Re: write ahead logging in standby (streaming replication)

Fujii Masao wrote:
> Personally, I think that semi-synchronous replication is sufficient for HA.
>   
Whether or not you think it's sufficient for what you have in mind, 
"synchronous replication" requires a return ACK from the secondary 
before you say things are committed on the primary.  If you don't do 
that, it's not true sync replication anymore; it's asynchronous 
replication.  Plenty of people decide that a local commit combined with 
a promise to synchronize as soon as possible to the slave is good enough 
for their apps, which as you say is getting referred to as 
"semi-synchronous replication" nowadays.  That's an awful name though, 
because it's not true--that's asynchronous replication, just aiming for 
minimal lag.  It's OK to say that's what you want, but you can't say 
it's really a synchronous commit anymore if you do things that way.

-- 
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com  www.2ndQuadrant.com