adhere to certain business rules, as determined by the database administrator or application developer. For example, assume that a business rule says that no row in the
inventory table can contain a numeric value greater than nine in the
sale_discount column. If an
UPDATE statement attempts to violate this integrity rule, then Oracle must roll back the invalid statement and return an error to the application. Oracle provides integrity constraints and database triggers to manage data integrity rules.
Database triggers let you define and enforce integrity rules, but a database trigger is not the same as an integrity constraint. Among other things, a database trigger does not check data already loaded into a table. Therefore, it is strongly recommended that you use database triggers only when the integrity rule cannot be enforced by integrity constraints.
An integrity constraint is a declarative way to define a business rule for a column of a table. An integrity constraint is a statement about a table's data that is always true and that follows these rules:
Integrity constraints are defined with a table and are stored as part of the table's definition in the data dictionary, so that all database applications adhere to the same set of rules. When a rule changes, it only needs be changed once at the database level and not many times for each application.
The following integrity constraints are supported by Oracle:
NULL: Disallows nulls (empty entries) in a table's column.
KEY: Disallows duplicate values in a column or set of columns.
KEY: Disallows duplicate values and nulls in a column or set of columns.
KEY: Requires each value in a column or set of columns to match a value in a related table's
KEYintegrity constraints also define referential integrity actions that dictate what Oracle should do with dependent data if the data it references is altered.
CHECK: Disallows values that do not satisfy the logical expression of the constraint.
Key is used in the definitions of several types of integrity constraints. A key is the column or set of columns included in the definition of certain types of integrity constraints. Keys describe the relationships between the different tables and columns of a relational database. Individual values in a key are called key values.
The different types of keys include:
KEYconstraint. A primary key's values uniquely identify the rows in a table. Only one primary key can be defined for each table.
SQL*Plus is a tool for entering and running ad-hoc database statements. It lets you run SQL statements and PL/SQL blocks, and perform many additional tasks as well. Through SQL*Plus, you can:
An Oracle server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.
The architectural features discussed in this section enable the Oracle server to support:
Figure 1-3 shows a typical variation of the Oracle server memory and process structures.
In a UNIX environment, most Oracle processes are part of one master Oracle process, rather than being individual processes. On Windows NT, all processes consist of at least one thread. A thread is an individual execution within a process. Threads enable concurrent operations within a process so that a process can run different parts of its program simultaneously on different processors. A thread is the most fundamental component that can be scheduled on Windows NT. In UNIX documentation, such as this book, whenever the word "process" is mentioned, it is considered a "thread" on Windows NT.
An Oracle server consists of an Oracle database and an Oracle server instance. Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The combination of the background processes and memory buffers is called an Oracle instance.
Some hardware architectures (for example, shared disk systems) enable multiple computers to share access to data, software, or peripheral devices. Real Application Clusters take advantage of such architecture by running multiple instances that share a single physical database. In most applications, Real Application Clusters enable access to a single database by users on multiple machines with increased performance.
Real Application Clusters are inherently high availability systems. The clusters that are typical of Real Application Clusters environments can provide continuous service for both planned and unplanned outages.
An Oracle server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.
Oracle creates and uses memory structures to complete several jobs. For example, memory stores program code being run and data shared among users. Two basic memory structures are associated with Oracle: the system global area and the program global area. The following subsections explain each in detail.
The System Global Area (SGA) is a shared memory region that contains data and control information for one Oracle instance. Oracle allocates the SGA when an instance starts and deallocates it when the instance shuts down. Each instance has its own SGA.
Users currently connected to an Oracle server share the data in the SGA. For optimal performance, the entire SGA should be as large as possible (while still fitting in real memory) to store as much data in memory as possible and to minimize disk I/O.
Database buffers store the most recently used blocks of data. The set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks. Because the most recently (and often, the most frequently) used data is kept in memory, less disk I/O is necessary, and performance is improved.
The redo log buffer stores redo entries--a log of changes made to the database. The redo entries stored in the redo log buffers are written to an online redo log, which is used if database recovery is necessary. The size of the redo log is static.
The shared pool contains shared memory constructs, such as shared SQL areas. A shared SQL area is required to process every unique SQL statement submitted to a database. A shared SQL area contains information such as the parse tree and execution plan for the corresponding statement. A single shared SQL area is used by multiple applications that issue the same statement, leaving more shared memory for other uses.
The large pool is an optional area that provides large memory allocations for Oracle backup and restore operations, I/O server processes, and session memory for the shared server and Oracle XA (used where transactions interact with more than one database).
A cursor is a handle (a name or pointer) for the memory associated with a specific statement. (Oracle Call Interface, OCI, refers to these as statement handles.) Although most Oracle users rely on automatic cursor handling of Oracle utilities, the programmatic interfaces offer application designers more control over cursors.
For example, in precompiler application development, a cursor is a named resource available to a program and can be used specifically to parse SQL statements embedded within the application. Application developers can code an application so it controls the phases of SQL statement execution and thus improves application performance.
The Program Global Area (PGA) is a memory buffer that contains data and control information for a server process. A PGA is created by Oracle when a server process is started. The information in a PGA depends on the Oracle configuration.
A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job or task. A process generally has its own private memory area in which it runs.
An Oracle server has two general types of processes: user processes and Oracle processes.
User processes are created and maintained to run the software code of an application program (such as a Pro*C/C++ program) or an Oracle tool (such as Enterprise Manager). User processes also manage communication with the server process through the program interface, which is described in a later section.
Oracle processes are invoked by other processes to perform functions on behalf of the invoking process. The different types of Oracle processes and their specific functions are discussed in the following sections.
