Scaling Slack and GitHub: Maude Lemaire on Backend Performance and Database Sharding

Maude Lemaire, principal engineer and author of "Refactoring at Scale," shares an insider view of scaling Slack and GitHub on The Pragmatic Engineer podcast. She spent nearly eight years leading Slack's backend performance infrastructure team, then joined GitHub in 2024 to tackle database scaling, and most recently worked on performance at Cursor, including time to first token. She explains Slack's 23-custom-emoji limit to avoid thundering herds, how load testing 500,000 WebSocket connections the naive way would have cost $150,000 per run, and why GitHub's Active Record abstraction and scatter queries capped it at 16 database shards.