Re: Transactions involving multiple postgres foreign servers

On Thu, Jul 27, 2017 at 6:58 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On a technical level, I am pretty sure that it is not OK to call
> AtEOXact_FDWXacts() from the sections of CommitTransaction,
> AbortTransaction, and PrepareTransaction that are described as
> "non-critical resource releasing". At that point, it's too late to
> throw an error, and it is very difficult to imagine something that
> involves a TCP connection to another machine not being subject to
> error. You might say "well, we can just make sure that any problems
> are reporting as a WARNING rather than an ERROR", but that's pretty
> hard to guarantee; most backend code assumes it can ERROR, so anything
> you call is a potential hazard. There is a second problem, too: any
> code that runs from here is not interruptible. The user can hit ^C
> all day and nothing will happen. That's a bad situation when you're
> busy doing network I/O. I'm not exactly sure what the best thing to
> do about this problem would be.
>

The remote transaction can be committed/aborted only after the fate of
the local transaction is decided. If we commit remote transaction and
abort local transaction, that's not good. AtEOXact* functions are
called immediately after that decision in post-commit/abort phase. So,
if we want to commit/abort the remote transaction immediately it has
to be done in post-commit/abort processing. Instead if we delegate
that to the remote transaction resolved backend (introduced by the
patches) the delay between local commit and remote commits depends
upon when the resolve gets a chance to run and process those
transactions. One could argue that that delay would anyway exist when
post-commit/abort processing fails to resolve remote transaction. But
given the real high availability these days, in most of the cases
remote transaction will be resolved in the post-commit/abort phase. I
think we should optimize for most common case. Your concern is still
valid, that we shouldn't raise an error or do anything critical in
post-commit/abort phase. So we should device a way to send
COMMIT/ABORT prepared messages to the remote server in asynchronous
fashion carefully avoiding errors. Recent changes to 2PC have improved
performance in that area to a great extent. Relying on resolver
backend to resolve remote transactions would erode that performance
gain.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company