Oracle creates server processes to handle requests from connected user processes. A server process communicates with the user process and interacts with Oracle to carry out requests from the associated user process. For example, if a user queries some data not already in the database buffers of the SGA, then the associated server process reads the proper data blocks from the datafiles into the SGA.
Oracle can be configured to vary the number of user processes for each server process. In a dedicated server configuration, a server process handles requests for a single user process. A shared server configuration lets many user processes share a small number of server processes, minimizing the number of server processes and maximizing the use of available system resources.
On some systems, the user and server processes are separate, while on others they are combined into a single process. If a system uses the shared server or if the user and server processes run on different machines, then the user and server processes must be separate. Client/server systems separate the user and server processes and run them on different machines.
Oracle creates a set of background processes for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. They asynchronously perform I/O and monitor other Oracle process to provide increased parallelism for better performance and reliability.
Each Oracle instance can use several background processes. The names of these processes are DBWn, LGWR, CKPT, SMON, PMON, ARCn, RECO, Jnnn, Dnnn, LMS, and QMNn.
The database writer writes modified blocks from the database buffer cache to the datafiles. Although one database writer process (DBW0) is sufficient for most systems, you can configure additional processes (DBW1 through DBW9 and DBWa through DBWj) to improve write performance for a system that modifies data heavily. The initialization parameter
DB_WRITER_PROCESSES specifies the number of DBWn processes.
Because Oracle uses write-ahead logging, DBWn does not need to write blocks when a transaction commits. Instead, DBWn is designed to perform batched writes with high efficiency. In the most common case, DBWn writes only when more data needs to be read into the SGA and too few database buffers are free. The least recently used data is written to the datafiles first. DBWn also performs writes for other functions, such as checkpointing.
The log writer writes redo log entries to disk. Redo log entries are generated in the redo log buffer of the SGA, and LGWR writes the redo log entries sequentially into an online redo log. If the database has a multiplexed redo log, then LGWR writes the redo log entries to a group of online redo log files.
At specific times, all modified database buffers in the SGA are written to the datafiles by DBWn. This event is called a checkpoint. The checkpoint process is responsible for signaling DBWn at checkpoints and updating all the datafiles and control files of the database to indicate the most recent checkpoint.
The system monitor performs recovery when a failed instance starts up again. With Real Application Clusters, the SMON process of one instance can perform instance recovery for other instances that have failed. SMON also cleans up temporary segments that are no longer in use and recovers terminated transactions skipped during recovery because of file-read or offline errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back online. SMON also coalesces free extents in the dictionary managed tablespaces to make free space contiguous and easier to allocate.
The process monitor performs process recovery when a user process fails. PMON is responsible for cleaning up the cache and freeing resources that the process was using. PMON also checks on dispatcher and server processes and restarts them if they have failed.
The archiver copies the online redo log files to archival storage after a log switch has occurred. Although a single ARCn process (ARC0) is sufficient for most systems, you can specify up to 10 ARCn processes by using the dynamic initialization parameter
LOG_ARCHIVE_MAX_PROCESSES. If the workload becomes too great for the current number of ARCn processes, then LGWR automatically starts another ARCn process up to the maximum of 10 processes. ARCn is active only when a database is in
ARCHIVELOG mode and automatic archiving is enabled.
The recoverer is used to resolve distributed transactions that are pending due to a network or system failure in a distributed database. At timed intervals, the local RECO attempts to connect to remote databases and automatically complete the commit or rollback of the local portion of any pending distributed transactions.
Job queue processes are used for batch processing. Job queue processes are managed dynamically. This enables job queue clients to use more job queue processes when required. The resources used by the new processes are released when they are idle.
Dispatchers are optional background processes, present only when a shared server configuration is used. At least one dispatcher process is created for every communication protocol in use (D000, . . ., Dnnn). Each dispatcher process is responsible for routing requests from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.
The Lock Manager Server process (LMS) is used for inter-instance locking in Real Application Clusters.
"Real Application Clusters: Multiple Instance Systems" for more information about the configuration of the lock process
Queue monitors are optional background processes that monitor the message queues for Oracle Advanced Queuing. You can configure up to 10 queue monitor processes.
The program interface is the mechanism by which a user process communicates with a server process. It serves as a method of standard communication between any client tool or application (such as Oracle Forms) and Oracle software. Its functions are to:
If the user and server processes are on different computers of a network, or if user processes connect to shared server processes through dispatcher processes, then the user process and server process communicate using Oracle Net Services. Dispatchers are optional background processes, present only in the shared server configuration.
Oracle Net Services is Oracle's mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed databases. Communication protocols define the way that data is transmitted and received on a network. In a networked environment, an Oracle database server communicates with client workstations and other Oracle database servers using Oracle Net Services software.
Oracle Net Services supports communications on all major network protocols, ranging from those supported by PC LANs to those used by the largest of mainframe computer systems.
Using Oracle Net Services, application developers do not need to be concerned with supporting network communications in a database application. If a new protocol is used, then the database administrator makes some minor changes, while the application requires no modifications and continues to function.
The following example describes the most basic level of operations that Oracle performs. This illustrates an Oracle configuration where the user and associated server process are on separate machines (connected through a network).
Chapter 8, "Process Architecture" for more information about Oracle configuration
There are two common ways to architect a database: client/server or multitier. As internet computing becomes more prevalent in computing environments, many database management systems are moving to a multitier environment.
Multiprocessing uses more than one processor for a set of related jobs. Distributed processing reduces the load on a single processor by allowing different processors to concentrate on a subset of related tasks, thus improving the performance and capabilities of the system as a whole.
An Oracle database system can easily take advantage of distributed processing by using its client/server architecture. In this architecture, the database system is divided into two parts: a front-end or a client and a back-end or a server.
