Event Sourcing: Storing and Reconstructing System State with Events
Tags: eventdriven, tutorial, discuss, aws
Published: 2023-06-28 19:00:00

Event Sourcing is a pattern used to persist and reconstruct the state of a system based on events, rather than by directly modifying the state. In this approach, all changes to the system's state are captured as a sequence of events, which are then stored in an event log. This allows the system to be reconstructed at any point in time by replaying the events in the log.

## How it works
In an event-sourced system, each change to the system's state is captured as an event object. Each event object contains all the information necessary to update the state of the system to reflect the change. Once an event is created, it is appended to the event log, which is an append-only data structure that records all events in the order they occurred.

When the system needs to be reconstructed, it reads all the events in the log and applies them in order to rebuild the state of the system. This allows the system to be reconstructed to any point in time by replaying events from the log up to that point.

## Benefits
Event sourcing offers several benefits over traditional data storage approaches:

- **Historical data**: Because all events are stored in the log, it is possible to view the entire history of the system's state at any point in time.
- **Auditing and compliance**: Because all changes to the system's state are recorded as events, it is easy to audit and track changes to the system, which can be useful for compliance purposes.
- **Scalability and performance**: Because the event log is an append-only data structure, it is highly scalable and can handle a large volume of events with high throughput.
- **Flexibility**: Because events contain all the information necessary to update the system's state, it is easy to evolve the system over time by adding, removing, or modifying events.

## Challenges
Event sourcing also comes with several challenges:

- **Complexity**: Event sourcing can be more complex to implement than traditional data storage approaches, especially when it comes to handling event versioning and evolution.
- **Data migration**: Because events contain all the information necessary to update the system's state, data migration can be more complex when using event sourcing.
- **Debugging and tracing**: Debugging and tracing can be more difficult in event-sourced systems, as the state of the system is reconstructed from events rather than directly from the data.

## Bonus Tip: Snapshots
Snapshots are a common optimization technique used in event sourcing systems. The idea is to periodically capture the state of an aggregate and store it as a snapshot. Then, when loading the aggregate, the system can start with the latest snapshot and apply only the events that occurred after it, reducing the number of events that need to be replayed. This can improve the performance of the system, especially for aggregates with a large number of events. The frequency of snapshotting should be carefully chosen based on the characteristics of the aggregate and the performance requirements of the system.

## What do you think?
Have you ever used Event Sourcing in your projects? What benefits and challenges did you experience?