C[omp]ute

From: andrew cooke <andrew@...>

Date: Wed, 6 Feb 2013 19:12:49 -0300

Calling EchoNest (see previous post) is expensive.  So I have written a very
general cache, backed by an SQLAlchemy database.

The code is at
https://github.com/andrewcooke/uykfg/tree/master/src/uykfg/support/cache

It has the following features:

 * Transparently wraps function calls (should only be used with pure
   functions, obviously!)

 * Can be used with multiple functions (called "owners" in the database
   schema).

 * Caches both values and exceptions.

 * Has separate expiry lifetimes for values and expcetions (typically
   exception lifetimes are shorter).

 * Uses fuzzy expiry lifetime to avoid cache stampede (+/-50%)

 * Supports LRU rejection when size exceeds some limit (data are stored
   as compressed binary blobs in the database)

 * Size and lifetimes can be specified separately for each owner

The main downside is that it is pretty heavyweight.  It's a no-brainer for web
calls, but you wouldn't want to use it to cache values that can be
re-calculated locally.

Also, it cannot cache values or exceptions that cannot be "pickled".

Andrew