> For the complete documentation index, see [llms.txt](https://gia-duong-duc-minh.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gia-duong-duc-minh.gitbook.io/notes/computer/microservices/cqrs.md).

# 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.

## 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](https://microservices.io/patterns/data/domain-event.html) published by the service that own the data.

![](https://502244816-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FXPmru3MV7ngRA9hdKlzd%2Fuploads%2Fgit-blob-a85cbf917fc39d5b9e3af868f4b7f4cfbfd972cc%2FQuerySideService.png?alt=media)

## Examples

* My book's FTGO example application has the `Order History Service`, which implements this pattern.
* There are [several Eventuate-based example applications](http://eventuate.io/exampleapps.html) that illustrate how to use this pattern.

## 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

## Related patterns

* The \[\[Database per Service]] pattern creates the need for this pattern
* The \[\[API Composition]] pattern is an alternative solution
* The \[Domain event] pattern generates the events
* CQRS is often used with \[\[Event sourcing]]

## See also

* [Eventuate](http://eventuate.io/), which is a platform for developing transactional business applications.
* My book [Microservices patterns](https://microservices.io/book) describes this pattern in a lot more detail.