Dual-Write to Outbox + CDC

Concepts covered: paDualWrite, paTransactionalOutbox, paCdc

The synthesis exercise is a real-shaped migration. A logistics company runs an order service backed by Postgres. The service publishes order_placed, order_shipped, and order_delivered events to a downstream warehouse, a recommendation system, and a customer notification service. Three years ago the team wired this up with the dual write inside each handler. The reconciliation job catches roughly 0.4% of events that drift, but enough leak through to drive customer complaints. The new principal engineer is asked to replace the dual write end to end without an outage. The Symptoms in Production The Target Architecture The order service writes to one durable medium. The outbox table holds a row per event the service intends to publish. Debezium reads the WAL and surfaces inserts on the outbox

About This Interactive Section

This section is part of the Ingestion Patterns: Advanced lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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