Hi,
Simon Riggs wrote:
> You're right that neither the data transfer nor data availability is
> entirely synchronous, but data transfer is synchronous at time of
> *commit*: it is recorded on multiple nodes at the same time.
I'm unsure what you mean by a "data transfer being synchronous". To what
other process or state should the data transfer be synchronous to?
> The term "synchronous replication" is already well used in the industry
> to mean synchronous commit, so I don't think we should change the name
> now. The project here is also known to everybody as "synch rep".
I understand very well, that you don't want to change the name. I've
been hesitant to "relabel" Postgres-R from synchronous to asynchronous
to eager.
However, that is a marketing decision [1], which should not be mixed
with the technical discussion here. Speaking of a "synchronous commit"
is utterly misleading, because the commit itself is exactly the thing
that's *not* synchronous.
It *is* an optimization to fully synchronous replication to defer commit
on the "slave" and only make sure that the transaction *can* be applied
at some time in the future.
However, this *does* have the drawback of transactions not being
immediately visible on the slave. Often enough, this is acceptable. But
it certainly matters to some applications developers.
> What is confusing is that "replication" itself is a much abused term and
> is used to describe technologies for HA, DR and data movement.
I absolutely agree to that. And I'm thus recommending to at least be
consistent and honest with the term "synchronous" and point out that WAL
writing is synchronous for the log shipping approach here (AFAIK). But
that the commit is asynchronous for performance reasons. In other words:
this approach is certainly (and hopefully, for performance reasons)
different from a fully synchronous approach. Even for marketing reasons,
it might make sense to point out that difference (.. "no, we are faster
than fully sync rep.").
Regards
Markus Wanner
[1]: Some people like the term "virtually synchronous" for marketing
purposes. That's at least half-ways technically correct.
