MVCC in the Cloud
At CHAR(10) Markus had a talk about Using MVCC for Clustered Database Systems and explained how Postgres-R does it. The scope of his project is to maintain a set of database servers in the same state, eventually.
Now, what does it mean to get “In the Cloud”? Well there are more than one answer I’m sure, mine would insist on including this “Elasticity” bit. What I mean here is that it’d be great to be able to add or lose nodes and stay online. Granted, that what’s Postgres-R is providing. Does that make it ready for the “Cloud”? Well it happens so that I don’t think so.
Once you have elasticity, you also want
scalability. That could mean lots of
Postgres-R already provides a great deal of it, at the connect
and reads level: you can do your business
unlimited on any node, the others
will eventually (
eagerly) catch-up, and you can do your
select on any node
too, reading from the same data set. Eventually.
What’s still missing here is the hard sell, write scalability. This is the idea that you don’t want to sustain the same write load on all the members of the “Cloud cluster”. It happens that I have some idea about how to go on this, and this time I’ve been trying to write them down. You might be interested into the MVCC in the Cloud part of my Next Generation PostgreSQL notes.
My opinion is that if you want to distribute the data, this is a problem that falls in the category of finding the data on disk. This problem is already solved in the executor, it knows which operating system level file to open and where to seek inside that in order to find a row value for a given relation. So it should be possible to teach it that some relation’s storage ain’t local, to get the data it needs to communicate to another PostgreSQL instance.
I would call that a remote tablespace. It allows for distributing both the data and their processing, which could happen in parallel. Of course that means there’s now some latency concerns, and that some JOIN will get slow if you need to retrieve the data from the network each time. For that what I’m thinking about is the possibility to manage a local copy of a remote tablespace, which would be a mirror tablespace. But that’s for another blog post.
Oh, if that makes you think a lot of SQL/MED, that would mean I did a good enough job at explaining the idea. The main difference though would be to ensure transaction boundaries over the local and remote data: it’s one single distributed database we’re talking about here.