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Around this time in 2015, W3C introduced WebAssembly, a small binary format that promises to bring near-native performance to the web. Since then it has been well received by web developers, with some going as far as to say that the “death of JavaScript is near.” It is also supported in all the major browsers including Mozilla, Chrome, Safari, and Edge. While WebAssembly was initially designed with the web in mind, it would be a waste not to take its performance and security benefits to go “beyond the web” environments as well.

This year we are seeing many initiatives pushing WebAssembly beyond the web. One of them is by Fastly, an edge cloud platform provider. Beginning this year, Fastly open sourced its WebAssembly compiler and runtime, named Lucet. With Lucet, Fastly’s edge cloud can execute tens of thousands of WebAssembly programs simultaneously.

Fastly CTO We had a great opportunity to interview Fastly’s CTO Tyler McMullen, who gave us insight into why and how they came up with Lucet, what sets it apart from other WebAssembly compilers, the inner workings and design decisions behind Lucet, and more.


Here are some of the highlights from the interview:

Benefits of WebAssembly beyond the Web

It is exciting to think that we will be able to experience near-native experience on the web. But WebAssembly also aims to solve another major concern of today’s times: security.

WebAssembly was designed for performance, and also for security. WebAssembly programs carry much stronger security guarantees than native code, with comparable performance. That makes it a great candidate for the edge cloud, where we can use the Lucet compiler and runtime to execute WebAssembly programs in isolation from each other, at a much lower resource and performance cost than competing approaches to multi-tenant isolation of native code, like processes, containers, or virtual machines.

Along with these security and performance benefits, the growing support for WebAssembly by compilers like LLVM (since its version 8 release) also makes it suitable for non-web environments. McMullen adds, “Besides security, the other aspect that makes WebAssembly attractive beyond the browser is maturing support by compilers, most notably the LLVM toolchain, used by the Clang C compiler and Rust language compiler, among others. Rather than having to build a new language, or a new compiler, to emit code with the security guarantees we need, we can use the WebAssembly output of any compiler. And it means that tons of existing programs can be compiled to WebAssembly with minimal modification.

How Lucet ensures security

With security being one of the major focus areas of Lucet, we asked McMullen how security in Lucet works. “WebAssembly provides a set of guarantees about the security and safety of the code that can be verified during compilation. But those guarantees only hold if verification and compilation are done correctly. Those guarantees also require the runtime to cooperate. So there are a lot of moving pieces here that need to work in concert with each other.

Lucet takes a security-by-contract approach to this problem. The compilation phase builds up a set of constraints for the runtime. Those constraints get embedded into the compiled artifact. The runtime then picks up those constraints and enforces them while loading and running the module. This lets us enforce things like which functions a module will be allowed to import for the embedding program, how much memory it will attempt to use, as well as the layout of that memory.

So, the security guarantees that Lucet provides end up being enforced with a combination of the compiler, runtime, and the embedding program.

Compilation in Lucet

Lucet is designed to compile a code written in C/Rust to WebAssembly and then compile this to native. So, why can’t we directly compile code written in C/Rust to native code? McMullen says that this will give you control over the behavior of the generated code.

If you used a typical C or Rust compiler you’d have relatively little in the way of guarantees about the behavior of the generated code. With Rust you’d have a bit more in that you could guarantee memory safety, but that’s not sufficient by itself. On the other hand, we could certainly create a new C or Rust compiler that guaranteed all the safety guarantees we’ve already discussed, but that would be a tremendous amount of work and would require still more work for each language you wanted to safely compile. We chose WebAssembly because it provides many of the safety and performance guarantees we’re looking for and — just as importantly — also has community support. Rather than reinventing the wheel over and over again, we as a community can work together toward a common goal.

Lucet is still in its early stages of development. McMullen shares what the Lucet team is up to now: “Prior to open sourcing Lucet, we focused on WebAssembly programs emitted by a couple of compilers – LLVM via Clang and Rustc, and AssemblyScript. Supporting that subset of WebAssembly was sufficient to launch Terrarium late last year, where users can create complex web services that are compiled and deployed on demand. Since the Lucet announcement, we’ve seen interest and contributions from other languages, including Swift, Golang, Zig, and Wam. We’ve fixed a bunch of the spec compliance issues that blocked these users, and are actively working on fixing the remaining ones now.

To support, or not to support JavaScript, that is the question

While building WebAssembly runtimes today, developers have two paths to choose from: either supporting JavaScript or not. Lucet follows the latter one, which helps it be simple yet performant.

Security and resource consumption also drove our design here. Modern, fast JavaScript engines are quite complex, require lots of RAM, startup time, and — in order to make them fast — highly advanced JIT compilers. These requirements run counter to what Fastly does. By dropping JavaScript, we can dramatically reduce the complexity and increase the performance of our system.

To be clear, reducing complexity isn’t just about making life easier on ourselves. By cutting out the massive complexity of JavaScript we can also reduce the attack surface and increase confidence in our safety guarantees.

