Cheap Caching with AutoDestruct
I’ve seen a couple references lately to using lazy attributes as a form of caching. This is a great approach to thinking about lazy attributes, as they share a number of characteristics with traditional caching: you only have to build a (potentially) expensive value once, and then only when you actually need it.
But what about when that lazily generated value is too old to trust?
A lazy attribute isn’t going to help you much then, as your instance is quite happy to keep on returning the same value forever once it has been built, unless you clear or change it manually. This is no good when, say, you’ve run a database query and you can really only expect your painfully contorted query to get the twitter ids of all the left handed Justin Beiber fans north of the Mason-Dixon line who own hypo-allergenic cats to be valid for, oh, say 55 minutes or so.
You could add an attribute to store the age of the value generated for the lazy attribute and check it either manually (boring!), or by wrapping the reader method (less boring, but still, unsightly).
Ok, method modifiers can be fun, but still… That’s a lot of annoying little code that, well, isn’t Moose there to help reduce that sort of code in our lives?
What we’re running into here is that while we implement one part of a cache (generate once, return many), lazy attributes don’t have any internal logic to determine when a value is no longer good. They don’t even have any concept of that, just “someone needed my value, so we’re going to get it and hang on to it until told otherwise”.
This is just the sort of behaviour an attribute trait can alter.
The MooseX::AutoDestruct Moose attribute trait allows us to specify an expiration date for our stored values. We can specify a time-to-live option at attribute creation, and then every time a value is set, the set time is stored. Every time the value is accessed, the attribute checks to make sure the value isn’t older than the set time to live, and if it is, clears the value. This allows the lazy value generation to kick in once more, without requiring any extra effort on the part of the user – just as one would expect.