Introduction to Architecture of Oracle Database

Oracle Database Architecture:

An Oracle server:

• Is a database management system that provides an open,     comprehensive, integrated approach to information management.

• Consists of an Oracle instance and an Oracle Database.

Oracle instance:

·        An instance is a set of memory structures that manage database files.

·        The instance consists of a shared memory area, called the system global area (SGA), and a set of background processes.

·        An instance can exist independently of database files.

Oracle Database:

·        A database is a set of files, located on disk, that store data.

·        These files can exist independently of a database instance.

Oracle Database Architecture:

·        The Oracle server is the key to information management.

·        In general, an Oracle server must reliably manage a large amount of data in a multiuser environment so that many users can concurrently access the same data.

·        All this must be accomplished while delivering high performance.

·        An Oracle server must also prevent unauthorized access and provide efficient solutions for failure recovery.

Database Structures:

1.     Memory structures

2.     Process structures

3.     Storage structures

·        Each running Oracle database is associated with an Oracle instance.

·        When a database is started on a database server, the Oracle software allocates a shared memory area called the System Global Area (SGA) and starts several Oracle background processes. This combination of the SGA and the Oracle processes is called an Oracle instance.

·        After starting an instance, the Oracle software associates the instance with a specific database. This is called mounting the database.

·        The database is then ready to be opened, which makes it accessible to authorized users.

·        Multiple instances can execute concurrently on the same computer, each accessing its own physical database.

·        An Oracle database 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 server.

·        Processes are jobs that work in the memory of these computers.

·        A process is defined as a “thread of control” or a mechanism in an operating system that can run a series of steps.

Oracle Memory Structures:

The basic memory structures associated with an Oracle instance include the following:

• System Global Area (SGA): Shared by all server and background processes

• Program Global Area (PGA): Private to each server and background process. There is one PGA for each process.

System Global Area (SGA):

The SGA is a memory area that contains data and control information for the instance.

The SGA includes the following data structures:

• Database buffer cache: Caches blocks of data retrieved from the database.

• Redo log buffer: Caches redo information (used for instance recovery) until it can be written to the physical redo log files stored on the disk.

• Shared pool: Caches various constructs that can be shared among users.

• Large pool: Is an optional area that provides large memory allocations for certain large processes, such as Oracle backup and recovery operations, and I/O server processes.

• Java pool: Is used for all session-specific Java code and data within the Java Virtual Machine (JVM).

• Streams pool: Is used by Oracle Streams.

·        When you start the instance by using Enterprise Manager or SQL*Plus, the amount of memory allocated for the SGA is displayed.

·        With the dynamic SGA infrastructure, the size of the database buffer cache, the shared pool, the large pool, the Java pool, and the Streams pool changes without shutting down the instance.

·        The Oracle database uses initialization parameters to create and configure memory structures.

·        For example: the SGA_TARGET parameter specifies the total amount of space available to the SGA. If you set SGA_TARGET to 0, Automatic Shared Memory Management is disabled.

Program Global Area (PGA):

·        A Program Global Area (PGA) is a memory region that contains data and control information for each server process.

·        An Oracle server process services a client’s requests.

·        Each server process has its own private PGA that is created when the server process is started.

·        Access to the PGA is exclusive to that server process, and the PGA is read and written only by the Oracle code acting on its behalf.

Process Structures:

·        User process: Is started at the time a database user requests a connection to the Oracle server

·        Server process: Connects to the Oracle instance and is started when a user establishes a session.

·        Background processes: Are started when an Oracle instance is started

·        When you invoke an application program or an Oracle tool, such as Enterprise Manager, the Oracle server creates a server process to execute the commands issued by the application.

·        The Oracle server also creates a set of background processes for an instance that interact with each other and with the operating system to manage the memory structures, asynchronously perform I/O to write data to disk, and perform other required tasks.

·        Which background processes are present depends on the features that are being used in the database.

