This is a document for PGroonga 2.X and 3.X. See PGroonga 1.x document when you're using old PGroonga.
pgroonga_standby_maintainer.max_parallel_wal_appliers_per_db parameterSince 3.1.2.
The pgroonga_standby_maintainer.max_parallel_wal_appliers_per_db parameter controls parallel level of the pgroonga_wal_apply() function execution by the pgroonga_standby_maintainer module.
If this value is 1 or larger, pgroonga_wal_apply() for each PGroonga index is executed in a new background worker process. If you have enough resource to run pgroonga_wal_apply() in parallel, you can reduce total WAL application time by specifying a large value such as 6.
Note that you may need to increase max_worker_processes when you specify a large value to this parameter. If you specify a large value, you may need to use many worker processes at the same time.
Note that this parameter controls parallel level per database. If you have 2 databases that use PGroonga and specify 6 to this parameter, 12 background worker processes may be used for pgroonga_wal_apply() at the same time.
The default value is 0. It means that pgroonga_wal_apply() is executed sequentially. No new background worker processes aren't used.
In postgresql.conf:
pgroonga_standby_maintainer.max_parallel_wal_appliers_per_db = max
max is 0 or positive integer that controls parallel level.
The default is 0.
Here is an example to use at most 6 background worker processes for parallel pgroonga_wal_apply() per database:
pgroonga_standby_maintainer.max_parallel_wal_appliers_per_db = 6