Postgresql HA questions

- Is there any way to do zero-downtime updates to Postgres? At first I was thinking I could just update the secondary, fail over to it, update the primary, and fail back, but then I realized that this would be much more complicated than my first thought, since I would have to either a) find a way to point all my apps that use the database to the secondary while updating, or b) re-ip  the machines. Add to that the difficulty with the fail-back, and it seems like it would be a simpler solution to simply take down the primary server (and all my companies apps) for the (assumedly short) duration of the update.

With my application servers, I have a system set up using corosync and pacemaker that allows for seamless fail-over between the two machines, with the IP address and all services moving smoothly between the two at will. Ideally, I would have a similar setup with my database servers, so the applications never even know that there was a switch. Is this possible with Postgresql at all? Does it make a difference that at least one app has an "always on" connection to the DB Server?

- When using named replication slots, I know the primary server will hold on to as many WAL files as the secondary needs to be able to recover to a consistent state, which runs the risk of filling up the hard drive with WAL files if the secondary is off-line for an extended period of time. As such, I have a system set up to monitor the number of WAL files on both the primary and secondary machines. Watching that, I have on occasion seen the number hit as high as 300 some odd files on the secondary, although it always comes back down fairly quickly (on the day I saw the count up at 378, it was back down to 30 within 10 minutes). At the moment I have 13 WAL files, even though my wall_keep_segments parameter is set to 5.  Interestingly (to me at least) I have not seen this same sort of variation on the primary - it generally stays right around 13 files. Is this sort of fluctuation normal, or should I be looking for some sort of an issue?

-What is the best way to monitor that the two servers are actually staying in sync? At the moment, I am looking at the value of the "active" field in the pg_replication_slots table. Is this sufficient to be sure replication is taking place properly, or should I do something like run a query on a frequently updated table and make sure the results are identical?

Thanks for any suggestions and information you can provide!