Outbox Pattern

Opening Scenario

Your orders-service owns a Postgres orders table. When a customer places an order, the service does two things in its request handler: INSERT INTO orders ... and kafkaProducer.send("order-placed", event). The bug ticket reads: "We charged a customer twice and shipped nothing." Investigation finds three production incidents in two weeks: an order row was committed but the Kafka send timed out (no event, downstream services didn't react); an order row failed to commit but the Kafka send succeeded (event published, no row, downstream charged a phantom order); a Kafka send succeeded but the response was lost on the network so the service retried, double-publishing. The team lead writes "USE OUTBOX" on the postmortem in red. Before nodding, you need to understand what outbox actually fixes, what it costs, and what alternatives lose against it.

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Question Progression

1. What's the actual problem with "DB write + Kafka send"?

Question. Why isn't INSERT followed by producer.send a correct way to publish an…