Where can I find the source code for AWS Lambda's dotnetcore3.1 runtime?

Question

AWS publishes a lot of their source code on GitHub, but so far I haven't been able to find the source for the dotnetcore3.1 Lambda Runtime.

I expect this source will be a console application responsible for startup and IoC (using the Lambda configuration to determine the handler class and function, using reflection to instantiate the handler, as well as the serializer specified on the lambda's assembly attribute). So far I have found a number of repositories containing things like NuGet libraries and dotnet CLI tooling--but haven't located the runtime itself.

Is the AWS Lambda dotnetcore3.1 runtime source publically available? Can you point me to the repo and .csproj? Thanks!

Answer 1

The source code for Lambda's dotnet runtime(s) is within the aws/aws-lambda-dotnet repository--it doesn't appear to have a dedicated netcoreapp3.1 library, or a preserved branch, but presumably the 3.1 runtime is contained within this repository's history.

The Amazon.Lambda.RuntimeSupport console application appears to be the entrypoint, which in turn instantiates and invokes the published Handler.

Answer 2

I am not 100% sure, but there is probably nothing like a dedicated ".NET Core Runtime" that is publicly available.

Let's explore how Lambda works to explain why I think that:

First there is an EC2 instance with a host operating system. That's the bare metal your resources are coming from (vCPU, RAM, disk, etc.).

Second, there is a thin management layer for microVMs. For AWS Lambda this is a project called Firecracker.

The microVMs that are started use a micro kernel. In case of AWS Lambda it is the OSv micro kernel. The micro kernel already has some support for runtimes:

OSv supports many managed language runtimes including unmodified JVM, Python 2 and 3, Node.JS, Ruby, Erlang as well as languages compiling directly to native machine code like Golang and Rust.

.NET Core is compiled so I think this falls into the same category. It is a "native" binary, that is self-contained and just started.

You mentioned reflection etc. to run the binary. I think it is not that complicated. This is were the last puzzle piece comes into place: environment variables.

There are a bunch of environment variables thar are set by the "runtime". You can find the full list in the AWS documentation: Defined runtime environment variables.

Every Lambda has a environment variable called _HANDLER which points to the handler. In your case this would be the binary. This would allow the runtime to just "run" your handler.

When the handler runs the underlying AWS SDK is starting some kind of webserver/socket (I think it depends on the framework/language) which exposes everything the "standard" runtime needs to communicate with your code.

So as far as I can tell there is not really a dedicated .NET Core runtime. It is pretty generic all the way down, which simplifies development for the folks at AWS I guess.