Chapter 44. Background Worker Processes

PostgreSQL can be extended to run user-supplied code in separate processes. Such processes are started, stopped and monitored by postgres, which permits them to have a lifetime closely linked to the server's status. These processes have the option to attach to PostgreSQL's shared memory area and to connect to databases internally; they can also run multiple transactions serially, just like a regular client-connected server process. Also, by linking to libpq they can connect to the server and behave like a regular client application.

Only modules listed in shared_preload_libraries can run background workers. A module wishing to run a background worker needs to register it by calling RegisterBackgroundWorker(BackgroundWorker *worker) from its _PG_init(). The structure BackgroundWorker is defined thus:

typedef void (*bgworker_main_type)(void *main_arg);
typedef struct BackgroundWorker
{
    char        bgw_name[BGW_MAXLEN];
    int         bgw_flags;
    BgWorkerStartTime bgw_start_time;
    int         bgw_restart_time;       /* in seconds, or BGW_NEVER_RESTART */
    bgworker_main_type bgw_main;
    Datum       bgw_main_arg;
} BackgroundWorker;

bgw_name is a string to be used in log messages, process listings and similar contexts.

bgw_flags is a bitwise-or'd bitmask indicating the capabilities that the module wants. Possible values are BGWORKER_SHMEM_ACCESS (requesting shared memory access) and BGWORKER_BACKEND_DATABASE_CONNECTION (requesting the ability to establish a database connection, through which it can later run transactions and queries). A background worker using BGWORKER_BACKEND_DATABASE_CONNECTION to connect to a database must also attach shared memory using BGWORKER_SHMEM_ACCESS, or worker start-up will fail.

bgw_start_time is the server state during which postgres should start the process; it can be one of BgWorkerStart_PostmasterStart (start as soon as postgres itself has finished its own initialization; processes requesting this are not eligible for database connections), BgWorkerStart_ConsistentState (start as soon as a consistent state has been reached in a hot standby, allowing processes to connect to databases and run read-only queries), and BgWorkerStart_RecoveryFinished (start as soon as the system has entered normal read-write state). Note the last two values are equivalent in a server that's not a hot standby. Note that this setting only indicates when the processes are to be started; they do not stop when a different state is reached.

bgw_restart_time is the interval, in seconds, that postgres should wait before restarting the process, in case it crashes. It can be any positive value, or BGW_NEVER_RESTART, indicating not to restart the process in case of a crash.

bgw_main is a pointer to the function to run when the process is started. This function must take a single argument of type void * and return void. bgw_main_arg will be passed to it as its only argument. Note that the global variable MyBgworkerEntry points to a copy of the BackgroundWorker structure passed at registration time.

Once running, the process can connect to a database by calling BackgroundWorkerInitializeConnection(char *dbname, char *username). This allows the process to run transactions and queries using the SPI interface. If dbname is NULL, the session is not connected to any particular database, but shared catalogs can be accessed. If username is NULL, the process will run as the superuser created during initdb. BackgroundWorkerInitializeConnection can only be called once per background process, it is not possible to switch databases.

Signals are initially blocked when control reaches the bgw_main function, and must be unblocked by it; this is to allow the process to customize its signal handlers, if necessary. Signals can be unblocked in the new process by calling BackgroundWorkerUnblockSignals and blocked by calling BackgroundWorkerBlockSignals.

Background workers are expected to be continuously running; if they exit cleanly, postgres will restart them immediately. Consider doing interruptible sleep when they have nothing to do; this can be achieved by calling WaitLatch(). Make sure the WL_POSTMASTER_DEATH flag is set when calling that function, and verify the return code for a prompt exit in the emergency case that postgres itself has terminated.

The worker_spi contrib module contains a working example, which demonstrates some useful techniques.