2.1. Process Architecture

PostgreSQL is a client/server type relational database management system with a multi-process architecture that runs on a single host.

A collection of multiple processes that cooperatively manage a database cluster is usually referred to as a ‘PostgreSQL server’. It contains the following types of processes:

  • The postgres server process is the parent of all processes related to database cluster management.
  • Each backend process handles all queries and statements issued by a connected client.
  • Various background processes perform processes perform tasks for database management, such as VACUUM and CHECKPOINT processing.
  • Replication-associated processes perform streaming replication. More details are described in Chapter 11.
  • Background worker processes supported from version 9.3, it can perform any processing implemented by users. For more information, refer to the official document.

In the following subsections describe the details of the first three types of processes.

This figure shows processes of a PostgreSQL server: a postgres server process, two backend processes, seven background processes, and two client processes. The database cluster, the shared memory, and two client processes are also illustrated.
Fig. 2.1. An example of the process architecture in PostgreSQL.

This figure shows processes of a PostgreSQL server: a postgres server process, two backend processes, seven background processes, and two client processes. The database cluster, the shared memory, and two client processes are also illustrated.

2.1.1. Postgres Server Process

As already described above, a postgres server process is a parent of all processes in a PostgreSQL server. In the earlier versions, it was called ‘postmaster’.

When you execute the pg_ctl utility with ‘start’ option, a postgres server process starts up. It then allocates a shared memory area in memory, starts various background processes, starts replication-associated processes and background worker processes if necessary, and waits for connection requests from clients. Whenever it receives a connection request from a client, it starts a backend process. (The started backend process then handles all queries issued by the connected client.)

A postgres server process listens to one network port, the default port is 5432. Although more than one PostgreSQL server can be run on the same host, each server must be set to listen to different port number, such as 5432, 5433, and so on.

2.1.2. Backend Processes

A backend process, also called a ‘postgres’ process, is started by the postgres server process and handles all queries issued by one connected client. It communicates with the client using a single TCP connection and terminates when the client disconnects.

Since a backend process is only allowed to operate on one database, you must explicitly specify the database you want to use when connecting to a PostgreSQL server.

PostgreSQL allows multiple clients to connect simultaneously. the configuration parameter max_connections controls the maximum number of the clients (default is 100).

If many clients, such as WEB applications, frequently connect and disconnect from a PostgreSQL server, it can increase the cost of establishing connections and creating backend processes, as PostgreSQL does not have a native connection pooling feature.

This can have a negative impact on the performance of the database server.

To deal with such a case, a connection pooling middleware such as pgbouncer or pgpool-II is usually used.

2.1.3. Background Processes

Table 2.1 shows a list of background processes.

In contrast to the postgres server process and the backend process, it is impossible to explain each of the functions simply. This is because these functions depend on the individual specific features and PostgreSQL internals.

Therefore, in this chapter, only introductions are made. Details will be described in the following chapters.

Table 2.1: background processes.
process description reference
background writer This process writes dirty pages on the shared buffer pool to a persistent storage (e.g., HDD, SSD) on a regular basis gradually. (In versions 9.1 or earlier, it was also responsible for the checkpoint process.) Section 8.6
checkpointer This process performs the checkpoint process in versions 9.2 or later. Section 8.6, Section 9.7
autovacuum launcher This process periodically invokes the autovacuum-worker processes for the vacuum process. (More precisely, it requests the postgres server to create the autovacuum workers.) Section 6.5
WAL writer This process writes and flushes the WAL data on the WAL buffer to persistent storage periodically. Section 9.9
statistics collector This process collects statistics information such as for pg_stat_activity and pg_stat_database, etc.  
logging collector (logger) This process writes error messages into log files.  
archiver This process executes archiving logging. Section 9.10

The actual processes of a PostgreSQL server are shown below.

In the following example, there is one postgres server process (pid is 9687), two backend processes (pids are 9697 and 9717), and several background processes listed in Table 2.1. See also Figure 2.1.

postgres> pstree -p 9687
-+= 00001 root /sbin/launchd
 \-+- 09687 postgres /usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
   |--= 09688 postgres postgres: logger process
   |--= 09690 postgres postgres: checkpointer process
   |--= 09691 postgres postgres: writer process
   |--= 09692 postgres postgres: wal writer process
   |--= 09693 postgres postgres: autovacuum launcher process
   |--= 09694 postgres postgres: archiver process
   |--= 09695 postgres postgres: stats collector process
   |--= 09697 postgres postgres: postgres sampledb idle
   \--= 09717 postgres postgres: postgres sampledb idle in transaction