Event Sourcing Implementation

The Core Idea

Event Sourcing is a pattern for storing the state of a system as a sequence of events. Instead of persisting the current state, we record every change that has occurred – each event represents an action taken.

This approach allows us to reconstruct the system’s state at any point in time by replaying these events, providing a complete audit trail and enabling various read models for different use cases.

Store Events as Immutable Records

To maintain data integrity, utilize optimistic concurrency control when updating event records. This means assuming that no other process has modified the record since you last read it.

For large aggregates, create snapshots – periodic summaries of the state – to reduce storage requirements and improve query performance for historical views.

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Log All Events for Audit

Event Sourcing is particularly well-suited for systems requiring comprehensive audit trails, allowing you to reconstruct the system’s state at any point in time. It's also ideal for complex business rules or scenarios where different read models are needed.

However, it may not be the best choice for simple CRUD operations without intricate logic, as the event-driven approach can add complexity.

Frequently asked questions

Should errors within commands generate events?

Errors within commands should not trigger events; instead, throw exceptions. For projection errors, implement a dead-letter queue for failed messages, log the errors, and maintain a replay mechanism to reprocess them.

What is an Event Store – a specialized database for storing events?

An Event Store is a specialized database designed specifically for storing event streams. Popular solutions include EventStore DB, Apache Kafka (as an event log), AWS EventBridge, and custom implementations on PostgreSQL or MongoDB – the best choice depends on your performance, scalability, and feature requirements.

How should I test aggregates using given-when-then?

Use the given-when-then testing methodology: Given a set of events, when a command is executed, then verify the resulting events. Test projections by replaying events and validating the read model state – this ensures consistency across different views.

Are events in Event Sourcing typically immutable?

Yes, events in Event Sourcing are generally immutable and never deleted. For compliance purposes, you can implement event archiving (moving older events to a cold storage) or event encryption – particularly important for GDPR compliance, where you might archive PII data after a defined retention period.

▶ Try it live

Everything above runs in your browser — open Hash Function Avalanche Visualizer and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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