Run Only Tasks Affected by a PR

As your workspace grows, re-testing, re-building an re-linting all projects becomes too slow. To address this Nx implements code change analysis to determine the minimum set of projects that were affected by the change. How does this work?

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nx affected -t <task>

When you run nx affected -t test, Nx uses Git to determine 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 determine 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.

Changing app1 can only affect app1
Changing lib can affect lib, app1 and app2

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.

Visualize Affected Projects

To visualize what is affected, run:

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nx affected:graph

Specify Which SHAs to Use to Calculate Affected Code

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

Typically, you want to set the base SHA not the the most recent commit on the main branch, but rather that latest commit that successfully passed in CI. In other words, in order to be certain that the repo is in a good state, we need to check all the changes that have happened since the last time the repo was in a good state. See the setup guide for your CI provider to learn how to calculate the last successful commit.

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.