Oracle Instance Management:

Oracle Instance Management:

·        An Oracle database server consists of an Oracle database and an Oracle instance.

·        An Oracle instance is made up of memory structures, known as the System Global Area (SGA), and background processes that handle much of the behind-the-scenes work involved in running an instance.

The most common background processes are the following:

• System Monitor (SMON): Performs crash recovery when the instance is started following a failure.

• Process Monitor (PMON): Performs process cleanup when a user process fails.

• Database Writer (DBWn): Writes modified blocks from the database buffer cache to the data files on the disk.

• Checkpoint (CKPT): Updates all the data files and control files of the database to indicate the most recent checkpoint

• LogWriter (LGWR): Writes redo log entries to the disk

• Archiver (ARCn): Copies redo log files to the archival storage when a log switch occurs.

Server Process and Database Buffer Cache:

·        When a query is processed, the Oracle server process looks in the database buffer cache for any blocks that it needs.

·        If the block is not found in the database buffer cache, the server process

reads the block from the data file and places a copy in the database buffer cache.

·        Because subsequent requests for the same block may find the block in memory, the requests may not require physical reads.

·        The Oracle server uses the least recently used algorithm to age out buffers

that have not been accessed recently to make room for new blocks in the database buffer cache.

Buffers in the buffer cache can be in one of the following four states:

·        Pinned: Multiple sessions are kept from writing to the same block at the same time. Other sessions wait to access the block.

·        Clean: The buffer is now unpinned and is a candidate for immediate aging out, if the current contents (data block) are not referenced again. Either the contents are in sync with the block contents stored on the disk or the buffer contains a consistent read (CR) snapshot of a block.

·        Free or unused: The buffer is empty because the instance has just started. This state is very similar to the clean state, except that the buffer has not been used.

·        Dirty: The buffer is no longer pinned but the contents (data block) have changed and must be flushed to the disk by DBWn before it can be aged out.

Physical Database Structure

The files that constitute an Oracle database are organized into the following:

·        Control files: Contain data about the database itself (that is, physical database structure information). These files are critical to the database. Without them, you cannot open data files to access the data within the database.

·        Data files: Contain the user or application data of the database.

·        Online redo log files: Allow for instance recovery of the database. If the database crashes and does not lose any data files, then the instance can recover the database with the information in these files.

The following additional files are important to the successful running of the database:

·        Parameter file: Is used to define how the instance is configured when it starts up.

·        Password file: Allows users to connect remotely to the database and perform administrative tasks.

·        Backup files: Are used for database recovery. You typically restore a backup file when a media failure or user error has damaged or deleted the original file.

·        Archive log files: Contain an ongoing history of the data changes (redo) that are generated by the instance. Using these files and a backup of the database, you can recover a lost data file. That is, archive logs enable the recovery of restored data files.

·        Trace files: Each server and background process can write to an associated trace file.When an internal error is detected by a process, the process dumps information about the error to its trace file. Some of the information written to a trace file is intended for the database administrator, whereas other information is for Oracle Support Services.

·        Alert log files: These are special trace files. They are also known as alert logs. The alert log of a database is a chronological log of messages and errors. Oracle recommends that you review these files.

Tablespaces and Data Files:

• Tablespaces consist of one or more data files.

• Data files belong to only one tablespace.

·        A database is divided into logical storage units called tablespaces, which can be used to group related logical structures together.

·        Each database is logically divided into one or more tablespaces.

·        One or more data files are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace.

SYSTEM and SYSAUX Tablespaces

•       The SYSTEM and SYSAUX tablespaces are mandatory tablespaces.

•       They are created at the time of database creation.

•       They must be online.

•       The SYSTEM tablespace is used for core functionality (for example, data dictionary tables).

•       The auxiliary SYSAUX tablespace is used for additional database components (such as the Enterprise Manager Repository).

