I have very little evidence to support this, but they probably use twemproxy. twemproxy is a proxy for redis that automatically manages routing commands to multiple redis servers efficiently.
They definitely don't use redis cluster, because redis cluster requires explicit client-side support. That support is only available in a limited number of clients today.
Oh, weird in what way? I was considering using it for horizontal redis scaling recently, under the impression that redis cluster was further away from release than it seems to be.
The code is just strange. To support Redis, it has a 2,500 line Redis adapter. It has special cases for each Redis command because it doesn't extract key position information from Redis directly.
Plus, despite the name and where it's hosted, Twitter never used it in production for Redis. Redis support was just added "because we can."
It probably works for very direct use cases, but it's one of those things that could probably bite you very severely under edge cases, and you'd be left on your own to figure out what happened.
Well, companies also use ejabberd, but that doesn't make ejabberd a valid piece of software either.
Then there's the massive problem of "everyone" using MongoDB and we know how much pain _that_ sucker causes. Playing popularity contests with software isn't useful. People install software either because of social reasons ("I don't know how anything works, but I hear everybody is using X, so I'm going to use X too!") or because they need the functionality ("redis is AMAZING at 30 different things!").
If install base at well known companies were a valid measure, we'd all be using Windows laptops while talking into our Kin and squirting our Zunes all over each other.
Never used it, but as far as I know twemproxy does not support the select command to select another Redis database on the current connection, which would be a good indicator of it being in use at Amazon or not.
They definitely don't use redis cluster, because redis cluster requires explicit client-side support. That support is only available in a limited number of clients today.