The client is the front-end database application, accessed by a user through the keyboard, display, and pointing device, such as a mouse. The client has no data access responsibilities. It requests, processes, and presents data managed by the server. The client workstation can be optimized for its job. For example, it might not need large disk capacity, or it might benefit from graphic capabilities.
Often, the client runs on a different computer than the database server, generally on a PC. Many clients can simultaneously run against one server.
The server runs Oracle software and handles the functions required for concurrent, shared data access. The server receives and processes the SQL and PL/SQL statements that originate from client applications. The computer that manages the server can be optimized for its duties. For example, it can have large disk capacity and fast processors.
A multitier architecture has the following components:
This architecture enables use of an application server to:
The identity of the client is maintained throughout all tiers of the connection.
A distributed database is a network of databases managed by multiple database servers that are used together. They are not usually seen as a single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified. The primary benefit of a distributed database is that the data of physically separate databases can be logically combined and potentially made accessible to all users on a network.
Each computer that manages a database in the distributed database is called a node. The database to which a user is directly connected is called the local database. Any additional databases accessed by this user are called remote databases. When a local database accesses a remote database for information, the local database is a client of the remote server. This is an example of client/server architecture.
A database link describes a path from one database to another. Database links are implicitly used when a reference is made to a global object name in a distributed database.
While a distributed database enables increased access to a large amount of data across a network, it must also hide the location of the data and the complexity of accessing it across the network. The distributed database management system must also preserve the advantages of administrating each local database as though it were not distributed.
|See Also :
Oracle9i Database Administrator's Guide for more information about distributed databases
Location transparency occurs when the physical location of data is transparent to the applications and users of a database system. Several Oracle features, such as views, procedures, and synonyms, can provide location transparency. For example, a view that joins table data from several databases provides location transparency because the user of the view does not need to know from where the data originates.
Site autonomy means that each database participating in a distributed database is administered separately and independently from the other databases, as though each database were a non-networked database. Although each database can work with others, they are distinct, separate systems that are cared for individually.
The Oracle distributed database architecture supports all DML operations, including queries, inserts, updates, and deletes of remote table data. To access remote data, you make reference to the remote object's global object name. No coding or complex syntax is required to access remote data.
For example, to query a table named
employees in the remote database named
sales, reference the table's global object name:
Oracle provides the same assurance of data consistency in a distributed environment as in a nondistributed environment. Oracle provides this assurance using the transaction model and a two-phase commit mechanism.
As in nondistributed systems, transactions should be carefully planned to include a logical set of SQL statements that should all succeed or fail as a unit. Oracle's two-phase commit mechanism guarantees that no matter what type of system or network failure occurs, a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database.
Replication is the process of copying and maintaining database objects, such as tables, in multiple databases that make up a distributed database system. Changes applied at one site are captured and stored locally before being forwarded and applied at each of the remote locations. Oracle replication is a fully integrated feature of the Oracle server. It is not a separate server.
Replication uses distributed database technology to share data between multiple sites, but a replicated database and a distributed database are not the same. In a distributed database, data is available at many locations, but a particular table resides at only one location. For example, the
employees table can reside at only the
db1 database in a distributed database system that also includes the
db3 databases. Replication means that the same data is available at multiple locations. For example, the
employees table can be available at
Distributed database systems often locally replicate remote tables that are frequently queried by local users. By having copies of heavily accessed data on several nodes, the distributed database does not need to send information across a network repeatedly, thus helping to maximize the performance of the database application.
Data can be replicated using materialized views.
Oracle supports materialized views that are hierarchical and updatable. Multitier replication provides increased flexibility of design for a distributed application. Using multitier materialized views, applications can manage multilevel data subsets with no direct connection between levels.
An updatable materialized view lets you insert, update, and delete rows in the materialized view and propagate the changes to the target master table. Synchronous and asynchronous replication is supported.
Figure 1-4 shows an example of multitier architecture, diagrammed as an inverted tree structure. Changes are propagated up and down along the branches connecting the outermost materialized views with the master (the root).
In Oracle9i conflict resolution routines are defined at the top level, the master site, and are pulled into the updatable materialized view site when needed. This makes it possible to have multitier materialized views. Existing system-defined conflict resolution methods are supported.
In addition, users can write their own conflict resolution routines. A user-defined conflict resolution method is a PL/SQL function that returns either
True indicates that the method was able to successfully resolve all conflicting modifications for a column group.
Oracle Streams enables the sharing of data and events in a data stream, either within a database or from one database to another. The stream routes specified information to specified destinations. Oracle Streams provides the capabilities needed to build and operate distributed enterprises and applications, data warehouses, and high availability solutions. You can use all the capabilities of Oracle Streams at the same time. If your needs change, you can implement a new capability of Streams without sacrificing existing capabilities.
Using Oracle Streams, you control what information is put into a stream, how the stream flows or is routed from database to database, what happens to events in the stream as they flow into each database, and how the stream terminates. By configuring specific capabilities of Streams, you can address specific requirements. Based on your specifications, Streams can capture and manage events in the database automatically, including, but not limited to, DML changes and DDL changes. You can also put user-defined events into a stream. Then, Streams can propagate the information to other databases or applications automatically. Again, based on your specifications, Streams can apply events at a destination database.
You can use Streams to:
You can configure a background capture process to capture changes made to tables, schemas, or the entire database. A capture process captures changes from the redo log and formats each captured change into a logical change record (LCR). The database where changes are generated in the redo log is called the source database.
A capture process enqueues LCR events into a queue that you specify. The queue can then share the LCR events within the same database or with other databases.
You can also enqueue user events into a queue explicitly with a user application. These explicitly enqueued events can be LCRs or user messages.
A background apply process can dequeue events from a queue. You can also dequeue events explicitly with a user application.
