Embroider Initiative: Progress Update

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In the blog post from November 2023, Chris Manson introduced to you Embroider, the new build pipeline that compiles Ember apps into spec-compliant, modern JavaScript. If this is the first time you hear about Embroider, we encourage you to read the introduction of the November blog post, which explains what Embroider is in more detail and presents the Embroider Initiative.

The main goal that was set in the last blog was to get our Ember apps building with Vite. Not only have we achieved that landmark, but we have moved onto the next phase, which brings the wonderful DX improvements that Vite can promise to all Ember apps.

anchorImproving how Ember works with Vite

Now that our challenge is no longer to get Ember working with Vite, we need to figure out how to optimize the Vite build and maintain the compatibility of all classic features. To find a solution, we need to dive into the challenge that an Ember build presents.

anchorHow we make it work: the rewritten app

By default, an Ember application cannot be built with Vite. The reason for this is because Ember performs some “magic” around resolving imports, relying on AMD modules and other specific build actions, which are incompatible with the Vite resolver. Let’s take an example: if you generated an Ember app using the default blueprint, you will get a file app/index.html which contains a reference to the CSS stylesheet “assets/vendor.css”. This file is supposed to contain the vendor styles that are automatically provided by the Ember addons you use. However, it doesn’t exist in your repo: your app/index.html points to a file that is literally nowhere in your app files. At build time, Ember will go through all the active Ember addons used in your app and will emit the assets/vendor.css file at some point in the build pipeline. This is an Ember-specific behavior that Vite cannot deal with. Vite expects the referenced file to exist, it cannot guess how to find it or write it.

To solve this issue, when the build starts, Embroider first produces an intermediate Ember application that we call the “rewritten app” and outputs it to node_modules/.embroider/rewritten-app. The rewritten app is slightly different from the initial Ember app in that it can be consumed by Vite. For instance, it has an index.html at the root and it does contain the assets/vendor.css. This approach allows us to build real-world applications with Vite, but also carries performance (and other) downsides: Vite prebuilds the app dependencies for optimization purposes, but the rewritten app is considered a dependency because it lives in a folder in node_modules. Therefore, changes in the Ember app that need to be reflected in the rewritten app imply changes in a dependency, so Vite has to go through bundling dependencies again and we lose the advantage of the prebuild.

anchorHow we want to make it work: “inversion of control”

To improve this situation and get all the benefits of using Vite, we have started a wide technical topic called “inversion of control”. The idea is the following: instead of having Embroider produce a rewritten app, and then passing over to Vite once the rewritten app is ready, Vite takes the lead, and when it’s unable to resolve Ember-specific requests, it asks Embroider to return the information without the need of a rewritten app.

A good example to illustrate this idea is file virtualization. Let’s go back to our vendor.css case. Before inversion of control, Embroider produces a rewritten app that concats all the styles provided by the classic addons into a file assets/vendor.css, and this file is referenced in the index.html. Vite has no trouble consuming that. But now, we want to get rid of the rewritten app step. To invert the control, we use a special identifier in the index.html that starts with @embroider. For instance: @embroider/core/vendor.css. Then, we implement our own Vite resolver plugin that is able to recognize this identifier and ask Embroider to return the content for this file. Embroider executes code that is very similar to the code initially used to create the assets/vendor.css in the rewritten app, but this time, it doesn’t write any file: it just returns the content to Vite as “virtual” content.

anchorOn going work

The “inversion of control” topic includes every task that is necessary to remove the rewritten app. Among these tasks, we have the virtualization of several files, handling the public assets provided by the app and addons, and also maintaining the compatibility of classic addons with Embroider by getting functionalities like content-for to work with Vite. A wide part of this work has already been done: all the app entry points are virtualized, the new authoring format for the index.html is ready, and we have identified the few pieces we still have to work on to maintain the compatibility with the classic world (the best example is FastBoot supports).

The task that will close out the “inversion of control” topic will be the one that actually turns off the writing of the rewritten app. From this moment, the initial Ember app will have to contain everything Vite needs to communicate with Embroider. This will also require us to update the application blueprint so newly generated Ember apps have everything in place to build with Vite. We're hoping to have a preview of this new blueprint ready in the next few days for people to try out.

anchorThe Embroider test suite now uses Vite

Embroider’s test suite relies on minimalist Ember app and addon templates that can be overridden and output according to the needs of each test scenario. Some of the new functionalities mentioned above required the test suite to build these output apps with Vite. It has been a long road, but we are finally there: all of Embroider's test suite uses Vite to get the app built.

For the new tests, we started to reorganize things in a way that allows us to assert that each functionality behaves as expected in build mode and dev mode: the approach that was previously used to start the Ember dev server in one specific test file has been refactored to a CommandWatcher helper that can be imported in tests to easily start Vite and wait for logs that say building is complete before asserting.

Also, we improved the existing audit system so that it can interface with both the Vite dev server and the build output. Because Vite behaves differently in dev mode and build (it essentially just uses Rollup during the build) we need to verify that some of the features we are implementing work both when Ember developers are developing their app and when they are deploying to production. Using this new audit system also allows us to more easily investigate the content of the build, e.g. you want to verify a specific piece of code has been included into the build, but you don’t know how Rollup is going to organize the files, so you don’t know what file to look into to verify that the code is present: the audit system is designed to figure this out for you, so you can focus on your test's purpose without struggling with Rollup internals.

anchorReducing the bus factor

In the section “Improving the bus factor” of the November blog post, we explained how challenging it is to contribute to the core of Embroider and we introduced the apprenticeship model we adopted to address the bus factor issue. This model still stands today: Chris pairs with Ed Faulkner every week and now has a solid knowledge of the heart of the Embroider codebase.

Following Andrey Mikhaylov’s departure in March, Marine Dunstetter joined the Embroider Initiative as a full-time engineer. We have extended the apprenticeship model by having Chris and Marine meet and pair regularly each week.

"Marine’s ramp-up was very successful and she has made many impactful contributions to the project over the past three months and we wouldn't be so close to completing the inversion of control work at the time of writing without her!" Chris Manson

anchorNext steps

We are now at the point of wrapping up the technical challenges. We should soon have a Vite demo that is a good approximation of the new way to write Ember apps and matches the future blueprint. The next phase will be all about communication and community work: we want this new fast Embroider + Vite experience to be the default experience for all Ember users.

To keep up to date with the latest developments on the Embroider Initiative, don't miss Chris Manson's talk at EmberConf on May 31st.

anchorIf you're facing challenges with Ember.js and need a helping hand, reach out!

Contact us!

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