Task Pipeline Configuration

Lerna delegates the running of npm scripts (forking processes etc) to Nx. The nx.json file is the place where you can configure how Nx does it.

:::tip

If you don't have nx.json, run npx lerna add-caching.

:::

Run Tasks in Parallel

If you want to increase the number of processes running the scripts to, say, 5 (by default, it is 3), pass the following:

npx lerna run build --concurrency=5

Define Task Dependencies (aka Task Pipelines)

Without our help Lerna cannot know what targets (scripts) have prerequisites and which ones don't. You can define task dependencies in the nx.json file:

{
  ...
  "targetDefaults": {
    "build": {
      "dependsOn": ["^build"]
    }
  }
}

With this, Lerna knows that before it can build a project, it needs to build all of its dependencies first. There are, however, no constraints on tests.

Once you define the targetDefaults property the sort flag is ignored.

This mechanism is very flexible. Let's look at this example:

{
  ...
  "targetDefaults": {
    "build": {
      "dependsOn": ["^build", "prebuild"]
    },
    "test": {
      "dependsOn": ["build"]
    }
  }
}

Note, older versions of Nx used targetDependencies instead of targetDefaults. Both still work, but targetDefaults is recommended.

The ^ symbol (a.k.a the caret symbol) simply means dependencies. Therefore whereas "test": { "dependsOn": ["build"] } means a particular project's "test" target depends on its own "build" target to have already completed before running, "build": { "dependsOn": ["^build"] } means that a particular project's "build" target depends on the "build" target of all of the project's dependencies to have already completed before running.

When running lerna run test --scope=myproj, the above configuration would tell Lerna to

1. Run the test command for myproj
2. But since there's a dependency defined from test -> build, Lerna runs build for myproj first.
3. build itself defines a dependency on prebuild (on the same project) as well as build of all the dependencies. Therefore, it will run the prebuild script and will run the build script for all the dependencies.

Note, Lerna doesn't have to run all builds before it starts running tests. The task orchestrator will run as many tasks in parallel as possible as long as the constraints are met.

Situations like this are pretty common:

Because we described the rules in nx.json, they will apply to all the projects in the repo. You can also define project-specific rules by adding them the project's package.json.

{
  ...
  "nx": {
    "targets": {
      "test": {
        "dependsOn": [
          "build"
        ]
      }
    }
  }
}