From 799bb299c705fea3fb5de990d7f8454a4054a908 Mon Sep 17 00:00:00 2001 From: Craig Ringer Date: Mon, 5 Sep 2016 16:33:37 +0800 Subject: [PATCH 11/21] Update comment on issues with logical decoding on standby --- src/backend/replication/logical/logical.c | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c index 1512be5..85f8f0e 100644 --- a/src/backend/replication/logical/logical.c +++ b/src/backend/replication/logical/logical.c @@ -88,16 +88,28 @@ CheckLogicalDecodingRequirements(void) errmsg("logical decoding requires a database connection"))); /* ---- - * TODO: We got to change that someday soon... + * To allow logical decoding on a standby we must ensure that: * - * There's basically three things missing to allow this: * 1) We need to be able to correctly and quickly identify the timeline a - * LSN belongs to + * LSN belongs to so we can follow timeline switches + * * 2) We need to force hot_standby_feedback to be enabled at all times so * the primary cannot remove rows we need. - * 3) support dropping replication slots referring to a database, in + * + * 3) ensure a replication slot is used to connect to the upstream so + * we know the catalog_xmin is persistent even over connection loss. + * + * 4) support dropping replication slots referring to a database, in * dbase_redo. There can't be any active ones due to HS recovery * conflicts, so that should be relatively easy. + * + * This means we can't allow logical decoding from a standby that's only + * configured for archive recovery. It would be OK to run temporarily in + * archive recovery during connectivity drops so long as we have a slot + * with a catalog_xmin set; it'd cause extra bloat on the master until we + * can reconnect, but that's unavoidable. We don't currently have any + * book-keeping about whether we have a slot unless it's in active use, + * though, so we have to assume there's no slot. * ---- */ if (RecoveryInProgress()) -- 2.5.5