Today’s post, the first one of 2017, is about the following feature of the upcoming Postgres 10:
commit: 3901fd70cc7ccacef1b0549a6835bb7d8dcaae43 author: Fujii Masao <email@example.com> date: Mon, 19 Dec 2016 21:15:30 +0900 Support quorum-based synchronous replication. This feature is also known as "quorum commit" especially in discussion on pgsql-hackers. This commit adds the following new syntaxes into synchronous_standby_names GUC. By using FIRST and ANY keywords, users can specify the method to choose synchronous standbys from the listed servers. FIRST num_sync (standby_name [, ...]) ANY num_sync (standby_name [, ...]) The keyword FIRST specifies a priority-based synchronous replication which was available also in 9.6 or before. This method makes transaction commits wait until their WAL records are replicated to num_sync synchronous standbys chosen based on their priorities. The keyword ANY specifies a quorum-based synchronous replication and makes transaction commits wait until their WAL records are replicated to *at least* num_sync listed standbys. In this method, the values of sync_state.pg_stat_replication for the listed standbys are reported as "quorum". The priority is still assigned to each standby, but not used in this method. The existing syntaxes having neither FIRST nor ANY keyword are still supported. They are the same as new syntax with FIRST keyword, i.e., a priority-based synchronous replication. Author: Masahiko Sawada Reviewed-By: Michael Paquier, Amit Kapila and me Discussion: <CAD21AoAACi9NeC_ecm+Vahm+MMA6nYh=Kqs3KB3np+MBOS_gZg@mail.gmail.com> Many thanks to the various individuals who were involved in discussing and developing this feature.
9.6 has introduced the possibility to specify multiple synchronous standbys by extending the syntax of synchronous_standby_names. For example values like ‘N (standby_1,standby_2, … ,standby_M)’ allow a primary server to wait for commit confirmations from N standbys among the set of M nodes defined in the list given by user, depending on the availability of the standbys at the moment of the transaction commit, and their reported WAL positions for write, apply or flush. In this case, though, the standbys from which a confirmation needs to be waited for are chosen depending on their order in the list of the parameter.
Being able to define quorum sets of synchronous standbys provides more flexibility in some availability scenarios. In short, it is possible to validate a commit after receiving a confirmation from N standbys, those standbys being any node listed in the M nodes of synchronous_standby_names. So this facility is actually useful for example in the case of deployments where there is a primary with two or more standbys to bring more flexibility in the way synchronous standbys are chosen. Be careful though that it is better to have a low latency between each node, but there is nothing new here…
In order to support this new feature, and as mentioned in the commit message, the grammar of synchronous_standby_names has been extended with a set of keywords.
- ANY maps to the quorum behavior, meaning that any node in the set can be used to confirm a commit.
- FIRST maps to the 9.6 behavior, giving priority to the nodes listed first (higher priority number defined).
Those can be used as follows:
# Quorum set of two nodes any 2(node_1,node_2) # Priority set of two nodes, with three standbys first 1(node_1,node_2,node_3)
Note as well that not using any keyword means ‘first’ for backward-compatibility. And that those keywords are case insensitive.
One last thing to know is that pg_stat_replication marks the standbys in a quorum set with… ‘quorum’. For example let’s take a primary with two standbys node_1 and node_2.
=# ALTER SYSTEM SET synchronous_standby_names = 'ANY 2(node_1,node_2)'; ALTER SYSTEM =# SELECT pg_reload_conf(); pg_reload_conf ---------------- t (1 row)
And here is how they show up to the user:
=# SELECT application_name, sync_priority, sync_state FROM pg_stat_replication; application_name | sync_priority | sync_state ------------------+---------------+------------ node_1 | 1 | quorum node_2 | 2 | quorum (2 rows)
Note that the priority number does not have much meaning for a quorum set, though it is useful to see them if user is willing to switch from ‘ANY’ to ‘FIRST’ to understand what would be the standbys that would be considered as synchronous after the switch (this is still subject to discussions on community side, and may change by the release of Postgres 10).