Control Task

Not all features are equally important. An analytics tracker failing is acceptable — a missing user session is not. When a critical feature fails, the app shouldn’t silently show a broken page; it should acknowledge the problem.

A control task is a pattern for this. It observes the statuses of critical tasks and returns a result — { passed: true } or { passed: false } — that other parts of your app can use to decide what to do.

Using task.status in context means the control task always runs — even when upstream tasks have failed or were skipped — which is what makes it a reliable observer.

const control = createTask({
  name: "control",
  run: {
    context: [fetchUser.status],
    fn: (ctx) => {
      const passed = ctx.every((status) => status === "done")

      return { ok: { passed } }
    },
  },
})

compose()
  .stage({ steps: [fetchUser, analytics] })
  .stage({ steps: [control] })
  .run()
  .then((scope) => scope.get(control.result))
  .then(console.log)

Tip

The scope.get call here is just for the sandbox. In a real app, the next stage would have two tasks — one for the success path, one for the failure path — each reading control.result from context and enabling itself based on passed.

Try uncommenting the failure in fetchUser to see the control task catch it:

import { compose, createTask } from "@grlt-hub/app-compose"

// Critical — the page can't work without this
const fetchUser = createTask({
  name: "fetchUser",
  run: {
    fn: () => {
      // 👇 Uncomment to simulate failure
      // throw new Error("[fetchUser]: failed")
      return { ok: { id: 1, name: "Alice" } }
    },
  },
})

// Non-critical — nice to have, but the page works without it
const analytics = createTask({
  name: "analytics",
  run: {
    fn: () => {
      throw new Error("service unavailable")
    },
  },
})

// Control task:
// observes critical tasks and decides if the app start succeeded
const control = createTask({
  name: "control",
  run: {
    context: [fetchUser.status],
    fn: (ctx) => {
      const passed = ctx.every((status) => status === "done")

      return { ok: { passed } }
    },
  },
})

compose()
  .stage({ steps: [fetchUser, analytics] })
  .stage({ steps: [control] })
  .run()
  .then((scope) => scope.get(control.result))
  .then(console.log)