Monday, May 24, 2010

An Excellent Developer Meeting

I'm really pretty fired up about the results of our PGCon 2010 Developer Meeting. Of course, the list of what everyone plans to work on is pretty interesting, and if we got even a fraction of those features we'd have a pretty awesome release. But that's not really what got me fired up. What I'm excited about is some of the new and innovative thinking on replication and clustering - or, at any rate, it was new to me.

Two concepts in particular stand out for me. First, we discussed the ability to give replication solutions a crash-recoverable view into transaction commit order, a point which Jan Wieck has since expanded into a full-blown design proposal. Jan believes that this infrastructure will be useful not only for his own project, Slony, but also for other replication solutions such as Londiste which also operate by replaying transactions in commit order. As I understand it, one of the major advantages of this approach is that it eliminates the need for a global shared counter to track the order of writes (which becomes a bottleneck). Instead, they can be tagged with their order within the top-level transaction, and then the transactions as a whole can be ordered using the transaction commit ordering information.

Second, I was very interested in our discussion of a global transaction manager, for which I unfortunately do not have a good link for further reading. One possible way of avoiding cross-node serialization anomalies in a distributed database environment is to have a single node which knows about all in-flight transactions and hands out snapshots that are coherent across the entire cluster. Postgres-XC takes this approach, but there might be value in integrating something like this into core PostgreSQL. We might imagine allowing one PostgreSQL instance to be configured as a "snapshot provider" and another instance to subscribe to it. Right now, it's not clear that there's enough benefit to core PostgreSQL from accepting a patch along these lines, but there are several ways that might change as our distributed computing capabilities improve. For example, if we had a significant SQL/MED implementation, we'd need to think about how to do serialization correctly across multiple nodes; there might also be applications as we work to expand the capabilities of Hot Standby.

If your eyes are glazing over at this point, you're probably not alone. These features are fairly esoteric. Still, I think the fact that we're starting to seriously talk about this topics and consider integrating some of them into core shows that we're starting to understand better what the real needs are for replication and clustering. As our understanding of those needs continues to improve, I expect to see more capabilities in core PostgreSQL, but perhaps even more importantly, an even stronger set of tools around core PostgreSQL that will make it progressively easier to scale horizontally. I don't expect this to happen overnight, but I feel like we're moving in the right direction.

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