You can configure an apply process to apply all of the events in a queue or only the events that you specify. You can also configure an apply process to call your own PL/SQL subprograms to process events.
The database where LCR events are applied and other types of events are processed is called the destination database. In some configurations, the source database and the destination database may be the same.
Other capabilities of Streams include the following:
Oracle Advanced Queuing provides an infrastructure for distributed applications to communicate asynchronously using messages. Oracle Advanced Queuing stores messages in queues for deferred retrieval and processing by the Oracle server. This provides a reliable and efficient queuing system without additional software such as transaction processing monitors or message-oriented middleware.
Messages pass between clients and servers, as well as between processes on different servers. An effective messaging system implements content-based routing, subscription, and querying.
A messaging system can be classified into one of two types:
Synchronous communication is based on the request/reply paradigm--a program sends a request to another program and waits until the reply arrives.
This model of communication (also called online or connected) is suitable for programs that need to get the reply before they can proceed with their work. Traditional client/server architectures are based on this model. The major drawback of this model is that the programs where the request is sent must be available and running for the calling application to work.
In the disconnected or deferred model, programs communicate asynchronously, placing requests in a queue and then proceeding with their work.
For example, an application might require entry of data or execution of an operation after specific conditions are met. The recipient program retrieves the request from the queue and acts on it. This model is suitable for applications that can continue with their work after placing a request in the queue -- they are not blocked waiting for a reply.
For deferred execution to work correctly in the presence of network, machine, and application failures, the requests must be stored persistently and processed exactly once. This is achieved by combining persistent queuing with transaction protection.
Heterogeneous Services is necessary for accessing a non-Oracle database system. The term "non-Oracle database system" refers to the following:
Heterogeneous Services makes it possible for users to do the following:
Heterogeneous Services is generally applied in one of two ways:
This section explains the software mechanisms used by Oracle to fulfill the following important requirements of an information management system:
A primary concern of a multiuser database management system is how to control concurrency, which is the simultaneous access of the same data by many users. Without adequate concurrency controls, data could be updated or changed improperly, compromising data integrity.
If many people are accessing the same data, one way of managing data concurrency is to make each user wait for a turn. The goal of a database management system is to reduce that wait so it is either nonexistent or negligible to each user. All data manipulation language statements should proceed with as little interference as possible, and destructive interactions between concurrent transactions must be prevented. Destructive interaction is any interaction that incorrectly updates data or incorrectly alters underlying data structures. Neither performance nor data integrity can be sacrificed.
Oracle resolves such issues by using various types of locks and a multiversion consistency model. Both features are discussed later in this section. These features are based on the concept of a transaction. It is the application designer's responsibility to ensure that transactions fully exploit these concurrency and consistency features.
Read consistency, as supported by Oracle, does the following:
The simplest way to think of Oracle's implementation of read consistency is to imagine each user operating a private copy of the database, hence the multiversion consistency model.
To manage the multiversion consistency model, Oracle must create a read-consistent set of data when a table is being queried (read) and simultaneously updated (written). When an update occurs, the original data values changed by the update are recorded in the database's undo records. As long as this update remains part of an uncommitted transaction, any user that later queries the modified data views the original data values. Oracle uses current information in the system global area and information in the undo records to construct a read-consistent view of a table's data for a query.
Only when a transaction is committed are the changes of the transaction made permanent. Statements that start after the user's transaction is committed only see the changes made by the committed transaction.
Note that a transaction is key to Oracle's strategy for providing read consistency. This unit of committed (or uncommitted) SQL statements:
By default, Oracle guarantees statement-level read consistency. The set of data returned by a single query is consistent with respect to a single point in time. However, in some situations, you might also require transaction-level read consistency. This is the ability to run multiple queries within a single transaction, all of which are read-consistent with respect to the same point in time, so that queries in this transaction do not see the effects of intervening committed transactions.
If you want to run a number of queries against multiple tables and if you are not doing any updating, you prefer a read-only transaction. After indicating that your transaction is read-only, you can run as many queries as you like against any table, knowing that the results of each query are consistent with respect to the same point in time.
Oracle also uses locks to control concurrent access to data. Locks are mechanisms intended to prevent destructive interaction between users accessing Oracle data.
Locks are used to ensure consistency and integrity. Consistency means that the data a user is viewing or changing is not changed (by other users) until the user is finished with the data. Integrity means that the database's data and structures reflect all changes made to them in the correct sequence.
Locks guarantee data integrity while enabling maximum concurrent access to the data by unlimited users.
Oracle locking is performed automatically and requires no user action. Implicit locking occurs for SQL statements as necessary, depending on the action requested.
Oracle's lock manager automatically locks table data at the row level. By locking table data at the row level, contention for the same data is minimized.
Oracle's lock manager maintains several different types of row locks, depending on what type of operation established the lock. The two general types of locks are exclusive locks and share locks. Only one exclusive lock can be placed on a resource (such as a row or a table); however, many share locks can be placed on a single resource. Both exclusive and share locks always allow queries on the locked resource but prohibit other activity on the resource (such as updates and deletes).
Under some circumstances, a user might want to override default locking. Oracle allows manual override of automatic locking features at both the row level (by first querying for the rows that will be updated in a subsequent statement) and the table level.
Database administrators occasionally need isolation from concurrent non-database administrator actions, that is, isolation from concurrent non-database administrator transactions, queries, or PL/SQL statements. One way to provide such isolation is to shut down the database and reopen it in restricted mode. The Quiesce Database feature provides another way of providing isolation: to put the system into quiesced state without disrupting users.
The database administrator uses SQL statements to quiesce the database. After the system is in quiesced state, the database administrator can safely perform certain actions whose executions require isolation from concurrent non-DBA users.
