ComLightInterop VS Rouille, Rust web server middleware

I have once made something remotely similar, to interop between C++ and C#: https://github.com/Const-me/ComLightInterop

I took different approach. Because I only needed to support these two languages, there’s no separate interface definition language, and no code generator for interfaces. Instead, users are expected to write both language projections manually.

Then there’s a runtime code generator on the .NET side of the interop which builds runtime callable proxy types for interfaces implemented in C++, also virtual tables for C# objects consumed by C++.

It’s pretty fast. Likely reason for that, MS designed both language and runtime this way since version 1.0. They needed that for their Windows Forms which consumes huge chunk of WinAPI.

I benchmarked a while ago when testing this library https://github.com/Const-me/ComLightInterop#performance On the computer I was using at that time (probably Ryzen 5 3600 CPU) the overhead was 15-20 nanoseconds per call.

That thing is COM, which is a small subset of C++ ABI. Technically it’s about the same as on Windows, i.e. C ABI with extra first argument for this pointer.

Once upon a time I made this library https://github.com/const-me/comlightInterop/ The native side of the interop is idiomatic C++, here’s an example https://github.com/Const-me/ComLightInterop/blob/master/Demo... The C# side of the interop is implemented through the built-in C interop, here’s the relevant part of the library https://github.com/Const-me/ComLightInterop/blob/master/ComL... I’ve tested Linux version of that library on AMD64, ARMv7, and ARM64 CPUs, but only with gcc compiler on the native side.

I like many parts of COM, but I believe that example mostly demonstrates bad parts, with IDL, registrations, and over-engineered support libraries.

There's nothing wrong with exporting factory functions from DLLs. Microsoft does it all the time, APIs like Direct3D, DirectDraw and Media Foundation don't come with type libraries are they aren't registered anywhere.

Speaking about support libraries, I once made my own: https://github.com/Const-me/ComLightInterop/tree/master/ComL... Compare examples from that article with this one: https://github.com/Const-me/ComLightInterop/blob/master/Demo... That source file is the complete DLL which implements a minimalistic COM object.

> Does it feel "brittle" to use

Yes and no.

No because when you try to do unsupported things like calling a method on an object which doesn’t support one, you gonna get an appropriate runtime exception.

Yes because if you fail lower-level things like local parameter allocation, you gonna get an appropriate runtime exception but that one is (1) too late, I’d prefer such things to be detected when you emit the code, not when trying to use the generated code (2) Lacks the context.

Overall, when I can I’m using that higher-level System.Linq.Expressions for runtime codegen. Things are much nicer at that level. I only using the low-level thing when I need to emit new types, like there: https://github.com/Const-me/ComLightInterop/blob/master/ComL...

> Do you have any good resources on writing dlls to consume via .net like you’re talking about?

For C APIs i.e. functions, structures and strings, the good resource is Microsoft documentation, the support is built-in, see “Consuming Unmanaged DLL Functions” section: https://docs.microsoft.com/en-us/dotnet/framework/interop/

For COM APIs i.e. sharing objects around see this library + demos: https://github.com/Const-me/ComLightInterop It’s only really needed on Linux because the desktop version of the framework has COM support already built-in, but it can be used for cross-platform things just fine, I tested that quite well i.e. not just with these simple demos.

> How do you deal with the managed memory when using the gc from .net

Most of the time, automatically.

When you calling C++ from C#, the runtime automatically pins arguments like strings or arrays. Pinning means until the C++ function returns, .NET GC won’t touch these things. This doesn’t normally make any copies: C++ will receive raw pointers/native references to the .NET objects.

Sometimes you do want to retain C# objects from C++ or vice versa i.e. keep them alive after the function/method returns. An idiomatic solution for these use cases is COM interop. IUnknown interface (a base interface for the rest of COM interfaces) allows to retain/release things across languages.

C++ interop is not supported in modern .NET out of the box, but wasn't too hard to implement as a library: https://github.com/Const-me/ComLightInterop

Your CRUD web application server almost certainly doesn't need async Rust. Using a blocking HTTP server is not "might be a good idea", it simply is a good idea.

I recommend Rouille for this: https://github.com/tomaka/rouille. In case you are worried about performance, check the benchmark. Blocking Rouille is faster than builtin async server in Node.js.

As you say, the majority of the web ecosystem in Rust has moved to async - but if you’re happy to stray a bit from the beaten path then rouille might do the trick.

In strongly typed languages like Rust, composing smaller libraries is usually quite painless, so you don't need a large framework.

Personally for backend Rust I use rouille[0] for the server (it's very simple and async-free), askama[1] for compile-time HTML templates and (if a SPA is unavoidable, as that is of course always to be avoided if at all possible) yew[2] for client-side WASM.

Now this stack is what I like personally, but there are many options that you can combine, some more full-featured than others. Check out https://www.arewewebyet.org/ for a partial overview.

[0]: https://github.com/tomaka/rouille

[1]: https://github.com/djc/askama

[2]: https://yew.rs/

rouille

I'd like to put in a word for a simple, sync framework such as rouille. The compile times are much, much better, the number of dependencies is much smaller, the stuff it's built on (the standard library) is extensively tested and extremely reliable. Kernel context switches are slower than userspace thread scheduling, but not much slower, and as long as your services aren't just shoving bytes from one place to another (i.e. actually doing some computation) the time taken for a context switch vanishes into noise. A lot of benchmarks test how quickly a web service can move bytes, which (if your business logic is non-trivial) actually isn't the most critical factor.

I don't have any Rust-relevant experience here, but if I wanted to build a web server in Rust and was okay with "reasonable" performance, I'd probably give rouille a try first.

Rouille is fairly solid in my experience. Save the pain of async and spend it building software that works. Honestly with Rust's lack of GC you get predictable response times already.

How would you differentiate it from let's say Rouille ?