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I have multiple PostgreSQL servers for a web application. Typically one master and multiple slaves in hot standby mode (asynchronous streaming replication).

I use PGBouncer for connection pooling: one instance installed on each PG server (port 6432) connecting to database on localhost. I use transaction pool mode.

In order to load-balance my read-only connections on slaves, I use HAProxy (v1.5) with a conf more or less like this:

listen pgsql_pool 0.0.0.0:10001
        mode tcp
        option pgsql-check user ha
        balance roundrobin
        server master 10.0.0.1:6432 check backup
        server slave1 10.0.0.2:6432 check
        server slave2 10.0.0.3:6432 check
        server slave3 10.0.0.4:6432 check

So, my web application connects to haproxy (port 10001), that load-balance connections on multiple pgbouncer configured on each PG slave.

Here is a representation graph of my current architecture:

haproxy > pgbouncer > postgresql

This works quite well like this, but I realize that some implements this quite differently: web application connects to a single PGBouncer instance that connects to HAproxy which load-balance over multiple PG servers:

pgbouncer > haproxy > postgresql

What's the best approach? The first one (my current one) or the second one? Are there any advantages of one solution over the other?

Thanks

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1 Answer

pgbouncer maintains connections in a pool with a postgres server. TCP connection establishment times are significant in a high-volume environment.

Clients making a large number of DB requests will have to setup a connection with a remote PGBouncer for each request. This is more expensive, than running PgBouncer locally (so the application connects to pgbouncer locally) and pgBouncer maintains a pool of connections with the remote PG server.

So, IMO, PGBouncer -> HAProxy -> PGServer seems to be better than, HAProxy -> PGBouncer -> PGServer, especially when the PGBouncer is local to the client application.

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