Question

We’re considering Slony and PGPool as alternatives to handle failover in our application -and it seems like since we’re gonna need at least two DB servers, we could take advantage of load balancing too-

Under which circumstances Slony is better than PGPool and viceversa?

Answer 1

This is anecdotal, so take it with a grain of salt.

PGPool and streaming WAL replication (with hot standby or not) works the way database replication ought to. Your application doesn’t need to know anything about replication or whether it is part of a cluster or whatnot, it just talks to the database as it would any other. Streaming replication is robust, and has the ability to fail back to WAL shipping if streaming breaks. PGPool makes managing this process easy and gives good heartbeats and monitoring info.

Slony, on the other hand, is an administrative tar-pit, which needs to add trigger functions and numerous other objects to your database to work. Furthermore, Slony doesn’t (easily) support the ability to replicate schema changes (DDL) in the same way it replicates ordinary insert/update/delete type operations (DML). Taken as a whole, these characteristics mean that, in many cases, your application needs to have special cases to handle Slony’s eccentricities. In my opinion, that’s bad: the application shouldn’t have to be aware of/make changes to deal with the database replication solution that it runs on. I spent the better part of a year hacking Slony to work as a stable replication solution, and eventually came to the conclusion that it’s a bad idea/bad replication mechanic implemented in an obtuse, illegible way, that is anything but stable or enterprise-ready. EDIT: as of PostgreSQL 9.3 you can install triggers (which Slony uses to detect changes) on DDL objects, which might allow Slony to replicate more aspects of a database.

That said, Slony does do two things better than streaming replication (administered via PGPool or no):

• Slony allows per-table or per-database replication. Streaming replication only permits the replication of an entire Postgres instance. If that kind of granularity is important to you, you might want Slony.
• Slony allows cascading (master -> slave -> slave) replication. Streaming replication only allows one level. EDIT: This is now supported in PostgreSQL native streaming replication, as of Postgres version 9.2.

At literally everything else, streaming replication is better and more stable.

But you’re not just asking about streaming replication: PGPool does a great deal more than that. It allows the balancing of read and write loads between read-only slave databases and masters, and the implementation of backup plans, as well as a whole host of other things. Especially when compared to Slony’s arcane command syntax and godawful administration scripts, PGPool wins in nearly every instance.

With regards to failover specifically, PGPool has heartbeat monitors and the ability to automatically route database traffic in a cluster. Slony only has a “fail over to slave now” command, leaving all of the monitoring and app-side routing up to you.

TL;DR: PGPool good. Slony bad.