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@2ndQuadrant.com  www.2ndQuadrant.com