[Pgpool-general] pgpool and Slony-I replication

[Tatsuo Ishii]

> I am planning to use pgpool to handle switchover/failover between two DB 
> servers replicating with Slony-I. I have a few questions, though, and I 
> haven't found any information on those topics, don't hesitate to point me 
> to the right place if I missed them.
> 
> The goal is to have two servers, without replication by pgpool, maybe with 
> load balancing, and handle voluntary switchovers (during the nightly vacuum 
> and other maintenances) as well as unexpected failovers.

Note that as of pgpool 2.5b1 does not support "no replication but uses
two PostgreSQL servers". I will add the functionarity to 2.5b2
soon. Following replies are supposed to be based on 2.5b2.

> Questions:
> - in this scenario, pgpool will switch writing from primary to secondary by 
> itself if the primary fails, right?

Correct.

> - likewise, pgpool will stop using the secondary (and send all reads and 
> writes to the primary) if it fails?

Yes.

> - what exactly are the criteria for "failure"? No connection? No response 
> to queries? What are the timeout thresholds?

"failue" includes:

- failed to connect to backend
- error on socket read/write

There's no timeout thresholds.

> - will pgpool forget about that host forever, or will it try to connect and 
> reuse the server whenever it comes back?

pgpool will forget about that host forever.

> - in some cases (during a switchover), reads can start from the new master 
> right away, but writes need to be blocked for a few seconds. Is there a way 
> to actually delay them? I.e. not send them to any of the servers, wait 
> until some event happens (the switchover is finished) and then send them to 
> the secondary? Reads should be done on the secondary right away, of course.

Currently there's no way to handle this situation.

> - how tested are the 2.5b1 "switch" features?

Sorry I don't understand the question.

> - do we agree that "pgpool switch" will stop sending requests to the 
> primary (and thus send both reads and writes to the secondary) and "pgpool 
> -s secondary switch" will stop sending requests to the secondary (and thus 
> send both reads and writes to the primary)?

Yes.

> - how can one "restore" the primary or secondary without restarting pgpool 
> (explicitly or implicitly)?

Currently there's no way to do that.

> I guess that's it. Any help would be greatly appreciated.
> 
> Thanks,
> 
> Jacques.
> 
> 
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