Nice combination of Redis and MySQL:
Redis has to keep all stored objects in memory, so just putting all data in there and forgetting about it was out of the question. We decided to only keep a few days of data in Redis and archive the results to MySQL. Daily API usage stats would be served directly by Redis, archived results on date ranges would be fetched from MySQL.
Note also what correct Redis data modeling means: usage of Redis data structures combined with smart keys (nb smart in the sense of keys carrying additional meta-information).