Enable

In this article, you’ll learn how to control when containers start using the enable option. While dependencies ensure that containers start when the required parts are ready, sometimes that’s not enough.

The enable option allows you to set custom conditions that decide if a container should start. This is useful when you need more control, like starting a container only when certain data is available or a specific state is active.

You’ll also see how enable can be used with feature toggles to turn features on or off in your app based on specific conditions.

Analogy from Cooking

Imagine you have dough and sauce, and you usually add cheese. But before you start making a pizza, you check if you have cheese.

If there’s no cheese, you don’t start.

This is how enable works. It helps you decide if you should start, even when everything else is ready.

Example

import { compose, createContainer } from '@grlt-hub/app-compose';

const hasCheese = () => false;

const dough = createContainer({
  id: 'dough',
  domain: 'ingredients',
  start: () => ({ api: { value: 'dough' } }),
});

const sauce = createContainer({
  id: 'sauce',
  domain: 'ingredients',
  start: () => ({ api: { value: 'sauce' } }),
});

const cheese = createContainer({
  id: 'cheese',
  domain: 'ingredients',
  start: () => ({ api: { value: 'cheese' } }),
  // The container will start
  // only if hasCheese() returns true
  enable: hasCheese,
});

const pizza = createContainer({
  id: 'pizza',
  domain: 'dish',
  dependencies: [dough, sauce, cheese],
  start: () => {
    return { api: { data: 'pizza' } };
  },
});

const { up } = await compose({
  stages: [['prepare', [pizza]]],
});

const result = await up();

console.log(JSON.stringify(result, undefined, 2));

When you run this code, you should see the following output in the console:

{
  "allDone": false,
  "stages": {
    "prepare": {
      "allDone": false,
      "containerStatuses": {
        // because one of its required dependencies (cheese) is OFF
        "pizza": "off",

        "dough": "done",
        "sauce": "done",

        // because the enable condition returned false
        "cheese": "off"
      }
    }
  }
}

The enable function should always return a boolean. If it’s not provided, it returns true by default.

Try modifying hasCheese and see what happens.

Try it

What’s Next?

We’ve learned how to control when containers start using enable. This helps start containers only when needed.

But what if you want to control the priority of starting containers? Sometimes, one container should start before another, even if they are not connected.

This is where stages help. They let you group containers and choose the order they start.