Postgres Signals

PostgreSQL
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# Chapter 65. Postgres Signals

> Note: Contributed by Massimo Dal Zotto

Postgres uses the following signals for communication between the postmaster and backends:

Table 65-1. Postgres Signals

Signal | postmaster Action | Server Action
--- | --- | ---
SIGHUP | kill(*,sighup) | read_pg_options
SIGINT | die | cancel query
SIGQUIT | kill(*,sigterm) | handle_warn
SIGTERM | kill(*,sigterm), kill(*,9), die | die
SIGPIPE | ignored | die
SIGUSR1 | kill(*,sigusr1), die | quickdie
SIGUSR2 | kill(*,sigusr2) | async notify (SI flush)
SIGCHLD | reaper | ignored (alive test)
SIGTTIN | ignored |
SIGTTOU | ignored |
SIGCONT | dumpstatus |
SIGFPE |  | FloatExceptionHandler

> Note: "`kill(*,signal)`" means sending a signal to all backends.

The main changes to the old signal handling are the use of SIGQUIT instead of SIGHUP to handle warns, SIGHUP to re-read the pg_options file and the redirection to all active backends of SIGHUP, SIGTERM, SIGUSR1 and SIGUSR2 sent to the postmaster. In this way these signals sent to the postmaster can be sent automatically to all the backends without need to know their pids. To shut down postgres one needs only to send a SIGTERM to postmaster and it will stop automatically all the backends.

The SIGUSR2 signal is also used to prevent SI cache table overflow which happens when some backend doesn't process SI cache for a long period. When a backend detects the SI table full at 70% it simply sends a signal to the postmaster which will wake up all idle backends and make them flush the cache.

The typical use of signals by programmers could be the following:

    # stop postgres
    kill -TERM $postmaster_pid

    # kill all the backends
    kill -QUIT $postmaster_pid

    # kill only the postmaster
    kill -INT $postmaster_pid

    # change pg_options
    cat new_pg_options > $DATA_DIR/pg_options
    kill -HUP $postmaster_pid

    # change pg_options only for a backend
    cat new_pg_options > $DATA_DIR/pg_options
    kill -HUP $backend_pid
    cat old_pg_options > $DATA_DIR/pg_options

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