Memcached patch: Item expiration signal to client and extension

I work as a developer in Metacafe. We use memcached as our caching system, and have been using it for a few years now with great results. A while ago, we thought of a neat little concept that made working with memcached much more convenient for us. We patched the memcached code in-house, and have been using this patched code for a while now. We are now offering the concept (and the source code) to the project.

The idea is as follows: currently, whenever an item which has expired is requested from a memcached server, the server immediately unlinks (deletes) the item internally and returns an empty (or null) item to the client. We propose, instead, returning an empty item to the client and extending the expiration time of the item by X seconds (we use 60 seconds), thus returning the expired item to all clients who ask for it in the next X seconds. This behavior repeats itself, after X seconds, and so forth. This behavior should be controlled by a command-line argument, and is off by default.

The benefit is that the client which receives the empty item can refresh the item however it deems fit. There is no race situation and no database “stampede”. The client can access a database and create a new item, storing it to server, knowing it is not racing another client. (Of course, if the server has just been launched or if the item is brand new – there might be a race, but this idea is not an attempt to fix this different issue.)

More information and the patch can be found here

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