·        Each Oracle database contains a SYSTEM tablespace and a SYSAUX tablespace. They are automatically created when the database is created.

·        The system default is to create a smallfile tablespace. You can also create bigfile tablespaces, which enable the Oracle database to manage

ultralarge files (up to 8 exabytes).

·        A tablespace can be online (accessible) or offline (not accessible).

·        The SYSTEM tablespace is always online when the database is open. It stores tables that support the core functionality of the database, such as the data dictionary tables.

·        The SYSAUX tablespace is an auxiliary tablespace to the SYSTEM tablespace.

·        The SYSAUX tablespace stores many database components, and it must be online for the correct functioning of all database components.

Segments, Extents, and Blocks:

·        Database objects, such as tables and indexes, are stored as segments in tablespaces.

·        Each segment contains one or more extents.

·        An extent consists of contiguous data blocks, which means that each extent can exist only in one data file.

·        Data blocks are the smallest unit of I/O in the database.

·        When the database requests a set of data blocks from the operating system (OS), the OS maps this to an actual file system or disk block on the storage device.

·        Because of this, you need not know the physical address of any of the data in your database. This also means that a data file can be striped or mirrored on several disks.

·        The size of the data block can be set at the time of the creation of the database. The default size of 8 KB is adequate for most databases.

·        If your database supports a data warehouse application that has large tables and indexes, then a larger block size may be beneficial.

·        If your database supports a transactional application where reads and writes are random, then specifying a smaller block size may be beneficial. The maximum block size depends on your OS.

·        The minimum Oracle block size is 2 KB and should rarely (if ever) be used.

Logical and Physical Database Structures:

An Oracle database is a collection of data that is treated as a unit.

The general purpose of a database is to store and retrieve related information. The database has logical structures and physical structures.

Tablespaces:

·        A database is divided into logical storage units called tablespaces, which group related logical structures together.

For example, tablespaces commonly group all of an application’s objects to

simplify some administrative operations. You may have a tablespace for application data and an additional one for application indexes.

Databases, Tablespaces, and Data Files:

·        Each database is logically divided into one or more tablespaces.

·        One or more data files are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace.

·        If it is a TEMPORARY tablespace, instead of a data file, then the tablespace has a temporary file.

Schemas:

·        A schema is a collection of database objects that are owned by a database user.

·        Schema objects are the logical structures that directly refer to the database’s data.

·        Schema objects include such structures as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links.

·        In general, schema objects include everything that your application creates in the database.

Data Blocks:

·        At the finest level of granularity, an Oracle database’s data is stored in data blocks.

·        One data block corresponds to a specific number of bytes of physical database space on the disk.

·        A data block size is specified for each tablespace when it is created.

·        A database uses and allocates free database space in Oracle data blocks.

Extents:

·        The next level of logical database space is called an extent.

·        An extent is a specific number of contiguous data blocks (obtained in a single allocation) that are used to store a specific type of information.

Segments:

·        The level of logical database storage above an extent is called a segment.

·        A segment is a set of extents allocated for a certain logical structure.

For example, the different types of segments include:

•       Data segments: Each nonclustered, non-indexed-organized table has a data segment. All of the table’s data is stored in the extents of its data segment. For a partitioned table, each partition has a data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster’s data segment.

•       Index segments: Each index has an index segment that stores all of its data. For a partitioned index, each partition has an index segment.

•       Undo segments: One UNDO tablespace is created by the database administrator to temporarily store undo information. The information in an undo segment is used to generate read-consistent database information and, during database recovery, to roll back

uncommitted transactions for users.

•       Temporary segments: Temporary segments are created by the Oracle database when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment’s extents are returned to the instance for future

use. Specify a default temporary tablespace for every user or a default temporary tablespace, which is used databasewide.

·        The Oracle database dynamically allocates space. When the existing extents of a segment are full, additional extents are added.

·        Because extents are allocated as needed, the extents of a segment may or may not be contiguous on the disk.