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Notes / Docker Sandboxes / Change Data Capture (CDC) with PostgreSQL WAL Logical Decoding
Docker Sandboxes · Tier 4: Architect & Chaos · Lab 54

Change Data Capture (CDC) with PostgreSQL WAL Logical Decoding

Extract real-time change events directly from the PostgreSQL Write-Ahead Log without polling. Use logical decoding replication slots to capture INSERT, UPDATE, and DELETE mutations, see why aborted transactions never leak, and drain the LSN stream safely.

Time30 minutes
StackPostgreSQL 16Ports 15452
MemoryAbout 90 MB
Before thisLab 23PostgreSQL Streaming Replication L
Download lab (.zip) View steps
Verified runWindows, Mac, LinuxBash and PowerShell
  1. 1PrepareWhat you will learn
  2. 2PredictGuess before you run
  3. 3RunStep by step
  4. 4ObserveWhat you should see
  5. 5Break itChange one thing
  6. 6Clean upLeave nothing running
On this page
  1. 00 / What You Will Learn
  2. 01 / Why It Matters
  3. 02 / What We Are Building
  4. 03 / Words You Need
  5. 04 / Predict First
  6. 05 / Step by Step
  7. 06 / What You Should See
  8. 07 / Look Inside
  9. 08 / Break It, Fix It
  10. 09 / In Real Projects
  11. 10 / Interview Corner
  12. 11 / Clean Up

00 /What You Will Learn

By the end you will be able to
  • Explain how Change Data Capture (CDC) eliminates dual-write race conditions and database polling.
  • Understand what the Write-Ahead Log (WAL) is and how logical decoding turns binary log records into structured events.
  • Create and manage a logical replication slot using test_decoding.
  • Verify why aborted transactions never emit events into the CDC stream.
  • Distinguish between peeking changes (pg_logical_slot_peek_changes) and consuming changes (pg_logical_slot_get_changes).
  • Avoid the dangerous WAL disk bloat trap caused by unconsumed or abandoned replication slots.

Before you start

Finish Lab 04 (SQL Basics) and Lab 05 (SQL Transactions and Isolation) first. You should know BEGIN, COMMIT, and ROLLBACK. Docker Desktop and Python 3 are needed.