Oracle includes security features that control how a database is accessed and used. For example, security mechanisms:
Associated with each database user is a schema by the same name. By default, each database user creates and has access to all objects in the corresponding schema.
Database security can be classified into two categories: system security and data security.
System security includes the mechanisms that control the access and use of the database at the system level. For example, system security includes:
System security mechanisms check whether a user is authorized to connect to the database, whether database auditing is active, and which system operations a user can perform.
Data security includes the mechanisms that control the access and use of the database at the schema object level. For example, data security includes:
INSERTstatements but not
DELETEstatements using the
The Oracle server provides discretionary access control, which is a means of restricting access to information based on privileges. The appropriate privilege must be assigned to a user in order for that user to access a schema object. Appropriately privileged users can grant other users privileges at their discretion. For this reason, this type of security is called discretionary.
Oracle manages database security using several different facilities:
Figure 1-5 illustrates the relationships of the different Oracle security facilities, and the following sections provide an overview of users, privileges, and roles.
Each Oracle database has a list of usernames. To access a database, a user must use a database application and attempt a connection with a valid username of the database. Each username has an associated password to prevent unauthorized use.
Each user has a security domain--a set of properties that determine such things as:
Each property that contributes to a user's security domain is discussed in the following sections.
A privilege is a right to run a particular type of SQL statement. Some examples of privileges include the right to:
The privileges of an Oracle database can be divided into two categories: system privileges and schema object privileges.
System privileges allow users to perform a particular systemwide action or a particular action on a particular type of schema object. For example, the privileges to create a tablespace or to delete the rows of any table in the database are system privileges. Many system privileges are available only to administrators and application developers because the privileges are very powerful.
Schema object privileges allow users to perform a particular action on a specific schema object. For example, the privilege to delete rows of a specific table is an object privilege. Object privileges are granted (assigned) to users so that they can use a database application to accomplish specific tasks.
Privileges are granted to users so that users can access and modify data in the database. A user can receive a privilege two different ways:
employeestable can be explicitly granted to the user
employeestable can be granted to the role named
CLERK, which in turn can be granted to the users
Because roles enable easier and better management of privileges, privileges are normally granted to roles and not to specific users. The following section explains more about roles and their use.
Oracle provides for easy and controlled privilege management through roles. Roles are named groups of related privileges that you grant to users or other roles.
"Introduction to Roles" for more information about role properties
Oracle provides a way to direct and limit the use of disk space allocated to the database for each user, including default and temporary tablespaces and tablespace quotas.
Each user is associated with a default tablespace. When a user creates a table, index, or cluster and no tablespace is specified to physically contain the schema object, the user's default tablespace is used if the user has the privilege to create the schema object and a quota in the specified default tablespace. The default tablespace feature provides Oracle with information to direct space use in situations where schema object's location is not specified.
Each user has a temporary tablespace. When a user runs a SQL statement that requires the creation of temporary segments (such as the creation of an index), the user's temporary tablespace is used. By directing all users' temporary segments to a separate tablespace, the temporary tablespace feature can reduce I/O contention among temporary segments and other types of segments.
Oracle can limit the collective amount of disk space available to the objects in a schema. Quotas (space limits) can be set for each tablespace available to a user. The tablespace quota security feature permits selective control over the amount of disk space that can be consumed by the objects of specific schemas.
Each user is assigned a profile that specifies limitations on several system resources available to the user, including the following:
Different profiles can be created and assigned individually to each user of the database. A default profile is present for all users not explicitly assigned a profile. The resource limit feature prevents excessive consumption of global database system resources.
Oracle permits selective auditing (recorded monitoring) of user actions to aid in the investigation of suspicious database use. Auditing can be performed at three different levels: Statement Auditing, Privilege Auditing, and Schema Object Auditing.
Statement auditing is the auditing of specific SQL statements without regard to specifically named schema objects. In addition, database triggers let a database administrator to extend and customize Oracle's built-in auditing features.
Statement auditing can be broad and audit all users of the system or can be focused to audit only selected users of the system. For example, statement auditing by user can audit connections to and disconnections from the database by the users
Privilege auditing is the auditing of powerful system privileges without regard to specifically named schema objects. Privilege auditing can be broad and audit all users or can be focused to audit only selected users.
Schema object auditing is the auditing of access to specific schema objects without regard to user. Object auditing monitors the statements permitted by object privileges, such as
DELETE statements on a given table.
For all types of auditing, Oracle allows the selective auditing of successful statement executions, unsuccessful statement executions, or both. This enables monitoring of suspicious statements, regardless of whether the user issuing a statement has the appropriate privileges to issue the statement. The results of audited operations are recorded in a table called the audit trail. Predefined views of the audit trail are available so you can easily retrieve audit records.
Fine-grained auditing allows the monitoring of data access based on content. For example, a central tax authority needs to track access to tax returns to guard against employee snooping. Enough detail is wanted to be able to determine what data was accessed, not just that
SELECT privilege was used by a specific user on a particular table. Fine-grained auditing provides this functionality.
In general, fine-grained auditing policy is based on simple user-defined SQL predicates on table objects as conditions for selective auditing. During fetching, whenever policy conditions are met for a returning row, the query is audited. Later, Oracle executes user-defined audit event handlers using autonomous transactions to process the event.
Fine-grained auditing can be implemented in user applications using the
DBMS_FGA package or by using database triggers.
People who administer the operation of an Oracle database system, known as database administrators (DBAs), are responsible for creating Oracle databases, ensuring their smooth operation, and monitoring their use.
