Re: Timeline following for logical slots

On Tue, Apr 5, 2016 at 10:34 PM, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>> Right, but right now you probably *aren't* do doing any kind of
>> logical decoding from a master server to a standby, because there's
>> squat in the core distribution that could make use of that capability.
>> So you never get as far as discovering this problem.
>
> OK, I'm starting to understand where this is going.
>
> pglogical and pglogical_output are irrelevant so long as they haven't yet
> landed in core. There is no logical replication worth considering, therefore
> anything that improves logical replication is not worth the effort because
> there's really nothing to improve. Why bother with enhancements like
> timeline following when nothing uses it.
>
> Right?

"Irrelevant" is a strong word, and I wouldn't personally go that far.
But they would be a lot *more* relevant if they landed in core, at
least IMHO.

>> Second, I'm not sure whether it was a good design decision to make
>> logical slots a special kind of object that sit off to the side,
>> neither configuration (like postgresql.conf) nor WAL-protected data
>> (like pg_clog and the data files themselves), but it was certainly a
>> very deliberate decision. I sort of expected them to be WAL-logged,
>> but Andres argued (not unconvincingly) that we'd want to have slots on
>> standbys, and making them WAL-logged would preclude that.
>
> Yeah. I understand the reasons for that decision. Per an earlier reply I
> think we can avoid making them WAL-logged so they can be used on standbys
> and still achieve usable failover support on physical replicas.

I think at one point we may have discussed doing this via additional
side-channel protocol messages. Is that what you are thinking about
now, or something else?

>> If you retype a column from text to integer,
>> you probably aren't storing anything in it other than integers, in
>> which case it is not necessarily the case that you are locked into
>> applying that change at a particular point in the change stream.
>
> ... if you're replicating in text format, not send/recv format, yeah. You
> get away with it in that particular case. But that *only* works where you're
> using text format and the types are convertable via text representation.
>
> It's also just one example. Adding a NOT NULL column bites you, hard, for
> example.
>
> Asking users to do this themselves is incredibly fragile and requires much
> more knowledge of PostgreSQL innards than I think is realistic. It's part of
> why Slony-I is difficult too.
>
> Sure, you can avoid having DDL replication if you're really, really careful
> and know exactly what you're doing. I don't think it's a pre-req for getting
> logical replication into core, but I do think it's a pretty strong
> requirement for taking the use of logical replication for HA and failover
> seriously for general use.

Well, we can agree to disagree on this. I don't think that it's all
that difficult to figure out how to change your schema in a
step-by-step way that allows logical replication to keep working while
the nodes are out of sync, but you don't have to agree and that's
fine. I'm not objecting to eventually adding that feature to core. I
do think it's a bad idea to be polishing that sort of thing before
getting some more basic facility into core.

>> To be honest, I was shocked that pglogical and pglogical_output didn't
>> go into this release. I assumed that you and other folks at
>> 2ndQuadrant were going to make a big push to get that done. I did
>> take a brief look at one of them - pglogical, I think - a week or two
>> ago but there were unaddressed review comments that had been pending
>> for months and there were a lot of fairly obvious things that needed
>> to be done before it could be seriously considered as a core
>> submission. Like, for example, rewriting the documentation heavily
>> and making it look like the rest of our docs, and putting it in SGML
>> format. The code seemed to need quite a bit of cleanup, too.
>
> Yes, I agree, and I think those comments are largely fair. Both Petr and I
> had more limited time to work on them than either of us had expected, so
> timely follow-up was a challenge. We also got them ready for initial
> submission later in the cycle than we'd hoped. That said, there was also
> very little review response or interest for a long time after they were
> published... and by the time there was I think both Petr and I were bogged a
> bit in other project work.
>
> In retrospect I think the separate submission of the output plugin was a
> mistake too, at least when called pglogical_output; in part because of the
> name it was completely ignored as meaningless without the downstream and
> written off as useful for nothing except logical replication. So no momentum
> was built by the time the downstream was published.

While I acknowledge that a logical output plugin has applications
beyond replication, I think replication is the preeminent one by a
considerable margin. Some people want it for other purposes, and we
should facilitate that. But that number of people is dwarfed by the
number who would use a seamless logical replication facility if we had
one available. And so that's the thing I think we should focus on
making work.

If I were doing that, I think I would attack it from a considerably
different direction than what has so far been proposed. I would try
to put the stuff in core, not contrib, and I would arrange to control
it using DDL, not function calls. For version one, I would cut all of
the stuff that allows data to be sent in any format other than text,
and just use in/outfuncs all the time.

> I think getting more people involved is useful in a number of ways. Not just
> time and resources, but also getting more agreement on the direction things
> should be going in and making sure that more people have a direct hand in
> the core and are happy with it. Working from a common repo where interested
> people can push directly would be an interesting start.
>
> I'll need to discuss this inside 2ndQ and follow up as there are things
> involved that I can't make determinations for myself.

Understood.

>> We have a long history of building features
>> incrementally. Parallel query in 9.6 doesn't parallelize every query
>> that can be parallelized
>
> Right. But it doesn't just arbitrarily break on what it doesn't understand.
> That's the difference here.

I do generally think that logical decoding relies too much on trying
to set up situations where it will never fail, and I've said from the
beginning that it should make more provision to cope with failure
rather than just trying to avoid it. If logical decoding never
breaks, great. But the approach I would favor is to set things up so
that it automatically reclones if there is a replication break, and
then as an optimization project, try to eliminate those cases one by
one.

> It's a big job, and there are lots of differing viewpoints on it.
>
> It does seem like having 2ndQ submit a patch, wait for feedback, be told
> "oh, don't do it that way, come back with <something else>", then wait for
> us to respond and address the issues isn't working. Certainly isn't going to
> get us there in a timely manner.

That isn't really my perception of how things have gone, but I admit I
may not have been following it as closely as I should have done. I'd
be happy to talk with you about this in person if you're going to be
at PGCONF.US or PGCon in Ottawa; or there's always Skype. I don't see
any reason why we can't all put our heads together and agree on a
direction that we can all live with.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company