The following commit has reached PostgreSQL 12, which brings more flexibility in managing replication with standbys distributed geographically:
commit: db361db2fce7491303f49243f652c75c084f5a19 author: Michael Paquier <firstname.lastname@example.org> date: Sat, 22 Sep 2018 15:23:59 +0900 Make GUC wal_sender_timeout user-settable Being able to use a value that can be changed on a connection basis is useful with clusters distributed geographically, and makes failure detection more flexible. A note is added in the documentation about the use of "options" in primary_conninfo, which can be hard to grasp for newcomers with the need of two single quotes when listing a set of parameters. Author: Tsunakawa Takayuki Reviewed-by: Masahiko Sawada, Michael Paquier Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1FAAD3AE@G01JPEXMBYT05
For some deployments, it matters to be able to change wal_sender_timeout depending on the standby and the latency with its primary (or another standby when dealing with a cascading instance). For example, a shorter timeout for a standby close to its primary allows faster problem detection and failover, while a longer timeout can become helpful for a standby in a remote location to judge correctly its health. In Postgres 11 and older versions, and this since wal_sender_timeout has been introduced since 9.1, this parameter can only be set at server-level, being marked as PGC_SIGHUP in its GUC properties. Changing the value of this parameter does not need an instance restart and the new value can be reloaded to all the sessions connected, including WAL senders.
The thread related to the above commit has also discussed if this parameter should be changed to be a backend-level parameter, which has the following properties:
- Reload does not work on it. Once this parameter is changed at connection time it can never change.
- Changing this parameter at server level will make all new connections using the new value.
- Role-level configuration is not possible.
Still, for default values, it is a huge advantage to be able to reload on-the-fly wal_sender_timeout depending on the state of an environment. So the choice has been made to make the parameter user-settable with PGC_USERSET so as it is possible to have values set up depending on the connected role (imagine different policies per role), and to allow the parameter to be reloaded for all sessions which do not enforce it at connection level. Coming back to the connection, the main advantage is that the value can be enforced using two different methods in primary_conninfo. First, and as mentioned previously, by connecting with a role which has a non-default value associated with it (this can be configured with ALTER ROLE). The second way is to pass directly the parameter “options” in a connection string, so as the following configuration gets used (single quotes are important!):
primary_conninfo = ' options=''-c wal_sender_timeout=60000'' ...'
Note as well that the SHOW command works across the replication protocol (since Postgres 10), so as it is possible to also check the effective value of a parameter at application level. Of course this is available for anything using the replication protocol, like a WAL archiver using pg_receivewal, logical workers, etc.
Making wal_sender_timeout configuration more flexible is extremely useful for many experienced users, so this is a great addition to Postgres 12.