Re: logical changeset generation v3 - Mailing list pgsql-hackers

On 2012-11-20 09:30:40 +0900, Michael Paquier wrote:
> On Mon, Nov 19, 2012 at 5:50 PM, Andres Freund <andres@2ndquadrant.com>wrote:

> > On 2012-11-19 16:28:55 +0900, Michael Paquier wrote:

> > > I am just looking at this patch and will provide some comments.
> > > By the way, you forgot the installation part of pg_receivellog, please see
> > > patch attached.
> >
> > That actually was somewhat intended, I thought people wouldn't like the
> > name and I didn't want a binary that's going to be replaced anyway lying
> > around ;)
> >
> OK no problem. For sure this is going to happen, I was wondering myself if
> it could be possible to merge pg_receivexlog and pg_receivellog into a
> single utility with multiple modes :)

Don't really see that, the differences already are significant and imo
are bound to get bigger. Shouldn't live in pg_basebackup/ either...

> Btw, here are some extra comments based on my progress, hope it will be
> useful for other people playing around with your patches.
> 1) Necessary to install the contrib module test_decoding on server side or
> the test case will not work.
> 2) Obtention of the following logs on server:
> LOG:  forced to assume catalog changes for xid 1370 because it was running
> to early
> WARNING:  ABORT 1370
> Actually I saw that there are many warnings like this.

Those aren't unexpected. Perhaps I should not make it a warning then...

A short explanation:

We can only decode tuples we see in the WAL when we already have a
timetravel catalog snapshot before that transaction started. To build
such a snapshot we need to collect information about committed which
changed the catalog. Unfortunately we can't diagnose whether a txn
changed the catalog without a snapshot so we just assume all committed
ones do - it just costs a bit of memory. Thats the background of the
"forced to assume catalog changes for ..." message.

The reason for the ABORTs is related but different. We start out in the
"SNAPBUILD_START" state when we try to build a snapshot. When we find
initial information about running transactions (i.e. xl_running_xacts)
we switch to the "SNAPBUILD_FULL_SNAPSHOT" state which means we can
decode all changes in transactions that start *after* the current
lsn. Earlier transactions might have tuples on a catalog state we can't
query.

Only when all transactions we observed as running before the
FULL_SNAPSHOT state have finished we switch to SNAPBUILD_CONSISTENT.
As we want a consistent/reproducible set of transactions to produce
output via the logstream we only pass transactions to the output plugin
if they commit *after* CONSISTENT (they can start earlier though!). This
allows us to produce a pg_dump compatible snapshot in the moment we get
consistent that contains exactly the changes we won't stream out.

Makes sense?