Sums it up pretty good.
Only thing is, memcached is kinda outdated, it's not safe, can put very easily your environment at risk in case of crash. For anybody starting to wander in the world of caching with PHP, i'd recommend Redis, much better, safer and on an other planet when it comes to features available.
You're right that memcached does not have security features (though I believe they added basic authentication). In fact, it couldn't be any simpler in its design, it really has only one feature at all when it comes right down to it. This limits its usage, but its simplicity is also a big strength as long as programmers and sysadmins don't lose sight of its purpose.
I have not used Redis (yet), but from what I understand it really solves a different problem. It's a key-value store. Though Memcached data is also accessed by keys, it is not a store, it's a very fast cache with minimal overhead.
Redis solves the same problem, while offering way more features. In terms of speed, Redis is in the same area.
Memcached is also a key-value store (quote from the official site "Memcached is an in-memory key-value store for small chunks of arbitrary data"). I've been slowly using Redis more and more over the past 6 months, and as an heavy memcached user, i can tell you that i don't really see a need for both, might as well take advantage of the useful features Redis as to offer on top of the "basic" Memcached-like stuff.
Redis is persistent, replicated storage, isn't it? No matter what buzzwords they use on their site, Memcached is a volatile cache, it should never be used to store otherwise irretrievable information. I know, people use it for stuff like session storage, but that's really not a good use case for it.
I don't want to attack Redis, so there is no need to defend it. Maybe memcached-like behavior is one of its many features, but I personally like Memcached exactly because of its simplicity.
Since Redis keeps the dataset in RAM, it offers performance in the same class as memcached -- though memcached is significantly faster. There's probably a way to run it without the persistence (or just decrease the snapshot frequency), though you could just set it up to clear the tables when the application restarts and not worry about it. If you're looking for a more "modern" replacement, it's a good choice.
Thank you for clearing that up. Because I never used Redis before I wasn't sure, but my impression was also that it is intended for persistent storage and that there is a difference between a cache and a storage engine.
