[Pgpool-general] Zimbra Detected SpamPgPool replication or Slony

[Pascal Cohen]

Hello all
I am new to PgPool (and also Slony).
I am currently studying the way to ensure more availability but also 
load balancing to reduce the load on a single DB whenever we would need it.
PgPool-II is very nice and is running well in the tests I did and the 
way to integrate it with PgPool-HA seems nice.
But I am concerned with the replication between several DBs.
I can handle that in two ways:
First use PgPool only as a load balancer and rely on Slony for 
replication or use PgPool as load balancer and replicator.
I would like to know what are the drawbacks and the advantages of each 
solution.

I would like also to get details on the PgPool replication behavior.
The simplest is probably a dummy example.
Let's say we have a master DB containing a table A and a transaction 
updates one row in A. Table A in S is synchronized with M.
Let's say then that I have two connections that get the updated row in A.

Using PgPool as load balancer between a Master M and a Slave S. I can 
have the following situation (not necessary much probable but 
technically possible):
Update of A in M.
First request on A goes on M.
Second request on A goes to S.
Slony performs synchro between M and S.
In that case my two requests may return different results.

Can that situation occurs also with PgPool replication like:
PgPool sends update of A in M .
First request on A goes on M.
Second request on A goes to S.
PgPool sends update to S.

My main concern is in fact that there can be transitional state where we 
can get different data depending on the targeted DB choosing PgPool 
replication or Slony replication.

Am I right ?

Thanks for your help

Regards

Pascal