memcached expiration time

Question

Memcached provides a cache expiration time option, which specifies how long objects are retained in the cache. Assuming all writes are through the cache I fail to understand why one would ever want to remove an object from the cache. In other words, if all write operations update the cache before the DB, then the cache can never contain a stale object, so why remove it?

One possible argument is that the cache will grow indefinitely if objects are never removed, but memcached allows you to specify a maximum size. Once this size is reached, memcached uses a least-recently-used (LRU) algorithm to determine which items to remove. To summarise, if a sensible maximum size has been configured, and all writes are through the cache, why would you want to expire objects after a certain length of time?

Thanks, Don

Answer 1

Expiration times are useful when you don't need precise information, you just want it to be accurate to within a certain time. So you cache your data for (say) five minutes. When the data is needed, check the cache. If it's there, use it. If not (because it expired), then go and compute the value anew.

Some cached values are based on a large set of data, and invalidating the cache or writing new values to it is impractical. This is often true of summary data, or data computed from a large set of original data.

Answer 2

I was curious about this myself, when I first started working with memcached. We asked friends who worked at hi5 and facebook (both heavy users of memcached).

They both said that they generally use something like a 3 hour default expire time as sort of a "just in case".

1. For most objects, it's not that expensive to rebuild them every 3 hours
2. On the off chance you've got some bug that causes things to stay cached that shouldn't otherwise, this can keep you from getting into too much trouble

So I guess the answer to the question "Why?" is really, "Why not?". It doesn't cost you much to have an expiration in there, and it will probably only help ensure you're not keeping stale data in the cache.