Slack: route by the channel, govern by the organisation

How a workplace messaging platform works at Slack's scale: tens of millions of connected clients and messages delivered across the world in 500 ms; every message stored on its channel's shard before the channel server that owns it on a consistent-hash ring sends one copy per subscribed gateway server; one copy of a shared channel, with each side's own name and privacy; slim boots from the Flannel edge cache; Vitess keyspaces sharded by the key each query uses, and the 2-22-22 incident, where a scatter query was hiding behind a cache; Kafka in front of the Redis job queue for 1.4 billion jobs on the busiest days; notifications decided per member and per device; Solr search filtered by what each person can read; and the first working day of 2021, when a network hub didn't scale in time.