Thursday, May 27 2010
PostgreSQL, pgcon, Extensions, backup, restore, 9.1

Back from PgCon2010

This year's edition has been the best pgcon ever for me. Granted, it's only my third time, but still :) As Josh said the "Hall Track" in particular was very good, and the Dev Meeting has been very effective!


This time I prepared some slides to present the extension design and I tried hard to make it so that we get to agree on a plan, even recognizing it's not solving all of our problems from the get go. I had been talking about the concept and design with lots of people already, and continued to do so while in Ottawa on Monday evening and through all Tuesday. So Wednesday, I felt prepared. It proved to be a good thing, as I edited the slides with ideas from several people I had the chance to expose my ideas to! Thanks Greg Stark and Heikki Linnakangas for the part we talked about at the meeting, and a lot more people for the things we'll have to solve later (Hi Stefan!).

So the current idea for extensions is for the backend support to start with a file in `pg_config --sharedir`/extensions/foo/control containing the foo extension's metadata. From that we know if we can install an extension and how. Here's an example:

name = foo
version = 1.0
custom_variable_classes = 'foo'
depends  = bar (>= 1.1), baz
conflicts = bla (< 0.8)

The other files should be install.sql, uninstall.sql and foo.conf. The only command the user will have to type in order for using the extension in his database will then be:


For that to work all that needs to happen is for me to write the code. I'll keep you informed as soon as I get a change to resume my activities on the git branch I'm using. You can already find my first attempt at a pg_execute_from_file() function there.

Building atop that backend support we already have two gentlemen competing on features to offer to distribute and package extensions! That will complete the work just fine, thanks guys.

Hot Standby

Heikki's talk about Built-in replication in PostgreSQL 9.0 left me with lots of thinking. In particular it seems we need two projects out of core to complete what 9.0 has to offer, namely something very simple to prepare a base backup and something more involved to manage a pool of standbys.


The idea I had listening to the talk was that it might be possible to ask the server, in a single SQL query, for the list of all the files it's using. After all, there's those pg_ls_files() and pg_read_file() functions, we could put them to good use. I couldn't get the idea out of my head, so I had to write some code and see it running: pg_basebackup is there at github, grab a copy!

What it does is very simple, in about 100 lines of self-contained python code it get all the files from a running server through a normal PostgreSQL connection. That was my first recursive query. I had to create a new function to get the file contents as the existing one returns text, and I want bytea here, of course.

Note that the code depends on the bytea representation in use, so it's only working with 9.0 as of now. Can be changed easily though, send a patch or just ask me to do it!

Lastly, note that even if pg_basebackup will compress each chunk it sends over the libpq connection, it won't be your fastest option around. Its only advantage there is its simplicity. Get the code, run it with 2 arguments: a connection string and a destination directory. There you are.

wal proxy, wal relay

The other thing that we'll miss in 9.0 is the ability to both manage more than a couple of standby servers and to manage failover gracefully. Here the idea would be to have a proxy server acting as both a walreceiver and a walsender. Its role would be to both archive the WAL and relay them to the real standbys.

Then in case of master's failure, we could instruct this proxy to be fed from the elected new master (manual procedure), the other standbys not being affected. Well apart than apparently changing the timeline (which will happen as soon as you promote a standby to master) while streaming is not meant to be supported. So the proxy would also disconnect all the slaves and have them reconnect.

If we need such a finesse, we could have the restore_command on the standbys prepared so that it'll connect to the proxy's archive. Now on failover, the standbys are disconnected from the stream, get a WAL file with a new timeline from the archive, replay it, and reconnect.

That means that for a full HA scenario you could get on with three servers. You're back to two servers at failover time and need to rebuild the crashed master as a standby, running a base backup again.

If you've followed the idea, I hope you liked it! I still have to motivate some volunteers so that some work gets done here, as I'm probably not the one to ask to as far as coding this is concerned, if you want it out before 9.1 kicks in!


We also had a nice Hall Tack session with Jan Wieck, Marko Kreen and Jim Nasby about how to get a single general (enough) queueing solution for PostgreSQL. It happens that the Slony queueing ideas made their way into PGQ and that we'd want to add some more capabilities to this one.

What we talked about was adding more interfaces (event producers, event format translating at both ends of the pipe) and optimising how many events from the past we keep in the queue for the subscribers, in a cascading environment.

It seems that the basic architecture of the queue is what PGQ 3 provides already, so it could even be not that much of a hassle to get something working out of the ideas exchanged.

Of course, one of those ideas has been discussed at the Dev Meeting, it's about deriving the transaction commit order from the place which already has the information rather than reconstructing it after the fact. We'll see how it goes, but it started pretty well with a design mail thread.

Other talks

I went to some other talks too, of course, unfortunately with an attention span far from constant. Between the social events (you should read that as beer drinking evenings) and the hall tracks, more than once my brain were less present than my body in the talks. I won't risk into commenting them here, but overall it was very good: in about each talk, new ideas popped into my head. And I love that.

Conclusion: I'm addicted.

The social aspect of the conference has been very good too. Once more, a warm welcome from the people that are central to the project, and who are so easily available for a chat about any aspect of it! Or just for sharing a drink.

Meeting our users is very important too, and pgcon allows for that also. I've met some people I'm used to talk to via IRC, and it was good fun sharing a beer over there.

All in all, I'm very happy I made it to Ottawa despite the volcano activity, there's so much happening over there! Thanks to all the people who made it possible by either organizing the conference or attending to it! See you next year, I'm addicted...