Re: Sync Rep: First Thoughts on Code

On Sat, Dec 13, 2008 at 1:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Robert Haas" <robertmhaas@gmail.com> writes:
>> I think we need to reserve the term "synchronous replication" for a
>> system where transactions that begin at the same time on the primary
>> and standby see the same tuples.  Clearly that is "more" synchronous
>
> We won't call it anything, because we never will or can implement that.
> See the theory of relativity: the notion of exactly simultaneous events

OK, fine.  I'll be more precise.  I think we need to reserve the term
"synchronous replication" for a system where transactions that begin
on the standby after the transactions has committed on the master see
the effects of the committed transaction.

> at distinct locations isn't even well-defined, because observers at yet
> other locations will disagree about what is "simultaneous".  And I'm
> not just making a joke here --- speed-of-light delays in a WAN are
> meaningful compared to current computer speeds.  In practice, the
> slave and the master will never commit at exactly the same time.
>
> I agree with the point made upthread that we should use the term
> "synchronous replication" the way it's commonly used in the industry.
> Inventing our own terminology might be fun but it's not really going
> to result in less confusion.

I just googled "synchronous replication" and read through the first
page of hits.  Most of them do not address the question of whether
synchronous replication can be said to have be completed when WAL has
been received by the standby not but yet applied.  One of the ones
that does is:

http://code.google.com/p/google-mysql-tools/wiki/SemiSyncReplicationDesign

...which refers to what we're proposing to call "Synchronous
Replication" as "Semi-Synchronous Replication" (or 2-safe replication)
specifically to distinguish it.  The other is:

http://www.cnds.jhu.edu/pub/papers/cnds-2002-4.pdf

...which doesn't specifically examine the issue but seems to take the
opposite position, namely that the server on which the transaction is
executed needs to wait only for one server to apply the changes to the
database (the others need only to know that they need to commit it;
they don't actually need to have done it).  However, that same paper
refers to two-phase commit as a synchronous replication algorithm, and
Wikipedia's discussion of two-phase commit:

http://en.wikipedia.org/wiki/Two-phase_commit_protocol

...clearly implies that the transaction must be applied everywhere
before it can be said to have committed.

The second page of Google results is mostly a further discussion of
the MySQL solution, which is mostly described as "semi-synchronous
replication".

Simon Riggs said upthread that Oracle called this "synchronous redo
transport".  That is obviously much closer to what we are doing than
"synchronous replication".

...Robert