Book: Event-Driven Architecture Pocket Guide My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub Picture the runbook entry every team eventually writes. It is 3 AM. PagerDuty fires. A consumer that ingests order.paid events from Kafka has just replayed the last several hours of a partition because a rebalance flipped the committed offset back to a stale checkpoint. Stripe webhooks downstream of that consumer fire charges again. A few hundred customers get billed twice. The on-call dev opens the runbook and finds a section called "Idempotency" with three bullet points and no code. This is the post that section should have been. The pattern is small. Maybe sixty lines if you count the table. Teams skip it because Kafka says "exactly-once semantics" on the box and engineers believe the box.…