Lesson 09 · 12 min
Concurrent changes: locks and upsert
Two buyers take the last item in the same millisecond. Who gets it?
When the app first reads a value and then writes a new one, two parallel requests can read the same thing and overwrite each other's changes — a lost update. Three techniques help: a change in one command with a condition (SET stock = stock - 1 WHERE stock > 0), locking the row with SELECT … FOR UPDATE inside a transaction, and INSERT … ON CONFLICT for "create or update".
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