Copy That | System Design Music Video (Replication)

What happens to your data when the machine holding it dies? This is a song about replication: keeping copies of the same data on several machines. Why we keep copies (survive failures, sit near users, spread the reads), why one leader decides the order of every write, synchronous vs asynchronous followers (and how an "OK" can still lose your write), replication lag and the reads it breaks, and failover: promoting a new leader, split brain, and the writes that never made it. Every frame is code-rendered and synced to the lyrics. Part of the System Design Music Videos series. Chapters 0:00 One copy is one bad day away 0:25 Your comment on one machine, then on three 0:40 Copies near users, and who decides the order 0:57 Chorus: every write goes to the leader 1:16 Synchronous: safe before you're told 1:31 Asynchronous: fast, but a confirmed write can vanish 1:50 Chorus 2:10 Replication lag 2:24 Read your own writes, stick to one copy 2:43 Failover: promote the follower furthest along 2:57 Split brain and lost writes 3:15 Final chorus: the new leader 3:33 Every follower's a little in the past Concepts follow chapter 6 of Designing Data-Intensive Applications (2nd edition) by Martin Kleppmann and Chris Riccomini. Highly recommended. James Reinhardt | software engineer writing about operating systems, data systems and the craft of building software.