Rebuilding projections in Axon Framework 3

CQRS (Command Query Responsibility Segregation) allows you to have separate models for reading and writing. Combining that pattern with Event Sourcing leads to a powerful capability: updating query (read) models, based on events. In real-time or rebuilding them from an existing collection of events. This post focuses on such projections, in applications that are built on Axon Framework.

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Upcasters or a versioned event store: pros and cons

In a previous article, I wrote a few things about upcasters. One of the significant downsides when implementing an upcaster is that it adds to our application’s technical debt. An alternative technique is the versioned event store (or versioned event stream), where the existing event store is copied and modified. In this post I’ll discuss the pros and cons of both approaches.

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Forget me please? Event sourcing and the GDPR

In May 2018, a new piece of EU legislation called the General Data Protection Regulation (GDPR) will come into effect. The GDPR attempts to regulate data protection for individuals within the EU and has very interesting and specific implications for applications that use event sourcing. In this article, I’ll discuss my thoughts on this subject and a few pointers for those implications.

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Using Tracking processors to replay events in Axon Framework 3

Replaying events is a crucial part in any event sourcing / cqrs application, to rebuild projections, generate new ones or seed external systems with data.

I’m a big fan of the Axon Framework. Even with its quirks and occasional (strange) bugs, it’s my go-to toolbox for my event sourcing & cqrs consulting and development work.

With the recent 3.0 release, Axon changed the way events can be replayed by introducing the Subscribing and Tracking event processors. The Subscribing processor follows the event stream in real-time, whereas the Tracking processor keeps track of events it has processed (using a token). This means that the Tracking processor can be stopped and resumed, and it will pick up processing where it left off.

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Event versioning (or why I sometimes modify the event store)

Requirements and applications evolve and change, leading to refactoring. For example, user registration now requires a first and last name, what was once an error may no longer be considered one, etc. In an event sourced application, that poses a few problems. In this post I’ll discuss a few strategies and my views on event versioning.

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