Postgres 9.5 will come up with an additional logging option making possible to log replication commands that are being received by a node. It has been introduced by this commit.
commit: 4ad2a548050fdde07fed93e6c60a4d0a7eba0622 author: Fujii Masao <email@example.com> date: Sat, 13 Sep 2014 02:55:45 +0900 Add GUC to enable logging of replication commands. Previously replication commands like IDENTIFY_COMMAND were not logged even when log_statements is set to all. Some users who want to audit all types of statements were not satisfied with this situation. To address the problem, this commit adds new GUC log_replication_commands. If it's enabled, all replication commands are logged in the server log. There are many ways to allow us to enable that logging. For example, we can extend log_statement so that replication commands are logged when it's set to all. But per discussion in the community, we reached the consensus to add separate GUC for that. Reviewed by Ian Barwick, Robert Haas and Heikki Linnakangas.
The new parameter is called log_replication_commands and needs to be set in postgresql.conf. Default is off to not log this new information that may surprise existing users after an upgrade to 9.5 and newer versions. And actually replication commands received by a node were already logged at DEBUG1 level by the server. A last thing to note is that if log_replication_commands is enabled, all the commands will be printed as LOG and not as DEBUG1, which is kept for backward-compatibility purposes.
Now, a server enabling this logging mode…
$ psql -At -c 'show log_replication_commands' on
… Is able to show replication commands in LOG mode. Here is for example the set of commands set by a standby starting up:
LOG: received replication command: IDENTIFY_SYSTEM LOG: received replication command: START_REPLICATION 0/3000000 TIMELINE 1
This will certainly help utilities and users running audit for replication, so looking forward to see log parsing tools like [pgbadger] (https://github.com/dalibo/pgbadger) make some nice outputs using this information.