Are these applications using memcached for caching or for something else? I don't see the point in putting Cassandra in as a level 1 or 2 cache replacement? Especially given as it does not support any reasonable expiration policy that would be of use in those circumstances.
Ryan On Mon, Apr 5, 2010 at 1:08 PM, Paul Prescod <pres...@gmail.com> wrote: > On Mon, Apr 5, 2010 at 5:29 AM, Ryan Daum <r...@thimbleware.com> wrote: > > It seems pretty clear to me that the full memcached protocol is not > > appropriate for Cassandra. The question is whether some subset of it is > of > > any use to anybody. The only advantage I can see is that there are a > large > > number of clients out there that can speak it already; but any app that > is > > making extensive use of it is probably doing so in a way that would > preclude > > Cassandra+Jmemcached from being a "drop-in" addition. > > Here are a couple of example projects for info. > > Django: > > http://docs.djangoproject.com/en/dev/topics/cache/ > > It says of "increment/decrement": "incr()/decr() methods are not > guaranteed to be atomic. On those backends that support atomic > increment/decrement (most notably, the memcached backend), increment > and decrement operations will be atomic. However, if the backend > doesn't natively provide an increment/decrement operation, it will be > implemented using a two-step retrieve/update." > > add() is implied to be atomic. > > Django itself does use add() in exactly one line of code that I can > find. I believe it is just an optimization (don't bother saving this > object if it already exists) and is not semantically meaningful. In > fact, I don't believe that there is a code path to the add() call but > I'm really not investigating very deeply. > > Rails: > > > http://github.com/rails/rails/blob/master/actionpack/lib/action_controller/caching/actions.rb > > Here is the complete usage of the cache_store object in Rails. > > actionpack/lib/action_controller/caching/fragments.rb > 44: cache_store.write(key, content, options) > 55: result = cache_store.read(key, options) > 66: cache_store.exist?(key, options) > 94: cache_store.delete_matched(key, options) > 96: cache_store.delete(key, options) > > actionpack/lib/action_controller/caching.rb > 79: cache_store.fetch(ActiveSupport::Cache.expand_cache_key(key, > :controller), options, &block) > > Fetch is an abstraction on top of read. delete_matched is not > supported by the memcached plugin and not used by Rails. > > So as far as I can see, Rails only uses write, read, exist? and delete. > > It does expose more functions to the actual application, but the Rails > framework does not use them. Most of them (including > increment/decrement) are not even documented, and not supported with > most cache stores. > > * > http://api.rubyonrails.org/classes/ActiveSupport/Cache/Store.html#M001029 > > I checked a few of my own apps. They use get/set/add/delete, but the > add is almost always used as an optimization. > > Paul Prescod >
