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 processes should exercise extreme caution. Only carefully audited modules should be permitted to run background worker processes. | 
  Background workers can be initialized at the time that
  PostgreSQL is started by including the module name in
  shared_preload_libraries.  A module wishing to run a background
  worker can register it by calling
  RegisterBackgroundWorker(BackgroundWorker *worker)
  from its _PG_init().  Background workers can also be started
  after the system is up and running by calling the function
  RegisterDynamicBackgroundWorker(BackgroundWorker
  *worker, BackgroundWorkerHandle **handle).  Unlike
  RegisterBackgroundWorker, which can only be called from within
  the postmaster, RegisterDynamicBackgroundWorker must be
  called from a regular backend.
 
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;
    char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
    char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
    Datum       bgw_main_arg;
    char        bgw_extra[BGW_EXTRALEN];
    int         bgw_notify_pid;
} BackgroundWorker;
bgw_name is a string to be used in log messages, process listings and similar contexts.
bgw_flags is a bitwise-or'd bit mask indicating the capabilities that the module wants. Possible values are:
Requests shared memory access. Workers without shared memory access cannot access any of PostgreSQL's shared data structures, such as heavyweight or lightweight locks, shared buffers, or any custom data structures which the worker itself may wish to create and use.
Requests 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 field can only safely be used to launch functions within the core server, because shared libraries may be loaded at different starting addresses in different backend processes. This will happen on all platforms when the library is loaded using any mechanism other than shared_preload_libraries. Even when that mechanism is used, address space layout variations will still occur on Windows, and when EXEC_BACKEND is used. Therefore, most users of this API should set this field to NULL. If it is non-NULL, it takes precedence over bgw_library_name and bgw_function_name.
bgw_library_name is the name of a library in which the initial entry point for the background worker should be sought. The named library will be dynamically loaded by the worker process and bgw_function_name will be used to identify the function to be called. If loading a function from the core code, bgw_main should be set instead.
bgw_function_name is the name of a function in a dynamically loaded library which should be used as the initial entry point for a new background worker.
   bgw_main_arg is the Datum argument
   to the background worker main function.  Regardless of whether that
   function is specified via bgw_main or via the combination
   of bgw_library_name and bgw_function_name,
   this main function should take a single argument of type Datum
   and return void.  bgw_main_arg will be
   passed as the argument.  In addition, the global variable
   MyBgworkerEntry
   points to a copy of the BackgroundWorker structure
   passed at registration time; the worker may find it helpful to examine
   this structure.
  
On Windows (and anywhere else where EXEC_BACKEND is defined) or in dynamic background workers it is not safe to pass a Datum by reference, only by value. If an argument is required, it is safest to pass an int32 or other small value and use that as an index into an array allocated in shared memory. If a value like a cstring or text is passed then the pointer won't be valid from the new background worker process.
bgw_extra can contain extra data to be passed to the background worker. Unlike bgw_main_arg, this data is not passed as an argument to the worker's main function, but it can be accessed via MyBgworkerEntry, as discussed above.
bgw_notify_pid is the PID of a PostgreSQL backend process to which the postmaster should send SIGUSR1 when the process is started or exits. It should be 0 for workers registered at postmaster startup time, or when the backend registering the worker does not wish to wait for the worker to start up. Otherwise, it should be initialized to MyProcPid.
Once running, the process can connect to a database by calling
   BackgroundWorkerInitializeConnection(char *dbname, char *username) or
   BackgroundWorkerInitializeConnectionByOid(Oid dboid, Oid useroid).
   This allows the process to run transactions and queries using the
   SPI interface.  If dbname is NULL or
   dboid is InvalidOid, the session is not connected
   to any particular database, but shared catalogs can be accessed.
   If username is NULL or useroid is
   InvalidOid, the process will run as the superuser created
   during initdb.
   A background worker can only call one of these two functions, and only
   once.  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.
  
   If bgw_restart_time for a background worker is
   configured as BGW_NEVER_RESTART, or if it exits with an exit
   code of 0 or is terminated by TerminateBackgroundWorker,
   it will be automatically unregistered by the postmaster on exit.
   Otherwise, it will be restarted after the time period configured via
   bgw_restart_time, or immediately if the postmaster
   reinitializes the cluster due to a backend failure.  Backends which need
   to suspend execution only temporarily should use an interruptible sleep
   rather than exiting; 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.
  
   When a background worker is registered using the
   RegisterDynamicBackgroundWorker function, it is
   possible for the backend performing the registration to obtain information
   regarding the status of the worker.  Backends wishing to do this should
   pass the address of a BackgroundWorkerHandle * as the second
   argument to RegisterDynamicBackgroundWorker.  If the
   worker is successfully registered, this pointer will be initialized with an
   opaque handle that can subsequently be passed to
   GetBackgroundWorkerPid(BackgroundWorkerHandle *, pid_t *) or
   TerminateBackgroundWorker(BackgroundWorkerHandle *).
   GetBackgroundWorkerPid can be used to poll the status of the
   worker: a return value of BGWH_NOT_YET_STARTED indicates that
   the worker has not yet been started by the postmaster;
   BGWH_STOPPED indicates that it has been started but is
   no longer running; and BGWH_STARTED indicates that it is
   currently running.  In this last case, the PID will also be returned via the
   second argument.
   TerminateBackgroundWorker causes the postmaster to send
   SIGTERM to the worker if it is running, and to unregister it
   as soon as it is not.
  
   In some cases, a process which registers a background worker may wish to
   wait for the worker to start up.  This can be accomplished by initializing
   bgw_notify_pid to MyProcPid and
   then passing the BackgroundWorkerHandle * obtained at
   registration time to
   WaitForBackgroundWorkerStartup(BackgroundWorkerHandle
   *handle, pid_t *) function.
   This function will block until the postmaster has attempted to start the
   background worker, or until the postmaster dies.  If the background runner
   is running, the return value will BGWH_STARTED, and
   the PID will be written to the provided address.  Otherwise, the return
   value will be BGWH_STOPPED or
   BGWH_POSTMASTER_DIED.
  
   If a background worker sends asynchronous notifications with the
   NOTIFY command via the Server Programming Interface
   (SPI), it should call
   ProcessCompletedNotifies explicitly after committing
   the enclosing transaction so that any notifications can be delivered.  If a
   background worker registers to receive asynchronous notifications with
   the LISTEN through SPI, the worker
   will log those notifications, but there is no programmatic way for the
   worker to intercept and respond to those notifications.
  
The src/test/modules/worker_spi module contains a working example, which demonstrates some useful techniques.
The maximum number of registered background workers is limited by max_worker_processes.