Re: write ahead logging in standby (streaming replication)

Fujii Masao wrote:
> Umm... what is your definition of "synchronous"? I'm planning to provide
> four synchronization modes as follows, for v8.5. Does this fit in your
> thought?
>
> The primary waits ... before returning "success" of a transaction;
> * nothing - asynchronous replication
> * recv ACK - semi-synchronous replication
> * fsync ACK - semi-synchronous replication
> * redo ACK - synchronous replication
>
> Or, in synchronous replication, we must wait a fsync and a redo ACK?
>
Right, those are the possibilities, all four of them have valid use
cases in the field and are worth implementing. I don't like the label
"semi-synchronous replication" myself, but it's a valuable feature to
implement, and that is unfortunately the term other parts of the
industry use for that approach.

But everyone needs to be extremely careful with the terminology here:
if you say "synchronous replication", that *only* means what you're
labeling "redo ACK" ("WAL ACK" really). "Synchronous replication"
should not be used as a group term that includes the semi-synchronous
variations, which are in fact asynchronous despite their marketing
name. If someone means semi-synchronous, but they say synchronous
thinking it's a shared term also applicable to the semi-synchronous
variations here, that's just going to be confusing for everyone.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com