Oracle9i Database Administrator's Guide for detailed information on database administration tasks
Enterprise Manager is a system management tool that provides an integrated solution for centrally managing your heterogeneous environment. Combining a graphical console, Oracle Management Servers, Oracle Intelligent Agents, common services, and administrative tools, Enterprise Manager provides a comprehensive systems management platform for managing Oracle products.
From the client interface, the Enterprise Manager Console, you can perform the following tasks:
This section covers the structures and mechanisms used by Oracle to provide:
In every database system, the possibility of a system or hardware failure always exists. If a failure occurs and affects the database, the database must be recovered. The goals after a failure are to ensure that the effects of all committed transactions are reflected in the recovered database and to return to normal operation as quickly as possible while insulating users from problems caused by the failure.
Several circumstances can halt the operation of an Oracle database. The most common types of failure are described in the following table.
Requires a database to be recovered to a point in time before the error occurred. For example, a user could accidentally drop a table. To enable recovery from user errors and accommodate other unique recovery requirements, Oracle provides exact point-in-time recovery. For example, if a user accidentally drops a table, the database can be recovered to the instant in time before the table was dropped.
Occurs when there is a logical failure in the handling of a statement in an Oracle program. When statement failure occurs, any effects of the statement are automatically undone by Oracle and control is returned to the user.
Results from a failure in a user process accessing Oracle, such as an abnormal disconnection or process termination. The background process PMON automatically detects the failed user process, rolls back the uncommitted transaction of the user process, and releases any resources that the process was using.
Occurs when a problem arises that prevents an instance from continuing work. Instance failure can result from a hardware problem such as a power outage, or a software problem such as an operating system failure. When an instance failure occurs, the data in the buffers of the system global area is not written to the datafiles.
After an instance failure, Oracle automatically performs instance recovery. If one instance in a Real Application Clusters environment, another instance recovers the redo for the failed instance. In a single-instance database, or in a Real Application Cluster database in which all instances fail, Oracle automatically applies all redo when you restart the database.
Media (disk) failure
Different files can be affected by this type of disk failure, including the datafiles, the redo log files, and the control files. Also, because the database instance cannot continue to function properly, the data in the database buffers of the system global area cannot be permanently written to the datafiles.
A disk failure requires you to restore lost files and then perform media recovery. Unlike instance recovery, media recovery must be initiated by the user. Media recovery updates restored datafiles so the information in them corresponds to the most recent time point before the disk failure, including the committed data in memory that was lost because of the failure.
Oracle provides for complete media recovery from all possible types of hardware failures, including disk failures. Options are provided so that a database can be completely recovered or partially recovered to a specific point in time.
If some datafiles are damaged in a disk failure but most of the database is intact and operational, the database can remain open while the required tablespaces are individually recovered. Therefore, undamaged portions of a database are available for normal use while damaged portions are being recovered.
Oracle uses several structures to provide complete recovery from an instance or disk failure: the redo log, undo records, a control file, and database backups. If compatibility is set to Oracle9i or higher, undo records can be stored in either undo tablespaces or rollback segments.
The redo log is a set of files that protect altered database data in memory that has not been written to the datafiles. The redo log can consist of two parts: the online redo log and the archived redo log.
The online redo log is a set of two or more online redo log files that record all changes made to the database, including both uncommitted and committed changes. Redo entries are temporarily stored in redo log buffers of the system global area, and the background process LGWR writes the redo entries sequentially to an online redo log file. LGWR writes redo entries continually, and it also writes a commit record every time a user process commits a transaction.
Optionally, filled online redo files can be manually or automatically archived before being reused, creating archived redo logs.
To enable or disable archiving, set the database in one of the following modes:
ARCHIVELOG: The filled online redo log files are archived before they are reused in the cycle.
NOARCHIVELOG: The filled online redo log files are not archived.
ARCHIVELOG mode, the database can be completely recovered from both instance and disk failure. The database can also be backed up while it is open and available for use. However, additional administrative operations are required to maintain the archived redo log.
If the database's redo log operates in
NOARCHIVELOG mode, the database can be completely recovered from instance failure but not from disk failure. Also, the database can be backed up only while it is completely closed. Because no archived redo log is created, no extra work is required by the database administrator.
Undo records can be stored in either undo tablespaces or rollback segments. Oracle uses the undo data for a variety of purposes, including accessing before-images of blocks changed in uncommitted transactions. During database recovery, Oracle applies all changes recorded in the redo log and then uses undo information to roll back any uncommitted transactions.
The control files of a database keep, among other things, information about the file structure of the database and the current log sequence number being written by LGWR. During normal recovery procedures, the information in a control file is used to guide the automated progression of the recovery operation. Oracle can multiplex the control file, that is, simultaneously maintain a number of identical control files.
Because one or more files can be physically damaged as the result of a disk failure, media recovery requires the restoration of the damaged files from the most recent operating system backup of a database.
You can either back up the database files with Recovery Manager, which is recommended, or use operating system utilities. Recovery Manager (RMAN) is an Oracle utility that manages backup and recovery operations, creates backups of database files (datafiles, control files, and archived redo log files), and restores or recovers a database from backups.
A data warehouse is a relational database designed for query and analysis rather than for transaction processing. It usually contains historical data derived from transaction data, but it can include data from other sources. It separates analysis workload from transaction workload and enables an organization to consolidate data from several sources.
In addition to a relational database, a data warehouse environment includes an extraction, transportation, transformation, and loading (ETL) solution, an online analytical processing (OLAP) engine, client analysis tools, and other applications that manage the process of gathering data and delivering it to business users.
Data warehouses and OLTP systems have very different requirements. Here are some examples of differences between typical data warehouses and OLTP systems:
Data warehouses are designed to accommodate ad hoc queries. You might not know the workload of your data warehouse in advance, so a data warehouse should be optimized to perform well for a wide variety of possible query operations.
