An Introduction to CQRS in NestJS
A common approach to NestJS development is to build services that controllers communicate with to access data. But this approach isn’t the only valid design pattern in NestJS. There are other design patterns, such as the CQRS design pattern.
CQRS is a design pattern that separates the read and write operations of an application. This separation can help improve scalability, performance, and maintainability.
Find out all about CQRS and how you can apply it when building a NestJS API.
What Is CQRS?
CQRSstands forcommand-query responsibility segregation. It implements the use ofcommandsto create, update, and delete data andqueriesto fetch data. This helps to eliminate the need to implement an application’s database calls into services.
It also enables a clear distinction between the logic of querying the database for data and performing other actions in an application.
The CQRS approach is useful indomain-driven design, which allows you to separate domain logic and infrastructural operations in your application. You can also use it to implement complex business logic, but this is not recommended for simpler applications.
Using CQRS in a NestJS API
it’s possible to use the CQRS design pattern in an API that you build in NestJS. To follow along, you need to haveNode.js installed on your computerand a recent version of NestJS.
Use the following steps to build a simple blogging application that implements the CQRS design pattern.
Create a Nest Project
Create a new Nest project and generate apostresource for a blog application. You can do this by running the following commands in a terminal:
Install Dependencies
After you’ve completed the steps above, run this terminal command to install the NestJS CQRS package:
Create a Post Service
Add the following code to yourposts.service.tsfile to define thePostServiceclass.
ThePostServicedefinescreateandfindByIdmethods to create a new post and get an existing post from its ID.
Define Commands and Queries
The next step is to define the queries and commands that are core to the CQRS design pattern.
In thepostsdirectory, create two new files:createPostCommand.command.tsandgetPostQuery.query.ts. The command file should look like this:
And the query definition file, like this:
Create Command and Query Handlers
After you have successfully defined your commands and queries, you need to create handlers for them. A handler is a function that runs a command or query and returns the outcome.
Create ahandlers.tsfile in yourpostdirectory and paste the following code in it:
In the samehandlers.tsfile, you can modify the import statements to include the ones below, to allow for working with queries. You may then implement the query handler as seen in the below code:
Register Handlers
The last step is to register the command and query handlers with the NestJS module.
This code registers thePostService,CommandHandlers, andQueryHandlersin theprovidersarray. The use of a spread operator (…) is to merge the arrays ofqueryhandlers andcommandhandlers into theprovidersarray.
Execute Commands and Queries
The registered commands and query handlers are usable in controllers. The following code is the implementation of apostscontroller which will accept HTTP requests and return required responses.
In the code above, theCommandBusexecutes theCreatePostCommandand creates a new post.
This code shows how to implement a controller that uses a query:
ThequeryBusexecutesGetPostQuerywhich gets the post with the given ID and returns it.
After completing all the steps above, you should now have a minimalist, working application to create and fetch blog posts.
Although the code here uses an array to store the created posts in memory, you’re more likely to use a database in production. You can either use aSQL database, or aNoSQL database like MongoDB, as NestJS supports both options.
Building APIs With the CQRS Design Pattern
Incorporating the CQRS design pattern into your NestJS application can aid in scalability, performance, and maintainability. CQRS allows for more efficient and optimized operations by separating the read and write operations an application carries out.
The @nestjs/cqrs package provides a building block for implementing CQRS in NestJS with commands and query handlers. Overall, CQRS is a powerful pattern that can help create more efficient and scalable applications, and you should weigh up your options before using it.
The observer pattern is a popular way of distributing events across a system. Get to know its ins and outs.
My iPhone does it all, but I still need my dumb phone.
This small feature makes a massive difference.
Don’t let someone else take over your phone number.
OneDrive is one of the best, but it has a catch.
So much time invested, and for what?
