Synchronizing slots from primary to standby

Hello,

The doc mentions that standby_slot_names is to only be used for waiting on physical slots. However, it seems like we calculate the flush_pos for logical slots as well in wait_for_standby_confirmation?

Re-posting some of my previous comment since it seems like it got lost...

In wait_for_standby_confirmation, should we move standby_slot_names -> namelist into the while (true) block so if we have wrong values set we fix it with a SIGHUP? Similarly, in slotsync.c, we never update standby_slot_names once the worker is launched.

If a logical slot was dropped on the writer, should the worker drop logical slots that it was previously synchronizing but are no longer present? Or should we leave that to the user to manage? I'm trying to think why users would want to sync logical slots to a reader but not have that be dropped if we're able to detect they're no longer present on the writer. Maybe if there was a use-case to set standby_slot_names one-at-a-time, you wouldn't want other logical slots to be dropped, but dropping sounds like reasonable behavior for '*'?

Is there a reason we're deciding to use one-worker syncing per database instead of one general worker that syncs across all the databases? I imagine I'm missing something obvious here.

As for how standby_slot_names should be configured, I'd prefer the flexibility similar to what we have for synchronus_standby_names since that seems the most analogous. It'd provide flexibility for failovers, which I imagine is the most common use-case.

John H

On 1/3/22, 5:49 AM, "Peter Eisentraut" <peter(dot)eisentraut(at)enterprisedb(dot)com> wrote:

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Here is an updated patch to fix some build failures. No feature changes.

On 14.12.21 23:12, Peter Eisentraut wrote:
> On 31.10.21 11:08, Peter Eisentraut wrote:
>> I want to reactivate $subject. I took Petr Jelinek's patch from [0],
>> rebased it, added a bit of testing. It basically works, but as
>> mentioned in [0], there are various issues to work out.
>>
>> The idea is that the standby runs a background worker to periodically
>> fetch replication slot information from the primary. On failover, a
>> logical subscriber would then ideally find up-to-date replication
>> slots on the new publisher and can just continue normally.
>
>> So, again, this isn't anywhere near ready, but there is already a lot
>> here to gather feedback about how it works, how it should work, how to
>> configure it, and how it fits into an overall replication and HA
>> architecture.
>
> Here is an updated patch. The main changes are that I added two
> configuration parameters. The first, synchronize_slot_names, is set on
> the physical standby to specify which slots to sync from the primary. By
> default, it is empty. (This also fixes the recovery test failures that
> I had to disable in the previous patch version.) The second,
> standby_slot_names, is set on the primary. It holds back logical
> replication until the listed physical standbys have caught up. That
> way, when failover is necessary, the promoted standby is not behind the
> logical replication consumers.
>
> In principle, this works now, I think. I haven't made much progress in
> creating more test cases for this; that's something that needs more
> attention.
>
> It's worth pondering what the configuration language for
> standby_slot_names should be. Right now, it's just a list of slots that
> all need to be caught up. More complicated setups are conceivable.
> Maybe you have standbys S1 and S2 that are potential failover targets
> for logical replication consumers L1 and L2, and also standbys S3 and S4
> that are potential failover targets for logical replication consumers L3
> and L4. Viewed like that, this setting could be a replication slot
> setting. The setting might also have some relationship with
> synchronous_standby_names. Like, if you have synchronous_standby_names
> set, then that's a pretty good indication that you also want some or all
> of those standbys in standby_slot_names. (But note that one is slots
> and one is application names.) So there are a variety of possibilities.