Re: Synchronizing slots from primary to standby

On Fri, Aug 25, 2023 at 2:15 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>
> Wait a minute ...
>
> From bac0fbef8b203c530e5117b0b7cfee13cfab78b9 Mon Sep 17 00:00:00 2001
> From: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
> Date: Sat, 22 Jul 2023 10:17:48 +0000
> Subject: [PATCH v13 1/2] Allow logical walsenders to wait for physical
> standbys
>
> @@ -2498,6 +2500,13 @@ ReorderBufferProcessTXN(ReorderBuffer *rb, ReorderBufferTXN *txn,
> }
> else
> {
> + /*
> + * Before we send out the last set of changes to logical decoding
> + * output plugin, wait for specified streaming replication standby
> + * servers (if any) to confirm receipt of WAL upto commit_lsn.
> + */
> + WaitForStandbyLSN(commit_lsn);
>
> OK, so we call this new function frequently enough -- once per
> transaction, if I read this correctly? So ...
>
> +void
> +WaitForStandbyLSN(XLogRecPtr wait_for_lsn)
> +{
> ...
>
> + /* "*" means all logical walsenders should wait for physical standbys. */
> + if (strcmp(synchronize_slot_names, "*") != 0)
> + {
> + bool shouldwait = false;
> +
> + rawname = pstrdup(synchronize_slot_names);
> + SplitIdentifierString(rawname, ',', &elemlist);
> +
> + foreach (l, elemlist)
> + {
> + char *name = lfirst(l);
> + if (strcmp(name, NameStr(MyReplicationSlot->data.name)) == 0)
> + {
> + shouldwait = true;
> + break;
> + }
> + }
> +
> + pfree(rawname);
> + rawname = NULL;
> + list_free(elemlist);
> + elemlist = NIL;
> +
> + if (!shouldwait)
> + return;
> + }
> +
> + rawname = pstrdup(standby_slot_names);
> + SplitIdentifierString(rawname, ',', &elemlist);
>
> ... do we really want to be doing the GUC string parsing every time
> through it? This sounds like it could be a bottleneck, or at least slow
> things down. Maybe we should think about caching this somehow.
>

Yes, these parsed lists are now cached. Please see v15
(https://www.postgresql.org/message-id/CAJpy0uAuzbzvcjpnzFTiWuDBctnH-SDZC6AZabPX65x9GWBrjQ%40mail.gmail.com)

thanks
Shveta