Re: Synchronizing slots from primary to standby

On Mon, Jan 22, 2024 at 3:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jan 19, 2024 at 3:55 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Fri, Jan 19, 2024 at 10:35 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > >
> > > Thank you for updating the patch. I have some comments:
> > >
> > > ---
> > > + latestWalEnd = GetWalRcvLatestWalEnd();
> > > + if (remote_slot->confirmed_lsn > latestWalEnd)
> > > + {
> > > + elog(ERROR, "exiting from slot synchronization as the
> > > received slot sync"
> > > + " LSN %X/%X for slot \"%s\" is ahead of the
> > > standby position %X/%X",
> > > + LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
> > > + remote_slot->name,
> > > + LSN_FORMAT_ARGS(latestWalEnd));
> > > + }
> > >
> > > IIUC GetWalRcvLatestWalEnd () returns walrcv->latestWalEnd, which is
> > > typically the primary server's flush position and doesn't mean the LSN
> > > where the walreceiver received/flushed up to.
> >
> > yes. I think it makes more sense to use something which actually tells
> > flushed-position. I gave it a try by replacing GetWalRcvLatestWalEnd()
> > with GetWalRcvFlushRecPtr() but I see a problem here. Lets say I have
> > enabled the slot-sync feature in a running standby, in that case we
> > are all good (flushedUpto is the same as actual flush-position
> > indicated by LogstreamResult.Flush). But if I restart standby, then I
> > observed that the startup process sets flushedUpto to some value 'x'
> > (see [1]) while when the wal-receiver starts, it sets
> > 'LogstreamResult.Flush' to another value (see [2]) which is always
> > greater than 'x'. And we do not update flushedUpto with the
> > 'LogstreamResult.Flush' value in walreceiver until we actually do an
> > operation on primary. Performing a data change on primary sends WALs
> > to standby which then hits XLogWalRcvFlush() and updates flushedUpto
> > same as LogstreamResult.Flush. Until then we have a situation where
> > slots received on standby are ahead of flushedUpto and thus slotsync
> > worker keeps one erroring out. I am yet to find out why flushedUpto is
> > set to a lower value than 'LogstreamResult.Flush' at the start of
> > standby. Or maybe am I using the wrong function
> > GetWalRcvFlushRecPtr() and should be using something else instead?
> >
>
> Can we think of using GetStandbyFlushRecPtr()? We probably need to
> expose this function, if this works for the required purpose.

GetStandbyFlushRecPtr() seems good. But do we really want to raise an
ERROR in this case? IIUC this case could happen often when the slot
used by the standby is not listed in standby_slot_names. I think we
can just skip such a slot to synchronize and check it the next time.

Here are random comments on slotsyncworker.c (v66):

---
The postmaster relaunches the slotsync worker without intervals. So if
a connection string in primary_conninfo is not correct, many errors
are emitted.

---
+/* GUC variable */
+bool enable_syncslot = false;

Is enable_syncslot a really good name? We use "enable" prefix only for
planner parameters such as enable_seqscan, and it seems to me that
"slot" is not specific. Other candidates are:

* synchronize_replication_slots = on|off
* synchronize_failover_slots = on|off

---
+ elog(ERROR,
+ "cannot synchronize local slot \"%s\" LSN(%X/%X)"
+ " to remote slot's LSN(%X/%X) as synchronization"
+ " would move it backwards", remote_slot->name,

Many error messages in slotsync.c are splitted into several lines, but
I think it would reduce the greppability when the user looks for the
error message in the source code.

---
+ SpinLockAcquire(&slot->mutex);
+ slot->data.database = get_database_oid(remote_slot->database, false);
+ namestrcpy(&slot->data.plugin, remote_slot->plugin);

We should not access syscaches while holding a spinlock.

---
+ SpinLockAcquire(&slot->mutex);
+ slot->data.database = get_database_oid(remote_slot->database, false);
+ namestrcpy(&slot->data.plugin, remote_slot->plugin);
+ SpinLockRelease(&slot->mutex);

Similarly, it's better to avoid calling namestrcpy() while holding a
spinlock, as we do in CreateInitDecodingContext().

---
+ SpinLockAcquire(&SlotSyncWorker->mutex);
+
+ SlotSyncWorker->stopSignaled = true;
+
+ if (SlotSyncWorker->pid == InvalidPid)
+ {
+ SpinLockRelease(&SlotSyncWorker->mutex);
+ return;
+ }
+
+ kill(SlotSyncWorker->pid, SIGINT);
+
+ SpinLockRelease(&SlotSyncWorker->mutex);

It's better to avoid calling a system call while holding a spin lock.

---
+ BackgroundWorkerUnblockSignals();

I think it's no longer necessary.

---
+ ereport(LOG,
+ /* translator: %s is a GUC variable name */
+ errmsg("bad configuration for slot synchronization"),
+ errhint("\"wal_level\" must be >= logical."));

There is no '%s' in errmsg string.

---
+/*
+ * Cleanup function for logical replication launcher.
+ *
+ * Called on logical replication launcher exit.
+ */

IIUC this function is never called by logical replication launcher.

---
+ /*
+ * The slot sync worker can not get here because it will only stop when it
+ * receives a SIGINT from the logical replication launcher, or when there
+ * is an error.
+ */
+ Assert(false);

This comment is not correct. IIUC the slotsync worker receives a
SIGINT from the startup process.

Regards,

--
Masahiko Sawada
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