Domain Events Versus Change Data Capture

The building of change data capture (CDC) and event based systems have recently come up several time in my discussions with people and in my online trawling. I sensed enough confusion around them that I figured this was worth talking about here.

CDC and event-based communication are two very different things which look similar to some extent, hence the confusion. Beware — confusing one for the other can lead to very difficult architectural situations.

What Are These Things?

Change Data Capture (CDC) typically alludes to a mechanism for capturing all changes happening to a system's data. The need for such a system is not difficult to imagine — audit for sensitive information, data replication across multiple DB instances or data centers, moving changes from transactional databases to data lakes/OLAP stores. Transaction management in ACID compliant databases is essentially CDC. A CDC system is a record of every change every made to an entity and the metadata of that change (changed by, change time etc).

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I have written about events on this blog before and have described them as announcements of something that has happened in the system domain, with relevant data about that occurrence. At a glance, this might seem to be the same as CDC — something changes in a system and this needs to be communicated to other systems — which is exactly what CDC is about.

However, there is a key distinction to be made here. Events are defined at a far higher level of abstraction than data changes because they are meaningful changes to the domain. Data representing an entity can change without it having any "business" impact on the overall entity that the data represents. There can be several sub-states of an order that an order management system might maintain internally but which do not matter to the outside world.

An order moving to these states would not generate events but changes would be logged in the CDC system. On the other hand, there are be states that the rest of the world cares about (created, dispatched etc) and the order management system explicitly exposes to the outside world. Changes to or from these states would generate events.