CQRS Pattern Inner Workings

CQRS Pattern Inner Workings

In this tutorial, we are going to discuss about the CQRS Pattern Inner Workings. Understanding the iCQRS Pattern Inner Workings involves delving into how commands and queries are processed, how events are handled, and how the read and write models interact. Here’s a detailed look at the internal mechanics of CQRS:

CQRS Pattern Inner Workings
1. Commands: Handling State Changes

Commands are requests to perform actions that modify the state of the system.

Steps for Handling Commands:

  1. Client Issues Command:
    • The client sends a command to the system via the Command API.
  2. Service Bus:
    • The command is routed through a service bus (or message bus) to the appropriate command handler.
  3. Command Handler:
    • The command handler receives the command and executes the corresponding business logic.
    • The handler interacts with the domain model to perform the required state changes.
  4. Domain Model:
    • The domain model encapsulates business rules and ensures consistency.
    • It interacts with aggregates, which are groups of related entities treated as a single unit for consistency.
  5. Persist State Changes:
    • The changes are persisted in the write database.
    • If using event sourcing, an event representing the state change is created and stored in the event store.
  6. Publish Events:
    • The event is published to the service bus to notify other parts of the system of the state change.
2. Queries: Retrieving Data

Queries are requests for information that do not modify the state.

Steps for Handling Queries:

  1. Client Issues Query:
    • The client sends a query to the system via the Query API.
  2. Service Bus:
    • The query is routed through a service bus to the appropriate query handler.
  3. Query Handler:
    • The query handler receives the query and retrieves the required data from the read database.
    • The read model is often denormalized and optimized for fast retrieval.
  4. Return Data:
    • The retrieved data is returned to the client in the desired format.
3. Event Handling

Events are used to keep the read model in sync with the write model and to facilitate communication between different parts of the system.

Steps for Handling Events:

  1. Event Creation:
    • When a state change occurs (via a command), an event is created to represent the change.
    • The event is stored in the event store if event sourcing is used.
  2. Publish Event:
    • The event is published to the service bus.
  3. Event Handlers:
    • Event handlers listen for specific events.
    • Upon receiving an event, an event handler updates the read model to reflect the state change.
  4. Update Read Model:
    • The read model is updated with the new data.
    • This ensures eventual consistency between the read and write models.
4. Data Models

Write Model:

  • Focuses on the consistency and integrity of data.
  • Normalized to reduce redundancy.
  • Contains complex business logic and validation rules.

Read Model:

  • Optimized for query performance.
  • Can be denormalized to simplify and speed up data retrieval.
  • Tailored to specific query needs.
The Query’s Quest

A query, in essence, is a request for data. Let’s say our user now wants to view their updated address. They issue a query: ‘Get User Address’. How does CQRS handle it?

Unlike commands, queries don’t require validation or event generation. They’re simply processed by the query handler, which fetches the required data. In this case, it fetches the user’s address from the system and displays it to the user.

Synchronizing Commands and Queries

We’ve explored the separate journeys of commands and queries. But how do these two sides stay synchronized? This is where Event Sourcing comes into play.

Remember the ‘User Address Updated’ event generated when the user updated their address? This event is replayed on the query side to update its state. In other words, the user’s updated address is reflected in the query model. Thus, Event Sourcing ensures that despite working separately, the command and query sides are always in sync.

Example: E-commerce Application

Command Workflow Example:

  1. A client sends a CreateOrderCommand.
  2. The service bus routes the command to the CreateOrderHandler.
  3. The CreateOrderHandler validates the command and interacts with the OrderAggregate in the domain model.
  4. The OrderAggregate performs the necessary state changes and persists the new order in the write database.
  5. An OrderCreatedEvent is created and stored in the event store.
  6. The event is published to the service bus.

Query Workflow Example:

  1. A client sends a GetOrderDetailsQuery.
  2. The service bus routes the query to the OrderQueryHandler.
  3. The OrderQueryHandler retrieves the order details from the read database.
  4. The order details are returned to the client.

Event Handling Example:

  1. An OrderCreatedEvent is published.
  2. The OrderCreatedEventHandler listens for this event.
  3. Upon receiving the event, the handler updates the read database to include the new order details.
  4. The read model is now in sync with the latest state changes.
Advantages and Challenges Revisited

Advantages:

  • Scalability: Independent scaling of read and write operations.
  • Performance: Optimized read models for fast data retrieval.
  • Flexibility: Independent evolution of command and query models.
  • Separation of Concerns: Clear division between business logic and data retrieval.

Challenges:

  • Complexity: Requires careful management of multiple models and data stores.
  • Eventual Consistency: Ensuring read models eventually reflect the latest state changes.
  • Development Overhead: Maintaining separate models and handling events can be resource-intensive.
Conclusion

The inner workings of the CQRS pattern involve a clear separation of commands and queries, robust event handling for synchronization, and optimized data models for specific operations. While CQRS introduces complexity, it provides significant benefits in scalability, performance, and maintainability, making it a powerful pattern for complex and high-traffic applications.

That’s all about the CQRS Pattern Inner Workings. If you have any queries or feedback, please write us email at contact@waytoeasylearn.com. Enjoy learning, Enjoy Microservices..!!

CQRS Pattern Inner Workings
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