OLTP systems support only predefined operations. Your applications might be specifically tuned or designed to support only these operations.
A data warehouse is updated on a regular basis by the ETL process (run nightly or weekly) using bulk data modification techniques. The end users of a data warehouse do not directly update the data warehouse.
In OLTP systems, end users routinely issue individual data modification statements to the database. The OLTP database is always up to date, and reflects the current state of each business transaction.
Data warehouses often use denormalized or partially denormalized schemas (such as a star schema) to optimize query performance.
OLTP systems often use fully normalized schemas to optimize update/insert/delete performance, and to guarantee data consistency.
A typical data warehouse query scans thousands or millions of rows.For example, "Find the total sales for all customers last month."
A typical OLTP operation accesses only a handful of records. For example, "Retrieve the current order for this customer."
Data warehouses usually store many months or years of data. This is to support historical analysis.
OLTP systems usually store data from only a few weeks or months. The OLTP system stores only historical data as needed to successfully meet the requirements of the current transaction.
Data warehouses and their architectures vary depending upon the specifics of an organization's situation. Three common architectures are:
Figure 1-6 shows a simple architecture for a data warehouse. End users directly access data derived from several source systems through the data warehouse.
In Figure 1-6, the metadata and raw data of a traditional OLTP system is present, as is an additional type of data, summary data. Summaries are very valuable in data warehouses because they pre-compute long operations in advance. For example, a typical data warehouse query is to retrieve something like August sales.
Summaries in Oracle are called materialized views.
Figure 1-6, you need to clean and process your operational data before putting it into the warehouse. You can do this programmatically, although most data warehouses use a staging area instead. A staging area simplifies building summaries and general warehouse management. Figure 1-7 illustrates this typical architecture.
Although the architecture in Figure 1-7 is quite common, you might want to customize your warehouse's architecture for different groups within your organization.
Do this by adding data marts, which are systems designed for a particular line of business. Figure 1-8 illustrates an example where purchasing, sales, and inventories are separated. In this example, a financial analyst might want to analyze historical data for purchases and sales.
A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, a materialized view contains the rows resulting from a query against one or more base tables or views. A materialized view can be stored in the same database as its base tables or in a different database.
Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Query rewrites are particularly useful in a data warehouse environment.
Oracle integrates Online Analytical Processing (OLAP) into the database to support business intelligence. This integration provides the power of a multidimensional database while retaining the manageability, scalability, and reliability of the Oracle database and the accessibility of SQL.
The relational management system and Oracle OLAP provide complementary functionality to support a full range of reporting and analytic applications. Applications developers can choose to use SQL OLAP functions for standard and ad-hoc reporting. When additional analytic functionality is needed, Oracle OLAP can be used to provide capabilities such as multidimensional calculations, forecasting, modeling, and what-if scenarios. These calculations enable developers to build sophisticated analytic and planning applications such as sales and marketing analysis, enterprise budgeting and financial analysis, and demand planning systems.
Data can be stored in either relational tables or multidimensional objects, whichever is more suitable in terms of performance and resources. Regardless of where the data is stored, it can be manipulated in the OLAP engine using either Java or SQL. There is no need for data replication between relational and multidimensional data sources.
Oracle OLAP consists of the following components:
Oracle9i OLAP User's Guide for more information about Oracle OLAP
Change data capture efficiently identifies and captures data that has been added to, updated, or removed from Oracle relational tables, and makes the change data available for use by applications.
Oftentimes, data warehousing involves the extraction and transportation of relational data from one or more source databases into the data warehouse for analysis. Change data capture quickly identifies and processes only the data that has changed, not entire tables, and makes the change data available for further use.
Change data capture does not depend on intermediate flat files to stage the data outside of the relational database. It captures the change data resulting from
DELETE operations made to user tables. The change data is then stored in a database object called a change table, and the change data is made available to applications in a controlled way.
Computing environments configured to provide nearly full-time availability are known as high availability systems. Such systems typically have redundant hardware and software that makes the system available despite failures. Well-designed high availability systems avoid having single points-of-failure. Any hardware or software component that can fail has a redundant component of the same type.
When failures occur, the failover process moves processing performed by the failed component to the backup component. This process remasters systemwide resources, recovers partial or failed transactions, and restores the system to normal, preferably within a matter of microseconds. The more transparent that failover is to users, the higher the availability of the system.
Oracle has a number of products and features that provide high availability. These include multiplexed redo log files, Recovery Manager (RMAN), Fast-Start Recovery, LogMiner, flashback query, partitioning, Transparent Application Failover, online reorganization, Oracle Replication, Oracle Data Guard and Standby Database, Real Application Clusters, and Oracle Real Application Clusters Guard. These can be used in various combinations to meet specific high availability needs.
Transparent Application Failover (TAF) enables an application user to automatically reconnect to a database if the connection fails. Active transactions roll back, but the new database connection, made by way of a different node, is identical to the original. This is true regardless of how the connection fails.
With Transparent Application Failover, a client notices no loss of connection as long as there is one instance left serving the application. The database administrator controls which applications run on which instances and also creates a failover order for each application.
During normal client/server database operations, the client maintains a connection to the database so the client and server can communicate. If the server fails, so then does the connection. The next time the client tries to use the connection the client issues an error. At this point, the user must log in to the database again.
With Transparent Application Failover, however, Oracle automatically obtains a new connection to the database. This enables users to continue working as if the original connection had never failed.
There are several elements associated with active database connections. These include:
Transparent Application Failover automatically restores some of these elements. However, you might need to embed other elements in the application code to enable transparent application failover.
Database administrators can perform a variety of online operations to table definitions, including online reorganization of heap-organized tables. This makes it possible to reorganize a table while users have full access to it.
