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Debezium Blog

SQL Server offers change data capture (CDC) through change tables - a special set of system tables that record modifications in selected “ordinary” tables. If you want to monitor changes in real time, you query these change tables periodically. That’s exactly how Debezium works today: it polls SQL Server’s change tables at configured intervals and turns the results into a continuous stream of CDC records. This approach works fine, but could we do better?

Captured tables are filled by the SQL Server Agent, which reads the transaction log, extracts changes, and stores them in change tables. In theory, we could skip the middleman and parse the transaction log directly. That’s how tools like OpenLogReplicator handle CDC for Oracle databases. Let’s peek inside the SQL Server internals and explore little bit how it works and stores the records.

In this post, we’ll:

  • Prepare a local SQL Server instance for experimentation

  • Explore the internal structure of the SQL Server transaction log

  • Understand how the records are stored on the disk

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