HLEN key

Storing hundreds of millions of simple key-value pairs in Redis

instagram-engineering.tumblr.com/post/12202313862/storing-hundreds-of-millions-of-simple-key-value

The Instagram Architecture Facebook Bought For A Cool Billion Dollars

http://highscalability.com/blog/2012/4/9/the-instagram-architecture-facebook-bought-for-a-cool-billio.html

The Architecture Twitter Uses To Deal With 150M Active Users, 300K QPS, A 22 MB/S Firehose, And Send Tweets In Under 5 Seconds

http://highscalability.com/blog/2013/7/8/the-architecture-twitter-uses-to-deal-with-150m-active-users.html

Using Redis as a Secondary Index for MySQL (sorted sets)

http://code.flickr.net/2013/03/26/using-redis-as-a-secondary-index-for-mysql/

Highly Available Real Time Push Notifications and You

http://code.flickr.net/2012/12/12/highly-available-real-time-notifications/