This online architecture provides the following capabilities:
ROWIDs (used in secondary indexes and mapping table on index-organized tables) also can be used.
ROWIDs stored in secondary indexes on IOTs. This allows online repair of invalid physical guesses
Oracle Data Guard maintains up to nine standby databases, each of which is a real-time copy of the production database, to protect against all threats--corruptions, human errors, and disasters. If a failure occurs on the production (primary) database, you can failover to one of the standby databases to become the new primary database. In addition, planned downtime for maintenance can be reduced because you can quickly and easily move (switch over) production processing from the current primary database to a standby database, and then back again.
To protect against unlogged direct writes in the primary database that cannot be propagated to the standby database, turn on
A Data Guard configuration is a collection of loosely connected systems, consisting of a single primary database and up to nine standby databases that can include a mix of both physical and logical standby databases. The databases in a Data Guard configuration can be connected by a LAN in the same data center, or--for maximum disaster protection--geographically dispersed over a WAN and connected by Oracle Network Services.
A Data Guard configuration can be deployed for any database. This is possible because its use is transparent to applications; no application code changes are required to accommodate a standby database. Moreover, Data Guard lets you tune the configuration to balance data protection levels and application performance impact; you can configure the protection mode to maximize data protection, maximize availability, or maximize performance.
As application transactions make changes to the primary database, the changes are logged locally in redo logs, which are sent to the standby databases by log transport services and applied by log apply services.
For physical standby databases, the changes are applied to each physical standby database that is running in managed recovery mode. For logical standby databases, the changes are applied using SQL regenerated from the archived redo logs.
A physical standby database is physically identical to the primary database. While the primary database is open and active, a physical standby database is either performing recovery (by applying logs), or open for reporting access. A physical standby database can be queried read-only when not performing recovery while the production database continues to ship redo data to the physical standby site.
Physical standby on disk database structures must be identical to the primary database on a block-for-block basis, because a recovery operation applies changes block-for-block using the physical ROWID. The database schema, including indexes, must be the same, and the database cannot be opened (other than for read-only access). If opened, the physical standby database will have different ROWIDs, making continued recovery impossible.
A logical standby database takes standard Oracle archived redo logs, transforms the redo records they contain into SQL transactions, and then applies them to an open standby database. Although changes can be applied concurrently with end-user access, the tables being maintained through regenerated SQL transactions allow read-only access to users of the logical standby database. Because the database is open, it is physically different from the primary database. The database tables can have different indexes and physical characteristics from their primary database peers, but must maintain logical consistency from an application access perspective, to fulfill their role as a standby data source.
Oracle Data Guard Broker automates complex creation and maintenance tasks and provides dramatically enhanced monitoring, alert, and control mechanisms. It uses background agent processes that are integrated with the Oracle database server and associated with each Data Guard site to provide a unified monitoring and management infrastructure for an entire Data Guard configuration. Two user interfaces are provided to interact with the Data Guard configuration, a command-line interface (
DGMGRL) and a graphical user interface called Data Guard Manager.
Oracle Data Guard Manager, which is integrated with Oracle Enterprise Manager, provides wizards to help you easily create, manage, and monitor the configuration. This integration lets you take advantage of other Enterprise Manager features, such as to provide an event service for alerts, the discovery service for easier setup, and the job service to ease maintenance.
LogMiner is a relational tool that lets administrators use SQL to read, analyze, and interpret log files. LogMiner can view both online and archived redo logs. The Enterprise Manager application LogMiner Viewer adds a GUI-based interface.
The ability of LogMiner to access data stored in redo logs helps you to perform many database management tasks. For example, you can do the following:
Real Application Clusters are inherently high availability systems. Clusters typical of Real Application Clusters environments can provide continuous service for both planned and unplanned outages.
Real Application Clusters builds higher levels of availability on top of the standard Oracle features. All single instance high availability features, such as Fast-Start Recovery and online reorganizations, apply to Real Application Clusters as well. Fast-Start Recovery can greatly reduce mean time to recover (MTTR) with minimal effects on online application performance. Online reorganizations reduce the durations of planned downtimes. Many operations can be performed online while users update the underlying objects. Real Application Clusters preserves all these standard Oracle features.
In addition to all the regular Oracle features, Real Application Clusters exploits the redundancy provided by clustering to deliver availability with n-1 node failures in an n-node cluster. In other words, all users have access to all data as long as there is one available node in the cluster.
Oracle Real Application Clusters Guard is an integral component of Real Application Clusters. Oracle Real Application Clusters Guard provides the following functions:
A database server that runs Real Application Clusters consists of the Oracle database, Real Application Clusters software, and the Oracle Net listeners that accept client requests. These software components run on each node of a cluster. They use the services provided by the hardware, the operating system, and the port-specific Cluster Manager. The Cluster Manager monitors and reports the health of the nodes in the cluster and controls pack behavior.
Oracle provides a single platform for creating, managing, and delivering personalized, rich content to any device. Corporate information assets - documents, spreadsheets, multimedia, presentations, e-mail, and HTML files - are easily accessible to all users, and there is no need for specialty servers or unrelated file systems. Automatic search capabilities can discover valuable content wherever it resides and whatever language it is in.
Oracle's content management features include the following:
Oracle provides access for creating and delivering content, while at the same time keeping content manageable. Not only can you create, manage, and deliver content through out-of-the-box interfaces like the Oracle Internet File System, but also through the Java, XML, and PL/SQL APIs.
A large amount of critical business information usually resides in documents, spreadsheets, email, and Web pages. This data often exists only on someone's laptop or in a departmental file server, obscured from the rest of the organization. The Oracle Internet File System creates a secure, scalable file service that reaches all your information.