Why bank-account aggregates are risky

Shawn McCool returns to the podcast to explore event sourcing - a pattern that fundamentally changed how he thinks about modeling software systems.

We start by tracing Shawn’s journey into event sourcing, beginning with his work on Laravel.io around 2012 when a friend introduced him to domain events. This discovery led him deeper into the DDD community and eventually to creating Event Sourcery, a free video course on domain modeling and event sourcing. Shawn shares how working with event sourcing broke him out of the “way we’ve always done it” mindset.

The conversation covers the core concepts - what event sourcing actually means (using events as the source of your model state), how it differs from event streaming and event-driven architectures, and when you might choose this approach. We explore the relationship between event sourcing and CQRS, discussing how separating read and write models enables independent evolution and scaling, while acknowledging the trade-offs around eventual consistency.

Topics include:

• What event sourcing means: events as the core of your model state
• The distinction between event sourcing, event streaming, and event-driven architectures
• Domain events versus CRUD: capturing intent and context
• CQRS: separating decision-making models from read models
• Eventual consistency: trade-offs between reliability and immediate feedback
• Synchronous versus asynchronous projections
• Why DDD isn’t a specific methodology but a pursuit of understanding
• Aggregates as lifecycles, not nouns - designing for short-lived event streams
• The bank account anti-pattern: why long-running aggregates are problematic
• Testing event sourced systems: given-when-then with events
• Projections as pure, side-effect-free transformations
• Versioning strategies and avoiding the pain of long-lived aggregates
• GDPR and data retention: designing systems that don’t need to keep data forever
• Implementation approaches: relational databases versus dedicated event stores
• Rolling your own versus using frameworks like EventSauce or Marten

Shawn emphasizes how building systems around short-lived aggregates - where event streams naturally terminate - eliminates entire classes of versioning and data retention problems. He shares insights from his work at a payment processing company, where the system is designed so that event streams exhaust within 30 minutes, making evolution and versioning remarkably straightforward.

Throughout the conversation, Shawn demonstrates how stepping away from noun-centric thinking and embracing events as the core building block leads to systems that are easier to understand, test, and evolve.

Show Links

• Shawn McCool’s Website
• Shawn McCool on X/Twitter
• Shawn McCool on LinkedIn
• Shawn McCool on GitHub
• Event Sourcery
• Laravel.io Community Portal
• Domain-Driven Design: Tackling Complexity in the Heart of Software by Eric Evans (Blue Book)
• Implementing Domain-Driven Design by Vaughn Vernon (Red Book)
• Versioning in an Event Sourced System by Greg Young
• Building on the BEAM: Exploring Erlang and Elixir, Part 1 with Shawn McCool
• Building on the BEAM: Exploring Erlang and Elixir, Part 2 with Shawn McCool
• CQRS Pattern
• Event Sourcing Pattern
• EventStorming
• EventSauce - PHP Event Sourcing Library
• Marten - .NET Event Store on PostgreSQL
• Kurrent (formerly Event Store DB)
• Shuhari - Japanese Training Philosophy
• Temporal Modelling by Mathias Verraes
• Active Record: How We Got Persistence Perfectly Wrong