This is a really good article where they talk about REDIS and ElasticSearch.
http://highscalability.com/blog/2014/1/6/how-hipchat-stores-and-indexes-billions-of-messages-using-el.html
Stats
60 messages per second.
1.2 Billion documents stored
4TB of EBS Raid
8 ElasticSearch servers on AWS
26 front end proxy serves. Double that in backend app servers.
18 people
.5 terabytes of search data.
Platform
Hosting: AWS EC2 East with 75 Instance currently all Ubuntu 12.04 LTS
Database: CouchDB currently for Chat History, transitioning to ElasticSearch. MySQL-RDS for everything else
Caching: Redis
Search: ElasticSearch
Queue/Workers server: Gearman (queue) and Curler, (worker)
Language: Twisted Python (XMPP Server) and PHP (Web front end)
System Configure: Open Source Chef + Fabric
Code Deployment: Capistrano
Monitoring: Sensu and monit pumping alerts to Pagerduty
Graphing: statsd + Graphite
Read more at: http://highscalability.com/blog/2014/1/6/how-hipchat-stores-and-indexes-billions-of-messages-using-el.html