Event Sourcing Pattern

1. Introduction

The Event Sourcing Pattern is a design pattern that involves storing the state of a system as a sequence of events. Instead of saving the current state, all changes (events) to the state are recorded, allowing for reconstruction of the state at any point in time.

2. Key Concepts

• Event: A record of a change that has occurred in the system.
• Event Store: A storage mechanism where events are persisted.
• Aggregate: A cluster of domain objects that can be treated as a single unit.
• Snapshot: A saved state of an aggregate to improve performance by reducing the number of events needed to reconstruct the state.

3. Implementation

To implement the Event Sourcing Pattern, follow these steps:

1. Define the events that can occur in your domain.
2. Create an event store to persist these events.
3. Implement an aggregate that applies events to update its state.
4. Allow the application to reconstruct the current state by replaying events.

class Account {
    private List changes = new ArrayList<>();
    private double balance;

    public void deposit(double amount) {
        balance += amount;
        changes.add(new DepositedEvent(amount));
    }

    public void withdraw(double amount) {
        balance -= amount;
        changes.add(new WithdrewEvent(amount));
    }

    public double getBalance() {
        return balance;
    }

    public List getChanges() {
        return changes;
    }
}
            

4. Best Practices

• Ensure events are immutable.
• Design events to be descriptive and include all necessary information.
• Implement versioning for events to handle changes in event structure over time.
• Consider using snapshots to improve performance.

5. FAQ

Flowchart

graph TD;
    A[Start] --> B{Event Occurs?};
    B -- Yes --> C[Store Event];
    B -- No --> D[No Action];
    C --> E[Update State];
    E --> F[End];