Oracle Database Architecture Overview

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High Level Overview of Oracle Database Architecture

An oracle database consists of Memory Structure, Set of Background Processes and Physical database files.

Memory structure (Commonly known as SGA and PGA ) together with Background Processes is known as database instance.When an instance is started, Oracle Database allocates a memory area and starts background processes.

The memory area allocated to Oracle database is divided into parts (As you can see in the above pic). Each part of memory stores specific kind of information and can be separately managed using corresponding initialization parameters

System global area (SGA)

The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. The SGA is shared by all server and background processes.

The space to all SGA components  except the redo log buffer are allocated/ deallocated in the form of granules.

Granules: is space in units of contiguous memory. Granule size varies based on OS, Oracle version and SGA size. You can check the  granule size using below query.

SQL> select bytes from v$sgainfo where name = 'Granule Size';
     BYTES
----------
  16777216

The most important SGA components are :

Database Buffer Cache: The database buffer cache, also called the buffer cache, is the memory area that stores copies of data blocks read from data files. Server Process is responsible to read the data from datafiles to Database Buffer Cache. The DBWR process is responsible to writes the dirty (changed) blocks back to datafile. 

To know more about oracle database buffer cache check out my post Database Buffer Cache in Details

Redo Log Buffer: The redo log buffer is a circular buffer in the SGA that stores redo entries for changes made to the database. Redo entries contain the information necessary to reconstruct, or redo, changes made to the database by DML or DDL operations. The server process prepares the change vector and writes the redo records in the redo buffer. The LGWR process writes the redo logs from redo buffer to online redo log file.

To know more about oracle database Redo Log Buffer check out my post Redo Log Buffer in Detail

Shared Pool: The shared pool is divided into following sub pools

• Library Cache: The library cache is a shared pool memory structure that stores executable SQL and PL/SQL code. This cache contains the shared SQL and PL/SQL areas and control structures such as locks and library cache handles. In a shared server architecture, the library cache also contains private SQL areas.
• Data Dictionary Cache: The data dictionary is a collection of database tables and views containing reference information about the database, its structures, and its users. Oracle Database accesses the data dictionary frequently during SQL statement parsing.
• Server Result Cache: The server result cache contains the SQL query result cache and PL/SQL function result cache.
• Reserved Pool: The reserved pool is a memory area in the shared pool that Oracle Database can use to allocate large contiguous chunks of memory

To know more about shared pool check out my post Database Shared Pool in Detail

Large Pool: The large pool is an optional memory area intended for memory allocations that are larger than is appropriate for the shared pool.

Java Pool: The Java pool is an area of memory that stores all session-specific Java code and data within the Java Virtual Machine (JVM).

Streams Pool: The Streams pool stores buffered queue messages and provides memory for Oracle Streams capture processes and apply processes. The Streams pool is used exclusively by Oracle Streams which is a deprecated feature now

Fixed SGA: The fixed area contains information about the state of the database and the instance, which the background processes need to access, information about locks etc. The size of the fixed SGA is set by Oracle Database and cannot be altered manually

Program Global Area (PGA)

The PGA is memory specific to an operating process or thread that is not shared by other processes or threads on the system. Because the PGA is process-specific, it is never allocated in the SGA. PGA can be divided into 2 parts  

• Private SQL Area: A private SQL area holds information about a parsed SQL statement and other session-specific information for processing. When a server process executes SQL or PL/SQL code, the process uses the private SQL area to store bind variable values, query execution state information, and query execution work areas.
  

• SQL Work Areas: A work area is a private allocation of PGA memory used for memory-intensive operations.

Process Architecture

A process is a mechanism in an operating system that can run a series of steps

Types of Processes

Processes are majorly categorized in 2 types. Client Process and Database Process

• Client processes : These are the processes which run the application or Oracle tool code. For Example PLSQL Developer, Toad, Oracle Client etc.
• Database Process: You can further divide the database processes into 3 categories

1. Server processes: Perform work based on a client request.
2. Slave processes: Perform additional tasks for a background or server process.
3. Background processes: Start with the database instance and perform maintenance tasks such as performing instance recovery, cleaning up processes, writing redo buffers to disk, and so on. There are some mandatory process, without which oracle database can not run. Below is the list of these mandatory process and a brief description about their job

Oracle Database Mandatory Background processes

Process Monitor Process (PMON)
The process monitor (PMON) monitors the other background processes and performs process recovery when a server or dispatcher process terminates abnormally. PMON is responsible for cleaning up the database buffer cache and freeing resources that the client process was using.

System Monitor Process (SMON)
The system monitor process (SMON) is in charge of a variety of system-level cleanup duties. eg
Performing instance recovery
Recovering terminated transactions
Cleaning up unused temporary segments
Coalescing contiguous free extents within dictionary-managed tablespaces

Database Writer Process (DBWn)
The database writer process (DBWn) writes the contents of database buffers to data files
Log Writer Process (LGWR)

The log writer process (LGWR) manages the redo log buffer. LGWR writes all redo entries from redo log buffer to online redo logfiles

Checkpoint Process (CKPT):The checkpoint process (CKPT) updates the control file and data file headers with checkpoint information and signals DBWn to write blocks to disk.

Recoverer Process (RECO): In a distributed database, the recoverer process (RECO) automatically resolves failures in distributed transactions. The RECO process of a node automatically connects to other databases involved in an in-doubt distributed transaction. When RECO reestablishes a connection between the databases, it automatically resolves all in-doubt transactions, removing from each database's pending transaction table any rows that correspond to the resolved transactions