What is a Postmaster PID file in PostgreSQL?

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  • As per the PostgreSQL architecture, the postmaster is the first background process that will starts when the cluster is started.
  • It is a parent process of all other background processes.
  • In case any of the other background processes got failed, The postmaster automatically restart the dead processes.
  • It also works as a listener. When the client’s request to the server first it will land into the postmaster and perform syntax and semantic checking, Once it is pass and spawn a new connection this process called “Pgprocess”.
  • The postmaster act as a listener well as the parent process, so it is also called “Supervision”.
ps -ef|grep postmasterpostgres 393 32729 0 22:28 pts/3 00:00:00 grep postmaster

Purpose of Postmaster PID file:

  • The postmaster PID file resides on the data directory when the cluster is up and running.
  • The purpose of this PID file uses to capture

1. the current postmaster process ID (PID),

2. cluster data directory path, postmaster start timestamp,

3. port number,

4. Unix domain socket directory path,

5. listen_address (IP address)

6. shared memory segment id.

  • The captured PID file details are being locked when the cluster was up and running. It protects to access the same port number or data directory for another cluster launch or running.
$cd $PGDATA$lsbase     pg_commit_ts  pg_ident.conf  pg_notify    pg_snapshots  pg_subtrans  PG_VERSION  postgresql.auto.conf  postmaster.pid
global   pg_dynshmem   pg_logical     pg_replslot  pg_stat       pg_tblspc    pg_wal      postgresql.conf
logfile  pg_hba.conf   pg_multixact   pg_serial    pg_stat_tmp   pg_twophase  pg_xact     postmaster.opts

To view the postmaster PID file:

$ cat postmaster.pid4693/u01/pgdatabase/data16156884855432/tmp*5432001 3604491ready

· Now I have just stopped my cluster services using pg_ctl

pg_ctl stop -mfwaiting for the server to shut down.... doneserver stopped

After the cluster is stopped of we verify that the postmaster PID file it was automatically disappeared from the data directory.

summary:

· The postmaster PID file will be generated automatically when the cluster services are up and running.

· It is a lock file that maintains the current cluster details i.e not allow to use of these details to run another cluster on the same port or data directory

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