Slack and GitHub scaling war stories: why Slack caps custom emojis at 23
The Pragmatic Engineer · rss · 2026-11-05
The Pragmatic Engineer interviews Maude Lemaire (author of Refactoring at Scale; 8 years leading backend performance at Slack, then database scaling at GitHub, now at Cursor) on scaling the world's largest software products.
- Slack limits custom emojis to 23 to avoid thundering herds: in a 100k-member workspace each emoji fans out to every user; event floods similarly crush power users logged into many busy channels
- Load-testing 500k WebSocket connections via headless Chrome would have cost $150k per run; she built Koi Pond, a Go tool simulating slimmed-down Slack clients
- GitHub's Rails monolith: the homepage triggered 100 SQL queries across three clusters; Active Record hid scatter queries that capped GitHub at 16 database shards
- For foundational work she personally verifies LLM output — "ambitious structures can only be built on stable foundations"; she advocates rotating platform and product engineers
- Episode sponsors include Entire, which reinvented the pull request as "Trails" for agentic development
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