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Configurable retention of members replication slots (#3108)

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Current problem of Patroni that strikes many people is that it removes replication slot for member which key is expired from DCS. As a result, when the replica comes back from a scheduled maintenance WAL segments could be already absent, and it can't continue streaming without pulling files from archive.
With PostgreSQL 16 and newer we get another problem: logical slot on a standby node could be invalidated if physical replication slot on the primary was removed (and `pg_catalog` vacuumed).
The most problematic environment is Kubernetes, where slot is removed nearly instantly when member Pod is deleted.

So far, one of the recommended solutions was to configure permanent physical slots with names that match member names to avoid removal of replication slots. It works, but depending on environment might be non-trivial to implement (when for example members may change their names).

This PR implements support of `member_slots_ttl` global configuration parameter, that controls for how long member replication slots should be kept when the member key is absent. Default value is set to `30min`.
The feature is supported only starting from PostgreSQL 11 and newer, because we want to retain slots not only on the leader node, but on all nodes that could potentially become the new leader, and they should be moved forward using `pg_replication_slot_advance()` function.

One could disable feature and get back to the old behavior by setting `member_slots_ttl` to `0`.
This commit is contained in:
Alexander Kukushkin
2024-08-23 14:50:36 +02:00
committed by GitHub
parent 8c5ab4c07d
commit 6d65aa311a
19 changed files with 227 additions and 81 deletions
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docs/faq.rst
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@@ -181,7 +181,10 @@ What is the difference between ``etcd`` and ``etcd3`` in Patroni configuration?
* API version 2 will be completely removed on Etcd v3.6.
I have ``use_slots`` enabled in my Patroni configuration, but when a cluster member goes offline for some time, the replication slot used by that member is dropped on the upstream node. What can I do to avoid that issue?
You can configure a permanent physical replication slot for the members.
There are two options:
1. You can tune ``member_slots_ttl`` (default value ``30min``, available since Patroni ``4.0.0`` and PostgreSQL 11 onwards) and replication slots for absent members will not be removed when the members downtime is shorter than the configured threshold.
2. You can configure permanent physical replication slots for the members.
Since Patroni ``3.2.0`` it is now possible to have member slots as permanent slots managed by Patroni.