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77 lines
3.5 KiB
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77 lines
3.5 KiB
Markdown
---
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title: "Request for Comments: New Embedded Protocol"
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author: Natalie Weizenbaum
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date: 2023-05-19 16:00 PST
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---
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If you're not an author of a host package for the Embedded Sass Protocol, you
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can skip this blog post—although if you're a big enough nerd, you may find it
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interesting regardless!
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We're planning to make a number of breaking changes to the Embedded Sass
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Protocol, and we want your feedback before we lock in the new way of doing
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things. We intend to make a number of breaking changes all at once to keep the
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total number of disruptions to a minimum.
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We're planning two major breaking changes:
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1. The Dart Sass embedded host will no longer be released as a separate
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executable. It will now be bundled into the main Dart Sass executable,
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accessible by running `sass --embedded`.
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2. Every packet in the embedded protocol now includes a compilation ID as part
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of the packet structure, rather than declaring it in the protocol buffer
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definitions.
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We're using this opportunity to also introduce three much smaller breaking
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changes:
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1. The specification for the embedded protocol and the protocol buffer
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definition have been moved to [the Sass language repository] so that they can
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be updated at the same time as changes to the language and the JS API.
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[the Sass language repository]: https://github.com/sass/sass/blob/main/spec/embedded-protocol.md
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2. The embedded protocol now explicitly declares optional fields using the
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protocol buffers language feature. This means that "default values" for
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various fields are no longer considered to be unset.
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3. The `CompilationSuccess.loaded_urls` field has been moved to
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`CompilationResult.loaded_urls` so that it's available even when a
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compilation fails. This allows watcher implementations to know which files to
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watch to redo a failed compilation.
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The repository-organization changes have already been made, but the changes to
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the protocol itself are fully documented [in a proposal in the language
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repository].
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[in a proposal in the language repository]: https://github.com/sass/sass/blob/main/proposal/embedded-protocol-2.md
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## Combining Executables
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The primary benefit of folding Embedded Dart Sass into the main Dart Sass
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executable is to provide embedded hosts an easy way to expose the standard Dart
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Sass command-line API to users. Now every user who installs any embedded host
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will have the full Dart Sass executable available to them at native Dart VM
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speeds.
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This also helps simplify the Sass team's organization by reducing the number of
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separate repositories and release processes we need to manage.
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## Wire-Level Compilation ID
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We're pulling the compilation ID out to the protocol level in order to provide
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better concurrency, particularly on the side of the embedded compiler. Sass
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compilations done by the embedded compiler don't share any state between one
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another, which means that they could in theory be run in totally separate worker
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threads. However, with the embedded protocol as it exists today, directing each
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message to the correct worker thread requires parsing the entire message on the
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main thread to determine which compilation it belongs to, then parsing it
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_again_ in the worker thread to actually handle it.
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Making the compilation ID part of the protocol itself solves this issue. Each
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endpoint can read the ID, look up the worker thread that's handling the
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compilation, and pass the message on to that thread without parsing the rest of
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the message. This makes concurrency both easier and more efficient, which will
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help ensure that large compilations happen as fast as possible.
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