In the myriad of WebAssembly runtimes, what sets Lucet apart

There are currently quite a few WebAssembly runtimes, for instance, Nebulet, Wasmjit, Life, including the ones very similar to Lucet like Wasmer and Wasmtime. We were curious to know what differences Lucet brings to the table.

Lucet was designed from the ground up for multi-tenant, highly concurrent use cases, which matches the runtime requirements of Fastly’s edge cloud. The major design decisions that differentiate it are all focused on performance and resource consumption in our use case, where we need to launch WebAssembly instances for each request our edge cloud handles. Adam Foltzer, a senior software engineer at Fastly, wrote a detailed post on our design and benchmarked its performance here.

Lucet shares a major component with the Wasmtime runtime, the Cranelift code generation engine. Wasmtime is currently designed for a single-tenant use case, and supports in-process compilation of WebAssembly, often called JIT. We are collaborating with the maintainers of Wasmtime on Cranelift, and on runtime implementations of the WebAssembly System Interface (WASI).

Why Fastly chose Rust for implementing Lucet

Looking at Rust’s memory and thread safety guarantees, a supportive community, and a quickly evolving toolchain, many major projects are being written or rewritten in Rust. One of them is Servo, an HTML rendering engine that will eventually replace Firefox’s rendering engine. Mozilla is also using Rust to rewrite many key parts of Firefox under Project Quantum. More recently, Facebook chose Rust to implement its controversial Libra blockchain.

And Fastly’s decision to choose Rust as Lucet’s implementation language was focused on security:

As for why we chose to write Lucet in Rust, the biggest reason was again safety. Writing compilers is complex work. Rust lets us take much of that complexity, describe it with types, and let the Rust compiler check our work in much deeper ways than other languages allow. It lets us focus on the problem we’re trying to solve, rather than the incidental issues of complex software.

Fastly on the future of Rust and WebAssembly

In the past few years, Fastly seems to be focusing on Rust and WebAssembly. McMullen believes these languages will be central to the future and will impact key domains in tech. While Rust enables developers to write both highly efficient and safe code, WebAssembly gives you the flexibility of writing code in your choice of language and platform.

With our role in the internet, efficiency is of utmost importance. That’s why, traditionally, the type of software we build has been done with lower level languages like C and C++. We still, today, write and maintain quite a bit of software in C. There are some problems where C is still the correct option. That domain of C — and to a lesser extent, processor-specific assembly code — has been largely unassailable for decades as we’ve developed languages that make writing software faster and easier, but at the cost of efficiency. That’s been a great detriment to the entire industry because of how easy it is to write unsafe C code. We believe that Rust has finally been the language to change that. It allows us to write highly efficient code while also providing incredible safety.

Now, WebAssembly. WebAssembly has the potential to provide something that we’ve never, in the history of computing, managed to accomplish: a common platform. It was designed to run in a browser, but manages to provide the other components that are needed: efficiency, safety, and platform-independence. We imagine a future in which a WebAssembly module can be run in a browser, on your watch, on your phone, on your TV, in the games you play, and inside server software. We’re still a ways off from that and many pieces are still needed. Lucet is our attempt at providing a WebAssembly compiler and runtime that is made to be used across many different use cases. The first one is Fastly’s edge, but we want to see many more.

Fastly on its other products and projects

Limitations in the legacy CDNs that Fastly’s edge cloud platform addresses

A CDN or Content Delivery Network consists of a geographically distributed group of servers that work together to ensure that content requested by a user reaches to them as fast as possible. However, it has many limitations like bulky XML based configuration files and specifications. McMullen adds, “Legacy CDNs suffer from a number of technical limitations that make them particularly ill-equipped to address changing consumer expectations, not to mention, developer and enterprise requirements. We’ve all had those online experiences when a site crashes or is non-responsive when we need it most, and our mission is to fuel the next modern digital experience, an experience that’s fast, secure, and reliable. By and large, traditional CDNs are black box solutions that are limited in their ability to provide real-time visibility and control, largely as a result of their outdated architecture, which adds cost and limits developers’ flexibility to expand on functionality.”

Fastly’s edge cloud platform is not that — rather, it aims to address these limitations by bringing data closer to the user. “As a result, developers have not been truly empowered to pursue digital transformations, despite many attempts for improvement within the industry,” he adds.

What other projects by Fastly we should look forward to

Fastly is continuously contributing towards making the internet better and safer by getting involved in projects like QUIC, Encrypted SNI, and standardizing WASI. Last year Fastly made three of its projects available on Fastly Labs: Terrarium, Fiddle, and Insights.

When asked what else it is working on, McMullen shared, “Fastly Labs is heavily dependent on experimentation. If the experiment goes well and we think it’ll be useful for others, then we release it. We have quite a few experiments currently underway, and many of them are around the items listed in the question: ESNI, QUIC, WASI, as well as others like DNS-over-HTTPS. More iteration on what we have now is also in the cards. Lucet has come a long way, but it still has so much room to grow. Expect to see some pretty compelling developments in performance, safety, and features there.

Follow Tyler McMullen on Twitter: @tbmcmullen

Learn more about Fastly and its edge-cloud platform at Fastly’s official website.

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