Introduction to Oracle Process architecture

We often say that the database instance, actually refers to the database memory structure and process structure. We install database software in large part by installing its memory and process component structure. In the previous section we introduced the Oracle Memory Architecture, which describes the next Oracle process architecture.

First look at the classification of Oracle processes:

1) User Process

is an application or tool connected to Oracle DB

2) database Process

Server processes: Connecting to Oracle instances, starting when a user establishes a session

Background process: Started when an Oracle instance is started

3) Daemon/application process

Network Listener Program

Gridinfrastructure Daemon Program

From a process perspective, you can generally divide the server model into two types:

Dedicated server: For each session, the dedicated server process provides services;

Shared server: You do not have to provide a dedicated server process for each connection. The dispatcher directs multiple incoming network session requests to the shared server process pool. The shared server process serves all client requests.

The General production Library is shared mode;

The next few major processes are outlined below:

Oracle process structure diagram (this image is from Oracle's official documentation)

Common background processes in non-RAC, non-ASM environments include:

• Database Write process (DBWn)

• Log write process (LGWR)

• Checkpoint process (CKPT)

• System Monitor process (Smon)

• Process Monitor process (Pmon)

• The recovery process (RECO)

• Job Queue Coordinator Program (CJQ0)

• Job dependent process (Jnnn)

• Archive process (ARCn)

• Queue monitor process (qmnn)

Description

DBW N The process is responsible for writing the modified buffer (gray data buffer) in the database buffer cache to the disk, and the db_writer_processes initialization parameter specifies the number of DBWn processes. The maximum number of DBWn processes is 36. If the user does not specify the number of processes during startup, Oracle DB will determine how to set the db_writer_processes based on the number of CPUs and processor groups

Note that n means that there can be more than one such process, but if it is a single-processor server, configuring n is useless.

The DBWn Process writes a gray data buffer to disk in the following cases:

When the server process cannot find a clean reusable buffer, push the checkpoint to the point;

Log Write Process (LGWR) is responsible for managing the redo log buffer, the redo log buffer entry is written to the redo log file on disk, large-scale system data block operation frequency is particularly high, there is bound to be a large number of logs written to the log buffer, Oracle must ensure that there is always enough space to write a new redo log, Therefore, the frequency of the log write process is very high and triggers the process to work in the following situations:

When a user process commits a transaction, such as a commit command;

The redo log buffer is one-third full;

DBWN Process writes the modified buffer to the disk;

every 3 seconds;

Checkpoint Process (CKPT)

A checkpoint is a data structure that defines the system change number (SCN) in the redo thread of the database. Checkpoints are recorded in the control file and in each data file header. They are a key element of the recovery operation, and Oracle DB must update the header of all data files to record the details of the checkpoint when it encounters a checkpoint.

This is done by the CKPT process. The SCN mechanism cleverly protects the Oracle data recovery mechanism. Here is just a brief mention, interested friends can find relevant technical information, understanding the principle of SCN, the database backup restore recovery has a considerable theoretical help.

System Monitor Process (Smon)

Perform recovery at instance startup;

Elimination of unused temporary segments;

Process Monitor Process (Pmon)

Performs a process recovery when a user process fails. Responsible for purging the database buffer cache and freeing the resources consumed by the user process; Pmon periodically checks the status of the dispatcher and server processes and restarts any dispatcher and server processes that have stopped running. It is the process steward of the database.

The recovery process (RECO)

is a background process for distributed database configuration that automatically resolves failures involving distributed transaction processing.

Archive Process (ARCn)

After a log switchover occurs, the archive process (ARCn) copies the redo log files to the specified storage device. An arcn process exists only if the database is in Archivelog mode and automatic archiving is enabled. Database logs are generally very large, while the database log is particularly important, as long as there is a complete log we can recover the lost data, so the loss of the log is a great risk, we need to periodically migrate it to offsite storage (tape, disk and other media), this process is called Log Archive, need to complete the archive process. Note that the Archivelog mode is enabled, which we will talk about later in the database backup and Restore section;

The Oracle process architecture is described here, and the next section describes the Oracle storage architecture. Welcome to pay attention to "know the way", your concern is the driving force we adhere to!

-------------------------------END-----------------------------------

"Know Youdao" here welcome friends who love to learn, focusing on the database and other IT knowledge sharing learning.

Please pay attention to "know Youdao" subscription number. Search the public number "to know Youdao", add attention!

or long press this QR code to add attention!
?

Recently shared:

The basic architecture of Oracle involves the basic theory of instance memory structure, process structure, physical storage structure, logical storage system and so on.

Introduction to Oracle Process architecture