83 lines
5.3 KiB
Markdown
83 lines
5.3 KiB
Markdown
---
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permalink: "/{{ year }}/{{ month }}/{{ day }}/leveraging-rust-to-build-cross-platform-mobile-libraries"
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title: "This Week in Glean: Leveraging Rust to build cross-platform mobile libraries"
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published_date: "2020-09-01 15:00:00 +0200"
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layout: post.liquid
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data:
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route: blog
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tags:
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- mozilla
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- rust
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---
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(“This Week in Glean” is a series of blog posts that the Glean Team at Mozilla is using to try to communicate better about our work. They could be release notes, documentation, hopes, dreams, or whatever: so long as it is inspired by Glean.)
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All "This Week in Glean" blog posts are listed in the [TWiG index](https://mozilla.github.io/glean/book/appendix/twig.html)
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(and on the [Mozilla Data blog](https://blog.mozilla.org/data/category/glean/)).
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This article is [cross-posted on the Mozilla Data blog](https://blog.mozilla.org/data/2020/09/01/twig-leveraging-rust/).
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---
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A couple of weeks ago I gave a talk titled ["Leveraging Rust to build cross-platform mobile libraries"][video].
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You can find my [slides as a PDF](https://fnordig.de/talks/2020/rustydays/slides.pdf).
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It was part of the [Rusty Days][rustydays] Webference, an online conference that was initially planned to happen in Poland, but had to move online.
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Definitely check out [the other talks][playlist].
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[rustydays]: https://rusty-days.org/
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[video]: https://www.youtube.com/watch?v=j5rczOF7pzg
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[slides]: https://fnordig.de/talks/2020/rustydays/slides.pdf
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[playlist]: https://www.youtube.com/watch?v=QaCvUKrxNLI&list=PLf3u8NhoEikhTC5radGrmmqdkOK-xMDoZ
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One thing I wanted to achieve with that talk is putting that knowledge out there.
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While multiple teams at Mozilla are already building cross-platform libraries, with a focus on mobile integration,
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the available material and documentation is lacking.
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I'd like to see better guides online, and I probably have to start with what we have done.
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But that should also be encouragement for those out there doing similar things to blog, tweet & speak about it.
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### Who else is using Rust to build cross-platform libraries, targetting mobile?
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I'd like to hear about it.
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Find me on Twitter ([@badboy\_](https://twitter.com/badboy_)) or drop me [an email](mailto:janerik@fnordig.de).
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## The Glean SDK
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I won't reiterate the full talk ([go watch it][video], really!), so this is just a brief overview of the Glean SDK itself.
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The Glean SDK is our approach to build a modern Telemetry library, used in Mozilla's mobile products and soon in Firefox on Desktop as well.
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The SDK consists of multiple components, spanning multiple programming languages for different implementations.
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All of the Glean SDK lives in the GitHub repository at [mozilla/glean](https://github.com/mozilla/glean).
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This is a rough diagram of the Glean SDK tech stack:
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On the very bottom we have `glean-core`, a pure Rust library that is the heart of the SDK.
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It's responsible for controlling the database, storing data and handling additional logic (e.g. assembling pings, clearing data, ..).
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As it is pure Rust we can rely on all Rust tooling for its development.
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We can write tests that `cargo test` picks up. We can generate the full API documentation thanks to `rustdoc`
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and we rely on `clippy` to tell us when our code is suboptimal.
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Working on `glean-core` should be possible for everyone that knows some Rust.
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On top of that sits `glean-ffi`.
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This is the FFI layer connecting `glean-core` with everything else.
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While glean-core is pure Rust, it doesn't actually provide the *nice API* we intend for users of Glean.
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That one is later implemented on top of it all.
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glean-ffi doesn't contain much logic.
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It's a translation between the proper Rust API of glean-core and C-compatible functions exposed into the dynamic library.
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In it we rely on the excellent [`ffi-support` crate](https://docs.rs/ffi-support/).
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ffi-support knows how to translate between Rust and C types, offers a nice (and safer) abstraction for C strings.
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glean-ffi holds *some* state: the instantiated global Glean object and metric objects.
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We don't need to pass pointers back and forth. Instead we use opaque handles that index into a map held inside the FFI crate.
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The top layer of the Glean SDK are the different [language implementations](https://mozilla.github.io/glean/book/dev/core/internal/implementations.html).
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Language implementations expose a nice ergonomic API to initialize Glean and record metrics in the respective language.
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Additionally each implementation handles some special cases for the platform they are running on, like gathering application and platform data or hooking into system events.
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The nice API calls into the Glean SDK using the exposed FFI functions of `glean-ffi`.
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Unfortunately at the moment different language implementations carry different amounts of actual logic in them.
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Sometimes metric implementations require this (e.g. we rely on the clock source of Kotlin for timing metrics),
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in other parts we just didn't move the logic out of the implementations yet.
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We're actively working on [moving logic into the Rust part where we can](https://bugzilla.mozilla.org/show_bug.cgi?id=1651382) and might eventually use some code generation to unify the other parts.
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[uniffi] is a current experiment for a multi-language bindings generator for Rust we might end up using.
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[uniffi]: https://github.com/mozilla/uniffi-rs
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