2 July 2013
Advisory locks are a very useful feature in PostgreSQL, and they just aren’t used enough.
Here’s a scenario: You have a bulk import job. While that job is running, there’s an analysis job you want to prevent from starting, and you don’t want a bulk import to start while the analysis job is running. But any number of bulk importers can run at the same time. How do you communicate this?
With an advisory lock!
Each of the bulk importers can take a shared advisory lock. Those locks don’t block each other, so they can run freely. But the analysis job takes an exclusive advisory lock of the same type. It will wait until all the importers are done, and the importers will not be issued their shared locks until it completes.
Even better, PostgreSQL cleans them up for you when a session terminates; you don’t have to worry about a lock lingering when you didn’t mean it to.
To make using advisory locks easier in Django, I have a small context manager than can be used to wrap code that should run with an advisory lock held; it makes it much easier to use them in your application. You can find it on GitHub, and it installs using