Re: Sync Rep: First Thoughts on Code

Hi,

Mark Mielke wrote:
> Might it not be true that anybody unfamiliar would be confused and that
> this is a bit of a straw man?

Might be. I've neglected the issue myself for a while.

> I don't think synchronous replication guarantees that it will be
> immediately visible. Even if it did push the change to the other
> machine, and the other machine had committed it, that doesn't guarantee
> that any reader sees it any more than if I commit to the same machine
> (no replication), I am guaranteed to see the change from another
> session.

AFAIK every snapshot taken after a transaction has acknowledged its
commit is guaranteed to see changes from that transaction. Isn't that a
pretty frequent and obvious user expectation?

> Synchronous replication only means that I can be assured that
> my change has been saved permanently by the time my commit completes. It
> doesn't mean anybody else can see my change or is guaranteed to see my
> change if the query from another session.

So you wouldn't be surprised if a transaction from two hours ago isn't
visible on another node, just because that node happens to be rather
busy with lots of other readers and maintenance tasks?

> If my application assumes that it can commit to one server, and then
> read back the commit from another server, and my application breaks as a
> result, it's because I didn't understand the problem.

Well, yeah, depends on user expectations. I'm surprised to hear that you
have that understanding of synchronous replication.

> Even if PostgreSQL
> didn't use the word "synchronous replication", I could still be
> confused. I need to understand the problem no matter what words are used.

As said, it depends on what the common understanding of "synchronous
replication" is. I've so far been under the impression, that these
potential lags are unexpected and confusing. Several people pointed me
at that problem and I've thus "relabeled" Postgres-R as not being
synchronous. I'm at least surprised to suddenly get pushed into the
other direction. :-)

However, I absolutely agree that it's not that important how we name it.
What is important, is that users and developers understand the difference.

Regards

Markus Wanner