Understanding Lambda in Rust

Given the announcement of the Lambda runtime, there is now an officially supported story around writing lambda functions in Rust. I wanted to try for myself and see the amount of effort needed to get a lambda function working, while also diving deeper into whats involved.

This blog post describes how to use the lambda runtime to create a lambda function in Rust, so I wont detail the actual process. Instead I will try to expand on what is actually happening behind the scenes and reasons for certain steps.

Here is some code that creates and invokes a lambda function.

Why we need "[[bin]] bootstrap" and "autobins = false"

Since we are going to include a custom runtime(tokio runtime), we need the executable to be called 'bootstrap'. These settings allow us to dictate these settings.

What is "glibc" and "musl" and whats the difference

This is a much larger topic but I will try to cover the relevant pieces. A computer comprises of layers of abstractions. So for example, if we wish to execute a task in our application, we request a thread from the kernel, which schedules our task to run on a core.

  hardware(cores) <-> kernel(thread management) <-> user space(application)

So to access resources of a computer we need to communicate with the kernel. The kernel in turn exposes the API via a C standard called the libc. glibc (GNU C Library) and musl are two different implementations of this standard.

glibc is the most commonly supported libc but Amazon instances require musl. From it's site: musl is lightweight, fast, simple, free, and strives to be correct in the sense of standards-conformance and safety.

static vs dynamic lib

I was not able to figure out if the musl lib is statically or dynamically linked. If I had to guess, I would say it is dynamically linked so as not to bloat the size of our lambda function. Additionally, since all functions should be targeting musl, it doesn't make sense to include it with each function.

what is a lambda runtime?

The lambda! macro takes an optional field: tokio runtime. As per docs creating a runtime does the following:

One can imagine configuring a runtime to specify the number of threads. Configuration options for the tokio runtime can be found in docs

Btw, as per AWS docs there is a default limit of 1024 threads/processes per function.

Why we need target.x86_64-unknown-linux-musl

The amazon linux instances require musl. For reference here is a list of all the target that the Rust compiler supports!

Why we need linker = "x86_64-linux-musl-gcc"

We need to download the linker so that we can locally compile for the musl target.


The smart folks at AWS have done a lot to make developing for lambda easy. However, it is always nice to understand where and how our code is actually running... yes, event in the world of lambda. In a future post I hope to explore firecracker, the micro-vm(yup written in Rust) where all lambda functions run.