Unraveling Event Sourcing: A Pattern for Resilient and Scalable Systems published 8/28/2023 | 3 min read

Unraveling Event Sourcing: The Key to Resilient and Scalable Systems

In an ever-evolving technology landscape, software developers continually face the challenge of building complex systems that not only meet the requirements today but are also resilient and adaptable to future changes. To meet this challenge, developers are exploring and introducing innovative architectural patterns. One such pattern that has gained increasing popularity lately is Event Sourcing (ES). Let's delve deeper into understanding ES and how it can help us design resilient and scalable applications.

What is Event Sourcing?

Event sourcing, at its core, revolves around the concept of capturing changes in the state of an application as a sequence of events. Instead of updating the current state of data in our system directly, we record all changes as immutable events over time. Then, we can reconstitute the current state of an entity by replaying these events.

The classic Todo application can give us a simple example. Usually, a Todo item might have fields 'task' as String and 'completed' as Boolean. When the user marks the task as completed, we usually just update the 'completed' field. In event sourcing, we would create an event called 'TaskCompleted' and append it to our event-store. The current state is derived by playing all related events, here, 'TaskCreated' and 'TaskCompleted' events.

  
// Traditional way
todo = { task: "Clean room", completed: true }

// Event Sourced way
events = [
  { type: "TaskCreated", data: { task: "Clean room" } },
  { type: "TaskCompleted", data: {} },
];

currentState = events.reduce(reducer, {}) // reducer will handle the state changes

// currentState will be { task: "Clean room", completed: true }

Benefits of Event Sourcing

Using event sourcing can lead to numerous long-term benefits. Let's take a look at some of them:

Audit Logging: As we persist every change as an event, we inherently build an audit log of all state changes happening to entities in our system, which can also be helpful in debriefing and debugging.
Temporal Querying: Event sourcing lets us travel through time. It allows us to query the state of an entity at any point in history, which can be tremendously helpful in debugging.
Replay Events: We can correct errors in system logic, replaying events through corrected logic.

Challenges with Event Sourcing

While event sourcing brings numerous striking benefits, it's also accompanied by specific challenges:

Event Schema Evolution: As the business needs evolve, so do the data associated with events. This can lead to complex migrations and increase overall system complexity.
Performance: Rebuilding the current state can be expensive as it involves replaying all the events.

Conclusion

Event Sourcing can be a powerful design pattern, especially in systems where accurate history is critical and temporal querying is needed. However, it's not a silver-bullet solution and like any architecture pattern, it needs to be carefully considered based on the specific needs of the project.

As software developers, it's crucial for us to keep up-to-date with current practices and architectural patterns, understand their intended use, and apply them wisely in designing our applications to make them more resilient, scalable, and maintainable.