Event Sourcing and CQRS: A Powerful Pair for Software Architecture!: Architect Guide

In the last article about CQRS, we covered how it can power the microservices architecture for scalability, flexibility, and performance. Today's blog post in Architect Guide will cover, what Event Sourcing is all about and how it's related to CQRS. We will also cover how you can combine these two to achieve complex distributed systems.

Event Sourcing and Command Query Responsibility Segregation (CQRS) are architectural patterns that are often used together to create scalable, resilient, and flexible software systems. 

CQRS:

CQRS is a pattern that separates the responsibility for handling commands (that change the state of the system) and queries (that retrieve data from the system). By segregating these two responsibilities, CQRS allows for different approaches to be taken for each task, leading to more efficient and effective code.

Event Sourcing:

Event Sourcing is a pattern that focuses on capturing the history of changes to the state of a system as a sequence of immutable events. Instead of storing the current state of the system, the events that led to the current state are stored, allowing for complete auditability and replay-ability of the system's history.

Put simply, Event sourcing is an architectural pattern for designing software systems that captures and stores every event or state change that occurs within the system. This allows for a detailed audit trail of all changes and enables the reconstruction of the system's state at any point in time. Here are the steps involved in implementing the event sourcing architecture:

Identify the Domain Model: The first step is to identify the domain model of the system. This involves defining the entities, their attributes, and their relationships.

Define Events: Once the domain model is identified, events should be defined for every state change that occurs within the system. Events should contain all the information necessary to fully describe the state change.

Store Events: The events should be stored in a data store such as a database or a message queue. Each event should be stored with a timestamp and a unique identifier.

Replay Events: To reconstruct the state of the system at any point in time, all the events should be replayed in the order they occurred. This involves querying the data store for all events and applying them to the domain model.

Implement Read Models: To enable fast and efficient querying of the system's state, read models should be implemented. Read models are denormalized views of the system's state that are optimized for specific queries.

Handle Concurrency: When multiple events occur concurrently, conflicts can arise. To handle concurrency, a mechanism such as optimistic locking or event versioning should be implemented.

Implement Command Handlers: Command handlers are responsible for processing incoming commands and generating the appropriate events. Command handlers should validate the input and ensure that the resulting events are consistent with the current state of the system.

Implement Event Handlers: Event handlers are responsible for applying events to the domain model and updating the read models. Event handlers should be idempotent, meaning that applying the same event multiple times should have the same result as applying it once.

By following these steps, a system can be designed using the event sourcing architecture, which allows for a detailed audit trail of all state changes and enables the reconstruction of the system's state at any point in time.

In the above diagram, the Command API receives commands from external clients and passes them to the Command Handlers. The Command Handlers validate and process the commands, generating events that describe the changes to the system. These events are then stored in the Event Store.

The Event Handlers / Projections listen for new events in the Event Store and update the Read Models and Query API accordingly. The Read Models are optimized for specific queries, and the Query API provides an interface for external clients to retrieve data from the system.

The example events listed include:

OrderCreatedEvent: This event is generated when a new order is created. It contains information about the order, such as the customer, shipping address, and order total.

OrderLineItemAddedEvent: This event is generated when a new line item is added to an existing order. It contains information about the line item, such as the product, quantity, and price.

OrderLineItemRemovedEvent: This event is generated when a line item is removed from an existing order. It contains information about the line item that was removed.

OrderShippedEvent: This event is generated when an order is shipped. It contains information about the shipment, such as the carrier, tracking number, and shipping date.

OrderDeliveredEvent: This event is generated when an order is delivered. It contains information about the delivery, such as the delivery date and the person who signed for the package.

By replaying all the events in the Event Store, the system's state can be reconstructed at any point in time. This enables auditing, debugging, and analysis of the system's behavior over time.

In the above diagram, the Event Store contains all the events that describe the changes to the system over time. The Event Handlers / Projections listen for new events in the Event Store and update the Read Models and Query API accordingly.

To reconstruct the system's state at any point in time, all the events in the Event Store are replayed in order. The Event Handlers / Projections read the events from the Event Store and apply them to the Read Models and Query API in the same order they were generated.

CQRS and Event Sourcing Combined:

When used together, Event Sourcing and CQRS provide a powerful toolset for building complex, distributed systems. In this architecture, commands are sent to the Command side of the system, which processes the command and creates an immutable event. The event is then stored in an Event Store, which serves as the single source of truth for the system's history.

On the Query side, the system reads from the Event Store to generate the current state of the system. By using Event Sourcing, the Query side can easily reconstruct the state of the system at any point in time, making it easier to perform complex queries and generate reports.

The Event Store can be used for other purposes, such as event-based messaging and event-driven architectures, allowing for a more decoupled and scalable system.

Summary:

Event Sourcing in CQRS is a powerful pattern that provides a scalable and resilient architecture for building complex systems. By separating commands and queries, and storing the history of changes to the state of the system as a sequence of immutable events, developers can create more flexible and efficient code, leading to a better user experience for end-users.