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Background workers are postmaster subprocesses that run arbitrary user-specified code. They can request shared memory access as well as backend database connections; or they can just use plain libpq frontend database connections. Modules listed in shared_preload_libraries can register background workers in their _PG_init() function; this is early enough that it's not necessary to provide an extra GUC option, because the necessary extra resources can be allocated early on. Modules can install more than one bgworker, if necessary. Care is taken that these extra processes do not interfere with other postmaster tasks: only one such process is started on each ServerLoop iteration. This means a large number of them could be waiting to be started up and postmaster is still able to quickly service external connection requests. Also, shutdown sequence should not be impacted by a worker process that's reasonably well behaved (i.e. promptly responds to termination signals.) The current implementation lets worker processes specify their start time, i.e. at what point in the server startup process they are to be started: right after postmaster start (in which case they mustn't ask for shared memory access), when consistent state has been reached (useful during recovery in a HOT standby server), or when recovery has terminated (i.e. when normal backends are allowed). In case of a bgworker crash, actions to take depend on registration data: if shared memory was requested, then all other connections are taken down (as well as other bgworkers), just like it were a regular backend crashing. The bgworker itself is restarted, too, within a configurable timeframe (which can be configured to be never). More features to add to this framework can be imagined without much effort, and have been discussed, but this seems good enough as a useful unit already. An elementary sample module is supplied. Author: Álvaro Herrera This patch is loosely based on prior patches submitted by KaiGai Kohei, and unsubmitted code by Simon Riggs. Reviewed by: KaiGai Kohei, Markus Wanner, Andres Freund, Heikki Linnakangas, Simon Riggs, Amit Kapila
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Changes between all PostgreSQL releases are recorded in the file HISTORY. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.