How would you build a chat service that needed to handle 7.5 million concurrent players, 27 million daily players, 11K messages per second, and 1 billion events per server, per day?

What could generate so much traffic? A game of course. League of Legends. League of Legends is a team based game, a multiplayer online battle arena (MOBA), where two teams of five battle against each other to control a map and achieve objectives.

For teams to succeed communication is crucial. I learned that from Michal Ptaszek, in an interesting talk on Scaling League of Legends Chat to 70 million Players (slides) at the Strange Loop 2014 conference. Michal gave a good example of why multiplayer team games require good communication between players. Imagine a basketball game without the ability to call plays. It wouldn’t work. So that means chat is crucial. Chat is not a Wouldn’t It Be Nice feature.

Michal structures the talk in an interesting way, using as a template the expression: Make it work. Make it right. Make it fast.

Making it work meant starting with XMPP as a base for chat. WhatsApp followed the same strategy. Out of the box you get something that works and scales well...until the user count really jumps. To make it right and fast, like WhatsApp, League of Legends found themselves customizing the Erlang VM. Adding lots of monitoring capabilities and performance optimizations to remove the bottlenecks that kill performance at scale.

Perhaps the most interesting part of their chat architecture is the use of Riak’s CRDTs (commutative replicated data types) to achieve their goal of a shared nothing fueled massively linear horizontal scalability. CRDTs are still esoteric, so you may not have heard of them yet, but they are the next cool thing if you can make them work for you. It’s a different way of thinking about handling writes.

Let’s learn how League of Legends built their chat system to handle 70 millions players...

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