CQRS stands for Command Query Responsibility Segregation. As the name suggests, we split the application into two parts: Command-Side and Query-Side. Now, one will ask, what do “command” and “query” mean?

  • Commands change the state of the object or entity, also called modifiers or mutators.
  • Queries return the state of the entity and do not change anything. Another term for them is “accessors.”

Why Is It Required?

There are issues with having a single view for both read and write sides:

  • Introduces the risk of data contention.
  • Managing permissions and security become complex as the same objects are exposed to both read and write operations.

How CQRS Solves This Problem

Benefits of CQRS

  • One can easily optimize the read side of the system separately from the write side, allowing scaling of each differently as per the load on the side. For example, read datastores often encounter greater load, and hence can be scaled without affecting the write datastores.
  • You can provide multiple views of your data for querying purposes depending on the use cases.

How CQRS Works

The diagram below provides the details of a CQRS-based system:

How to Implement Event Sourcing and CQRS

Eventuate is one such example. It is an application platform consisting of two products, one of which is Eventuate ES, a microservices framework that implements an event sourcing-based programming and persistence model.

Lagom is an open-source microservice framework built on top of the Akka and Play frameworks. Lagom Persistence makes use of Event Sourcing and CQRS to help achieve decoupled architecture.

You can find real-life examples of how Lagom handles Event Sourcing and CQRS in the following blogs:

Axon is a lightweight framework that helps developers build scalable and extensible applications based on the CQRS principles by providing implementations of the most important building blocks, such as aggregates, repositories, and event buses.

Happy blogging!

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