Re: [PATCH 10/16] Introduce the concept that wal has a 'origin' node

On Tue, Jun 19, 2012 at 3:18 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> More seriously: Even if we don't put MM in core I think putting the basis for
> it in core so that somebody can build such a solution reusing the existing
> infrastructure is a sensible idea. Imo the only thing that requires explicit
> support which is hard to add outside of core is prevention of loops (aka this
> patch). Everything else should be doable reusing the hopefully modular pieces.

I don't think prevention of loops is hard to do outside of core
either, unless you insist on tying "logical" replication so closely to
WAL that anyone doing MMR is necessarily getting the change stream
from WAL.  In fact, I'd go so far as to say that the ONLY part of this
that's hard to do outside of core is change extraction.  Even
low-level apply can be implemented as a loadable module.

>> Right.  If we decide we need this, and if we did decide to conflate
>> the WAL stream, both of which I disagree with as noted above, then we
>> still don't need it on every record.  It would probably be sufficient
>> for local transactions to do nothing at all (and we can implicitly
>> assume that they have master node ID = local node ID) and transactions
>> which are replaying remote changes to emit one record per XID per
>> checkpoint cycle containing the remote node ID.
> Youve gone from a pretty trivial 150 line patch without any runtime/space
> overhead to something *considerably* more complex in that case though.
>
> You suddently need to have relatively complex logic to remember which
> information you got for a certain xid (and forget that information afterwards)
> and whether you already logged that xid and you need to have to decide about
> logging that information at multiple places.

You need a backend-local hash table inside the wal reader process, and
that hash table needs to map XIDs to node IDs.  And you occasionally
need to prune it, so that it doesn't eat too much memory.  None of
that sounds very hard.

> Btw, what do you mean with "conflating" the stream? I don't really see that
> being proposed.

It seems to me that you are intent on using the WAL stream as the
logical change stream.  I think that's a bad design.  Instead, you
should extract changes from WAL and then ship them around in a format
that is specific to logical replication.

--
Robert Haas
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