Introduced in PostgreSQL 9.4, [pg_recvlogical] (https://www.postgresql.org/docs/devel/static/app-pgrecvlogical.html) has the ability to control the creation of logical replication slots from which logical changes can be streamed. Note that in the case this is a mandatory condition when using [logical decoding] (https://www.postgresql.org/docs/devel/static/logicaldecoding.html). [pg_receivexlog] (https://www.postgresql.org/docs/devel/static/app-pgreceivexlog.html) does not have in 9.4 any control on the physical replication slots it may stream from (to ensure that the WAL segment files this utility is looking for are still retained on the server side). This feature has been added for 9.5 with the following commit:
commit: d9f38c7a555dd5a6b81100c6d1e4aa68342d8771 author: Andres Freund <email@example.com> date: Mon, 6 Oct 2014 12:51:37 +0200 Add support for managing physical replication slots to pg_receivexlog. pg_receivexlog already has the capability to use a replication slot to reserve WAL on the upstream node. But the used slot currently has to be created via SQL. To allow using slots directly, without involving SQL, add --create-slot and --drop-slot actions, analogous to the logical slot manipulation support in pg_recvlogical. Author: Michael Paquier
This simply introduces two new options allowing to create or drop a physical replication slot, respectively –create-slot and –drop-slot. The main difference with pg_recvlogical is that those additional actions are optional (not –start option introduced as well for backward-compatibility). Be careful of a couple of things when using this feature though. First, when a slot is created, stream of the segment files begins immediately.
$ pg_receivexlog --create-slot --slot physical_slot -v -D ~/xlog_data/ pg_receivexlog: creating replication slot "physical_slot" pg_receivexlog: starting log streaming at 0/1000000 (timeline 1)
The slot created can then be found in the system view pg_replication_slots.
=# select slot_name, plugin, restart_lsn from pg_replication_slots ; slot_name | plugin | restart_lsn ---------------+--------+------------- physical_slot | null | 0/1000000 (1 row)
Then, when dropping a slot, as process can stream nothing it exits immediately, and slot is of course not more:
$ pg_receivexlog --drop-slot --slot physical_slot -v pg_receivexlog: dropping replication slot "physical_slot" $ psql -c 'SELECT slot_name FROM pg_replication_slots' slot_name ----------- (0 rows)
Deletion and creation of the replication slot is made uses the same replication connection as the one for stream and uses the commands CREATE_REPLICATION_SLOT and DROP_REPLICATION_SLOT from the [replication protocol] (https://www.postgresql.org/docs/devel/static/protocol-replication.html), resulting in a light-weight implementation. So do not hesitate to refer to this code when implementing your own client applications, src/bin/pg_basebackup/streamutil.c being particularly helpful.