How Affected Works

First thing!

Before reading this guide, check out the mental model guide. It will help you understand how computation caching fits into the rest of Nx.

Overview

When you run nx test app1, you are telling Nx to run the app1:test task plus all the tasks it depends on.

When you run nx run-many -t test -p app1 lib, you are telling Nx to do the same for two tasks app1:test and lib:test.

When you run nx run-many -t test, you are telling Nx to do this for all the projects.

As your workspace grows, retesting all projects becomes too slow. To address this Nx implements code change analysis to get the min set of projects that need to be retested. How does it work?

When you run nx affected -t test, Nx looks at the files you changed in your PR, it will look at the nature of change (what exactly did you update in those files), and it uses this to figure the list of projects in the workspace that can be affected by this change. It then runs the run-many command with that list.

For instance, if my PR changes lib, and I then run nx affected -t test, Nx figures out that app1 and app2 depend on lib, so it will invoke nx run-many -t test -p app1 app2 lib.

affected

Nx analyzes the nature of the changes. For example, if you change the version of Next.js in the package.json, Nx knows that app2 cannot be affected by it, so it only retests app1.

Project Graph

To visualise what is affected, run:

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nx affected:graph

CI

The SHAs you pass must be defined in the git repository. The main and HEAD SHAs are what you normally use while developing. Most likely you will want to provision other SHAs in your CI environment.

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nx affected:build --base= origin/main --head=$PR_BRANCH_NAME # where PR_BRANCH_NAME is defined by your CI system

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nx affected:build --base= origin/main~1 --head= origin/main # rerun what is affected by the last commit in main

You can also set the base and head SHAs as env variables:

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NX_BASE=origin/main~1

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NX_HEAD=origin/main

Ignoring Files from Affected Commands

Nx provides two methods to exclude glob patterns (files and folders) from affected:* commands.

  • Glob patterns defined in your .gitignore file are ignored.
  • Glob patterns defined in an optional .nxignore file are ignored.

Not Using Git

If you aren't using Git, you can pass --files to any affected command to indicate what files have been changed.