RFC: Sidecar Tasks

[FrozenPandaz]

Problem

E2E tasks, dev-server tasks, and other kinds of tasks need a task to run alongside when they run. This problem is not solved by dependsOn because dependent tasks must complete before the task is run.

Prior Art

https://kubernetes.io/docs/concepts/workloads/pods/sidecar-containers/

Proposed Solution (not final)

Nx can have a notion of tasks which run alongside the task being run.

{
  name: 'app-e2e',
  targets: {
    e2e: {
      command: 'playwright test',
      sidecars: ['app:serve']
    }
  }
}

In this configuration, the app-e2e:e2e task would specify that it would like app:serve to be running before it runs as well as while it is running. Running nx e2e app-e2e would execute the following chain of events:

1. A task graph is constructed with app-e2e:e2e as well as any dependsOn, sidecar, and any dependent tasks of sidecar tasks.
2. Tasks begin to run starting with dependsOn tasks.
3. After all dependsOn tasks are completed, sidecar tasks are run WITH app-e2e:e2e

This would also be considered for running multiple tasks. When running nx run-many -t e2e multiple e2e tasks could depend on the same web server. Nx could try to schedule this in a way where the sidecar task is started once and then multiple e2e tasks with the same sidecar task are run in parallel with the same sidecar task.

Considerations

Starting dev servers and databases might not be instantaneous but also do not complete. At the same time, tasks do not currently have a notion of "ready". Tasks can be built in a way where they can be started without sidecar tasks being truly ready and handle when that sidecar is ready. I imagine most applications are already built like this as they would need this sort of functionality when running in production. Thus, I'm not sure if tasks need to introduce the notion of "ready" to enable sidecar tasks.

Examples

Starting a web-server to e2e test

Currently in Nx, the e2e task is responsible for starting the web-server which it is testing. With sidecars, this can be changed to Nx being responsible for starting the web server while the e2e task is responsible for waiting for the server to be ready. Cypress, Playwright and other e2e tasks runners already do this.

Starting backends alongside a web server

{
  name: 'frontend',
  targets: {
    serve: {
      command: 'vite dev',
      sidecars: ['content-backend:serve', 'auth-backend:serve']
    }
  }
}

This would start the required backends while running a frontend application.

Starting a db alongside a backend server

{
  name: 'api',
  targets: {
    serve: {
      command: 'node ./server.js',
      sidecars: ['database:start']
    }
  }
}

Publishing to a local-registry before running e2e tests

{
  name: 'angular-e2e',
  targets: {
    serve: {
      command: 'jest e2e/angular',
      sidecars: ['@nx/source:publish']
    }
  }
}

Note: The core team has thought a bit about this but have not committed nor had the time to implement it yet. This discussion is to gauge interest for this feature.

[Ambroos]

Looking at our monorepo this would be useful as described!

There are some more complex things that do need to have a concept of 'ready'. Codegen in watch mode is a common one. For example, we currently integrate GraphQL Codegen via a Vite plugin in dev mode since there's no clean way to do this outside of it right now. The node executor has waitUntilTargets that does something like this but is not super flexible (waits for any console output as far as I can tell).

I'd like something like this, where the sidecars/targets can take some optional configuration to configure a wait for ready:

{
  name: "api",
  targets: {
    serve: {
      command: "node ./server.js",
      sidecars: [
        "database:start",
        {
          target: "api:codegen:watch",
          waitForOutput: /✔ Generate outputs/, // or just true to wait for any console output
          waitTimeout: 10, // assume failure if the ready condition isn't met in time
        },
      ],
    },
  },
}

[wjthieme]

Another example might be where you have a dev command that relies on other packages. i.e.

package1:watch `tsc --watch`
package2:watch `tsc --watch`
package3:dev `command with hot reload`

Having package3:dev dependsOn: ["^watch"] won't work since package1:watch and package2:watch never exit. In an ideal situation you'd run package3:dev with package1:watch and package2:watch as sidecars.

Currently you could do this with nx watch but that reloads the package3:dev command which defeats the purpose of the hot reload it already has built in.

[leggomuhgreggo]

Question: would it make sense to include this as part of the existing dependsOn feature, using the object syntax?

      "dependsOn": [
        {
          "target": "^dev", 
+         "sidecar: true
        }
      ]

[mike-fam]

Yes this would be helpful. We've got a 'portal' app that redirects to other 'product' apps, and the portal app needs to set up the bootstrap data for the product apps. It's quite difficult to mock the data for all use cases for the product apps. Having an easy way to also run the portal app whenever a product app is run would be very helpful.

[FrozenPandaz]

Thank you for everyone's feedback here. It's much appreciated.

I'm going to close this RFC in favor of #29025 which is a more broad proposal including an equivalent of sidecar tasks. Please Give that RFC a read! 🙏