Ok to use memcache in this way? or need a system re-architecture?

Question

Hi all,

I have a "score" i need to calculate for multiple items for multiple users. Each user has many many scores unique to them, and calculating can be time/processor intensive. (the slowness isn't on the database end). To deal with this, I'm making extensive use of memcached. Without memcache some pages would take 10 seconds to load! Memcache seems to work well because the scores are very small pieces of information, but take awhile to compute. I'm actually setting the key to never expire, and then I delete them on the occasional circumstances the score changes.

I'm entering a new phase on this product, and am considering re-architecting the whole thing. There seems to be a way I can calculate the values iteratively, and then store them in a local field. It'll be a bit similar to whats happening now, just the value updates will happen faster, and the cache will be in the real database, and managing it will be a bit more work (I think I'd still use memcache on top of that though).

if it matters, its all in python/django.

Is intending on the cache like this bad practice? is it ok? why? should I try and re-architect things?

Answer 1

If it ain't broke...don't fix it ;^) It seems your method is working, so I'd say stick with it. You might look at memcachedb (or tokyo cabinet) , which is a persistent version of memcache. This way, when the memcache machine crashes and reboots, it doesn't have to recalc all values.

Answer 2

You're applying several architectural patterns here, and each of them certainly has a place. There's not enough information here for me to evaluate whether your current solution needs rearchitecting or whether your ideas will work. It does seem likley to me that as your understanding of the user's requirements grows you may want to improve things.

As always, prototype, measure performance, consider the trade off between complexity and performance - you don't need to be as fast as possible, just fast enough.

Caching in various forms is often the key to good performance. The question here is whether there's merit in persisting the caclulated, cahced values. If they're stable over time then this is often an effective strategy. Whether to persist the cache or make space for them in your database schema will probably depend upon the access patterns. I there are various query paths then a carefully designed database scheme may be appropriate.

Answer 3

Rather than using memcached, try storing the computed score in the same place as your other data; this may be simpler and require fewer boxes.

Memcached is not necessarily the answer to everything; it's intended for systems which need to read-scale very highly. It sounds like in your case, it doesn't need to, it simply needs to be a bit more efficient.