Scaling Elixir at Discord for Massive Online User Loads

In this thorough blog post, Yuliy Pisetsky from Discord's engineering team outlines the technical challenges and strategies involved in scaling Discord's Elixir-based systems. The server architecture, consisting of individual Elixir processes per guild and session, needed to efficiently fan out updates like messages or voice channel joins to potentially millions of concurrent users. As Discord's user base and activity levels grew dramatically, the team tackled performance issues that scaled quadratically with user count. They implemented a range of optimizations, including passive sessions to reduce workload, and a relay system to distribute fan-out tasks across multiple machines, improving throughput limits. They also employed worker processes and ETS (Erlang Term Storage) to maintain server responsiveness during heavy operations. Additionally, the blog post discusses the importance of detailed system performance analysis and the continuous monitoring and adjustment to address new bottlenecks as they arise. The article serves as an insightful case study on applying performance tuning and system design principles within the context of a massively multi-user platform using Elixir.