Lesson 2 of 4 · 40 min
Platform Events and the at-least-once contract
The publish gotchas, replay ids, and why every subscriber ends up idempotent.
Course content · 4 lessons
In this lesson
Request-reply fell over the moment billing went down for maintenance and took our transaction with it. This lesson decouples the two systems with a Platform Event — and then spends most of its time on the contract nobody reads: delivery is at least once, ordering is per-partition, and the 72-hour replay window is shorter than a long weekend plus a broken pager.
By the end the subscriber is idempotent, stores its replay id after every batch, and the demo deliberately delivers one event twice so you can watch the external-id upsert shrug it off.