On May 12, 12:56 pm, Henrik Schröder <[email protected]> wrote:
> If you went with the application cache in your case, how would you handle
> cache invalidations? Would you have to develop a mechanism to tell all of
> your front end machines that a certain key has expired, or isn't your
> application affected if it gets stale data from the cache? Yes, memcached is
> slower than a local application cache, but it's synchronized which means
> that if one of your front end machines update or delete a key, that change
> is visible immediately to your entire application. To compare a local
> application cache with memcahed, you would have to factor in both the
> network overhead of fetching something from memcached, and the network
> overhead of synchronizing your local application caches. The latter case
> scales really, really badly if you have a lot of front end machines.
>
> /Henrik
If we went with a local cache, it wouldn't be shared between machines
- in reality, a lot of the front end machines would end up with almost
'identical' caches (as they often run the same queries) - but it
wouldn't be shared.
Expiry would be the same as memcached - when data's added to the local
cache, it gets an 'expire by' time stored with it.
By the sound of it, memcached will work - it'll cost me network calls
(but not to the MySQL server) - the only remaining doubt I have is
over the data being stored in it.
Not a lot of our data can be agregated before being cached - so we're
left again, looking in the cache for very small bits of data (a few
bytes to a few hundred bytes).
I'll have to sit down and work out where in the application it's
likely to sit, and whether the cost of the network calls etc. to get a
few bytes, is actually worth it.
I guess having a 'localhost' cache would avoid some of the overhead,
as would UDP.
I have a feeling this is all going to end in a 'write it and see' kind
of solution :)
At least in my head it seems like it might play out - afterall, even
if the SQL queries usually return very little data - some of them are
quite expensive for the server to run, and very static in nature - so
we're trading cycles on the front end (making it less efficient) to
push that load to the front end, rather than the MySQL servers.
