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Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

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A Heavyweight Python Cache

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

Mods to Cache

From: andrew cooke <andrew@...>

Date: Sun, 10 Feb 2013 20:03:13 -0300

Since writing the above I've changed the cache slightly.

Expired values now remain in the cache (unless size becomes too large and they
are ejected by the LRU mechanism).  When the key to expired value is requested
the call is made and, if successful, the expried value is replaced.  But if
the call fails and the exceptin raised is "Fallback", then the expired value
is retained and returned as a result.

This is useful for web services and the like which may fail due to network
issues.  If those errors are caught and wrapped in "Fallback" then the
existign cache value is used, since it's better to use an old value than none
at all.

If the call raises an exception that is not a Fallback instance then that is
treated as a valid "result", cached (replacing the expired value) and
returned.  As before.

Andrew

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