Re: Minimal logical decoding on standbys

On Tue, 11 Jun 2019 at 12:24, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
>
> On Mon, 10 Jun 2019 at 10:37, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> >
> > On Tue, 4 Jun 2019 at 21:28, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > >
> > > Hi,
> > >
> > > On 2019-06-04 15:51:01 +0530, Amit Khandekar wrote:
> > > > After giving more thought on this, I think it might make sense to
> > > > arrange for the xl_running_xact record to be sent from master to the
> > > > standby, when a logical slot is to be created on standby. How about
> > > > standby sending a new message type to the master, requesting for
> > > > xl_running_xact record ? Then on master, ProcessStandbyMessage() will
> > > > process this new message type and call LogStandbySnapshot().
> > >
> > > I think that should be a secondary feature. You don't necessarily know
> > > the upstream master, as the setup could be cascading one.
> > Oh yeah, cascading setup makes it more complicated.
> >
> > > I think for
> > > now just having to wait, perhaps with a comment to manually start a
> > > checkpoint, ought to suffice?
> >
> > Ok.
> >
> > Since this requires the test to handle the
> > fire-create-slot-and-then-fire-checkpoint-from-master actions, I was
> > modifying the test file to do this. After doing that, I found that the
> > slave gets an assertion failure in XLogReadRecord()=>XRecOffIsValid().
> > This happens only when the restart_lsn is set to ReplayRecPtr.
> > Somehow, this does not happen when I manually create the logical slot.
> > It happens only while running testcase. Working on it ...
>
> Like I mentioned above, I get an assertion failure for
> Assert(XRecOffIsValid(RecPtr)) while reading WAL records looking for a
> start position (DecodingContextFindStartpoint()). This is because in
> CreateInitDecodingContext()=>ReplicationSlotReserveWal(), I now set
> the logical slot's restart_lsn to XLogCtl->lastReplayedEndRecPtr. And
> just after bringing up slave, lastReplayedEndRecPtr's initial values
> are in this order : 0/2000028, 0/2000060, 0/20000D8, 0/2000100,
> 0/3000000, 0/3000060. You can see that 0/3000000 is not a valid value
> because it points to the start of a WAL block, meaning it points to
> the XLog page header (I think it's possible because it is 1 + endof
> last replayed record, which can be start of next block). So when we
> try to create a slot when it's in that position, then XRecOffIsValid()
> fails while looking for a starting point.
>
> One option I considered was : If lastReplayedEndRecPtr points to XLog
> page header, get a position of the first record on that WAL block,
> probably with XLogFindNextRecord(). But it is not trivial because
> while in ReplicationSlotReserveWal(), XLogReaderState is not created
> yet.

In the attached v6 version of the patch, I did the above. That is, I
used XLogFindNextRecord() to bump up the restart_lsn of the slot to
the first valid record. But since XLogReaderState is not available in
ReplicationSlotReserveWal(), I did this in
DecodingContextFindStartpoint(). And then updated the slot restart_lsn
with this corrected position.

Since XLogFindNextRecord() is currently disabled using #if 0, removed
this directive.

> Or else, do you think we can just increment the record pointer by
> doing something like (lastReplayedEndRecPtr % XLOG_BLCKSZ) +
> SizeOfXLogShortPHD() ?

I found out that we can't do this, because we don't know whether the
xlog header is SizeOfXLogShortPHD or SizeOfXLogLongPHD. In fact, in
our context, it is SizeOfXLogLongPHD. So we indeed need the
XLogReaderState handle.

>
> Do you think that we can solve this using some other approach ? I am
> not sure whether it's only the initial conditions that cause
> lastReplayedEndRecPtr value to *not* point to a valid record, or is it
> just a coincidence and that lastReplayedEndRecPtr can also have such a
> value any time afterwards. If it's only possible initially, we can
> just use GetRedoRecPtr() instead of lastReplayedEndRecPtr if
> lastReplayedEndRecPtr is invalid.

So now as the v6 patch stands, lastReplayedEndRecPtr is used to set
the restart_lsn, but its position is later adjusted in
DecodingContextFindStartpoint().

Also, modified the test to handle the requirement that the logical
slot creation on standby requires a checkpoint (or any other
transaction commit) to be given from master. For that, in
src/test/perl/PostgresNode.pm, added a new function
create_logical_slot_on_standby() which does the reqiured steps.

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company