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
g...@2ndquadrant.com www.2ndQuadrant.com
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