What is the Event Sourcing architecture pattern?

Event Sourcing is an architectural pattern that involves capturing all changes to an application's state as a sequence of events. This approach differs from traditional data storage models where application state is updated in place. In event sourcing, every change to the application’s state is recorded as a separate event. These events are stored in an event log (or a write-ahead log) to represent the system's state at any point in time.

Event sourcing provides several benefits, including:

1. Immutable state: Event sourcing ensures that the data is immutable, as events are never deleted or modified. This provides a complete audit trail of all changes that have occurred over time.

2. Reproducible state: Since all events are immutable, it's easy to reproduce the state of the application system at any point in the past.

3. Scalability: Event sourcing is highly scalable due to its ability to process large amounts of data. It's easier to scale because every write is an insertion rather than an update.

4. Flexibility: Event sourcing allows different views of the same data to coexist, making it easier to model complex business rules.

Event sourcing requires more work upfront to model your domain, but benefits quickly become apparent as the application evolves. It's becoming a more popular approach in modern application development, especially for applications with complex business rules, high scalability requirements, or requirements for auditing and compliance.

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