Background Worker Processes

PostgreSQL 9.3.5 Documentation
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# Chapter 45. 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.

Warning
There are considerable robustness and security risks in using background worker processes because, being written in the `C` language, they have unrestricted access to data. Administrators wishing to enable modules that include background worker process should exercise extreme caution. Only carefully audited modules should be permitted to run background worker processes.

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)(Datum 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 `Datum` 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.

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