Lately a discussion on the memcached-mailing list has started where for example the guys behind facebook.com and bloglines.com are participating and sharing some of their experiences. I’m don’t think this is rocket science, but I’d like to quote some of the things that are being said and provide some links to the relevant discussions.
About the general “would you want to bet your up time on memcached as an infrastructure component?”-question:
We consider memcached a critical part of our infrastructure. The benefit of memcached in a typical setup is to reduce the amount of database hardware you need to support an application; if you have enough database horsepower to run unimpaired with most of your memcached servers out of service, then there's probably no point using memcached at all, since it without a doubt adds extra complexity to your application code. [<a href="http://lists.danga.com/pipermail/memcached/2007-May/004105.html">link</a>]
If you shard all you data, etc. etc., is memcached still worth it?
*Question*: And you would split (federate) your database into 100 chunks (the remaining 100 would be hot spares of the first 100 and could even be used to serve reads), wouldn't that take care of all your database load needs and pretty much eliminate the need for memcache? Wouldn't 50 such boxes be enough in reality? *Answer*: Don't forget about latency. At Hi5 we cache entire user profiles that are composed of data from up to a dozen databases. Each page might need access to many profiles. Getting these from cache is about the only way you can achieve sub 500ms response times, even with the best DBs. [<a href="http://lists.danga.com/pipermail/memcached/2007-May/004112.html">link</a>]
Also, there is a lot of talk about a FUSE (File system in user space) file system based on top of memcached. Not only would that make caching available for those applications you do not control (black box) but it would have some really great advantages for your generic PHP app:
Over the last two weeks i spent a lot of time discussing a memcachefs (fuse-based) with two fellow geeks - applications that came to mind were (a) the smarty cache (b) php sessions; for both cases, losing files (as a whole, not random parts inside) is ok and readdir is irrelevant, which allows cutting a lot of corners. [<a href="http://lists.danga.com/pipermail/memcached/2007-May/004197.html">link</a>]