Microservice Architecture

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Pattern: Command Query Responsibility Segregation (CQRS)

Context

You have applied the Microservices architecture pattern and the Database per service pattern. As a result, it is no longer straightforward to implement queries that join data from multiple services. Also, if you have applied the Event sourcing pattern then the data is no longer easily queried.

Problem

How to implement a query that retrieves data from multiple services in a microservice architecture?

Solution

Define a view database, which is a read-only replica that is designed to support that query. The application keeps the replica up to data by subscribing to Domain events published by the service that own the data.

Examples

Resulting context

This pattern has the following benefits:

  • Supports multiple denormalized views that are scalable and performant
  • Improved separation of concerns = simpler command and query models
  • Necessary in an event sourced architecture

This pattern has the following drawbacks:

  • Increased complexity
  • Potential code duplication
  • Replication lag/eventually consistent views

See also

  • Eventuate, which is a platform for developing transactional business applications.

  • My book Microservices patterns describes this pattern in a lot more detail.

About Microservices.io

Microservices.io is brought to you by Chris Richardson. Experienced software architect, author of POJOs in Action, the creator of the original CloudFoundry.com, and the author of Microservices patterns.

Chris helps clients around the world adopt the microservice architecture through consulting engagements, and training classes and workshops.

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