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The patterns


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Pattern: Event sourcing

Problem

You have applied the Event-driven architecture pattern. In order to be reliable, services must atomically publish events whenever their state changes. It is not viable to use a distributed transaction that spans the database and the message broker. How to reliably/atomically publish events whenever state changes?

Solution

A good solution to this problem is to use event sourcing. Event sourcing persists the state of a business entity such an Order or a Customer as a sequence of state-changing events. Whenever the state of a business entity changes, a new event is appended to the list of events. Since saving an event is a single operation, it is inherently atomic. The application reconstructs an entity’s current state by replaying the events.

Applications persist events in an event store, which is a database of events. The store has an API for adding and retrieving an entity’s events. The event store also behaves like a message broker. It provides an API that enables services to subscribe to events. When a service saves an event in the event store, it is delivered to all interested subscribers.

Example

The following diagram shows how the e-commerce system could use event sourcing to persist orders.

Instead of simply storing the current state of each order as a row in an ORDERS table, the application persists each Order as a sequence of events. The CustomerService can subscribe to the order events and update its own state.

For more details, see the Eventuate Platform, which is an application platform based on event sourcing and CQRS. There are several example applications

Resulting context

Event sourcing has several benefits:

  • It solves one of the key problems in implementing an event-driven architecture and makes it possible to reliably publish events whenever state changes.
  • Because it persists events rather than domain objects, it mostly avoids the object‑relational impedance mismatch problem.
  • It provides a 100% reliable audit log of the changes made to a business entity
  • It makes it possible to implement temporal queries that determine the state of an entity at any point in time.
  • Event sourcing-based business logic consists of loosely coupled business entities that exchange events. This makes it a lot easier to migrate from a monolithic application to a microservice architecture.

Event sourcing also has several drawbacks:

  • It is a different and unfamiliar style of programming and so there is a learning curve.
  • The event store is difficult to query since it requires typical queries to reconstruct the state of the business entities. That is likely to be complex and inefficient. As a result, the application must use Command Query Responsibility Segregation (CQRS) to implement queries. This in turn means that applications must handle eventually consistent data.

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