League of Legends' chat service architecture

League of Legends players collectively send millions of messages every day. They're asking friends to duo-queue, suggesting a team comp on the champ select screen, and thanking opponents for a good game. On July 21st of this year (I picked a day at random), players forged 1.7 million new friendships in the game - that's a lot of love! And each time players send a message they trigger a number of operations on the back-end technology that powers Riot chat.In the previous episode of this series on chat, I discussed the protocol we chose to communicate between client and server: XMPP (Extensible Messaging and Presence Protocol). Today I'll dive into the mechanisms in place on the server-side and the architecture of the infrastructure, and Iill discuss the work weive done to ensure that our servers are scalable and robust. Like the last article, I hope itill be interesting to anyone building out chat features to a distributed client base.