Service worker in a webpack module federation environment

Question

I have a small setup with webpack module federation, one container app and one remote. The remote is exposing itself completely and the container is consuming it to show it after clicking a navigation link like /users.

The container is reachable at localhost:8080, the remote at localhost:3001

Now, the remote is using mockup service worker (https://mswjs.io/), which works fine when visiting the app directly. When called through the container, an error comes up:

Uncaught (in promise) Error: [MSW] Failed to register a Service Worker for scope ('http://localhost:8080/') with script ('http://localhost:8080/mockServiceWorker.js'): Service Worker script does not exist at the given path.

I also tried setting the scope with

worker.start({
  serviceWorker: {
    options: {
      scope: "http://localhost:3001",
    },
  },
})

which resolves in an error

Failed to register a ServiceWorker: The origin of the provided scope ('http://localhost:3001') does not match the current origin ('http://localhost:8080').

Does anyone have experiences with this? Using service workers in remotes with module federation? How to call them in the container?

Answer 1

I was also experimenting with MSW (Mock Service Worker) for webpack 5 and module federation last couple of days. It may not apply to your specific use case, but below is what I did to make MSW work with a module federation app.

For mocking in a browser in a development environment:

Set up and initiate MSW in the container app.

• place worker.start() in bootstrap.js in the container app (I put it before ReactDOM render as my app is using react).
• place mocks folder also in the container app (e.g. under src).
• Add endpoints that you want to intercept and mock in handlers.js in the mocks folder.
• run npx msw init ./ --save in the root of the host app (container). Note that the directory of the installation should be the root - mockServiceWorker.js should be in the same location as the index.html (its location is specified in webpack config).

This worked for me regardless of which child MFE apps calls the api endpoints you are intercepting with the MSW.

Below is the repo of a little experiment/example I did with Webpack5 Module Federation and MSW. https://github.com/nfabacus/module-federation-example

For mocking in a node environment (Jest Unit Tests):

I did not test specifically with the module federation, but I think you can just follow the instruction in the MSW setup page for node (https://mswjs.io/docs/getting-started/install). Just install msw for each MFE and set up the tests. When I tested with normal react app (CRA), it worked fine (https://github.com/nfabacus/msw-cra-example). I don't think there is any difference between module federation and single SPA in terms of implementing msw with unit tests.