Hey, all, The configuration parameter max_replication_slots is most notably used to control how many replication slots can be created on a server, but it also controls how many replication origins can be tracked on the subscriber side.
This is noted in the Configuration Settings section in the Logical Replication Chapter [1], but it is not mentioned in the documentation the parameter itself [2]. The attached patch adds an extra paragraph explaining its effect on subscribers. Using max_replication_slots for sizing the number available of replication origin states is a little odd, and is actually noted twice in the source code [3] [4]: > XXX: Should we use a separate variable to size this rather than > max_replication_slots? > XXX: max_replication_slots is arguably the wrong thing to use, as here > we keep the replay state of *remote* transactions. But for now it > seems sufficient to reuse it, rather than introduce a separate GUC. This is a different usage of max_replication_slots than originally intended, managing resource usage on the subscriber side, rather than the provider side. This manifests itself in the awkwardness of the documentation, where max_replication_slots is only listed in the Sending Server section, and not mentioned in the Subscribers section. Given this, I think introducing a new parameter would make sense (max_replication_origins? slightly confusing because there's no limit on the number of records in pg_replication_origins; tracking of replication origins is displayed in pg_replication_origin_status). I'd be happy to make a patch for a new GUC parameter, if people think it's worth it to separate the functionality. Until then, however, the addition to the documentation should help prevent confusion. - Paul [1]: https://www.postgresql.org/docs/13/logical-replication-config.html [2]: https://www.postgresql.org/docs/13/runtime-config-replication.html#GUC-MAX-REPLICATION-SLOTS [3]: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/logical/origin.c;h=685eaa6134e7cad193b583ff28284d877a6d8055;hb=HEAD#l162 [4]: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/logical/origin.c;h=685eaa6134e7cad193b583ff28284d877a6d8055;hb=HEAD#l495
max_replication_slots_subscriber_doc_v00.diff
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