[openstack-dev] [oslo] memoizer aka cache

Shawn Hartsock hartsock at acm.org
Thu Jan 23 16:07:47 UTC 2014

I would like to have us adopt a memoizing caching library of some kind
for use with OpenStack projects. I have no strong preference at this
time and I would like suggestions on what to use.

I have seen a number of patches where people have begun to implement
their own caches in dictionaries. This typically confuses the code and
mixes issues of correctness and performance in code.

Here's an example:

We start with:

def my_thing_method(some_args):
    # do expensive work
    return value

... but a performance problem is detected... maybe the method is
called 15 times in 10 seconds but then not again for 5 minutes and the
return value can only logically change every minute or two... so we
end up with ...


def my_thing_method(some_args):
    key = key_from(some_args)
     if key in _GLOBAL_THING_CACHE:
         return _GLOBAL_THING_CACHE[key]
          # do expensive work
          _GLOBAL_THING_CACHE[key] = value
          return value

... which is all well and good... but now as a maintenance programmer
I need to comprehend the cache mechanism, when cached values are
invalidated, and if I need to debug the "do expensive work" part I
need to tease out some test that prevents the cache from being hit.
Plus I've introduced a new global variable. We love globals right?

I would like us to be able to say:

def my_thing_method(some_args):
    # do expensive work
    return value

... where we're clearly addressing the performance issue by
introducing a cache and limiting it's possible impact to 10 seconds
which allows for the idea that "do expensive work" has network calls
to systems that may change state outside of this Python process.

I'd like to see this done because I would like to have a place to
point developers to during reviews... to say: use "common/memoizer" or
use "Bob's awesome memoizer" because Bob has worked out all the cache
problems already and you can just use it instead of worrying about
introducing new bugs by building your own cache.

Does this make sense? I'd love to contribute something... but I wanted
to understand why this state of affairs has persisted for a number of
years... is there something I'm missing?

# Shawn.Hartsock - twitter: @hartsock - plus.google.com/+ShawnHartsock

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