James McMurray's Blog

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Writing a simple AWS Lambda Custom Runtime in Rust

I was recently reading more about the lambda_runtime crate and came across this issue where it is mentioned that the #[lambda] procedural macro can be misleading and cause problems if used naively.

In this post we will implement our own simple custom runtime for AWS Lambda in Rust, and understand the reasons behind this issue.

The code used in this blog post is available on Github in my micro_lambda repository.

Opinions expressed are solely my own and do not express the views or opinions of my employer.

AWS Lambda

AWS Lambda is a serverless computing service. I have previously covered setting up serverless data pipelines from Excel files on S3, and from email attachments with SES and Workmail using Rust and the lambda_runtime crate.

Custom Runtimes

A Custom Runtime allows you to run any x86_64 binary as an AWS Lambda function, by providing the runtime code to interact with the AWS Lambda Custom Runtime API.

The idea is you first retrieve some of the Lambda function settings provided to the executable as environment variables, run any one-time or shared initialisation code, and then call the handler function (i.e. the function that will be called for each invocation), passing it the AWS Lambda event JSON.

In summary, the runtime code should:

  1. Read relevant environment variables, in our case only AWS_LAMBDA_RUNTIME_API is necessary since we will include the runtime and handler function together in our final binary.
  2. Run any initialisation code - note in our case this could be defined either as part of the runtime library crate, or in the application code prior to passing the handler function to the Lambda runtime.
  3. Loop indefinitely, polling the Lambda Runtime API for new invocations and sending these to the handler function - then POST the output to either the invocation response or error endpoint.

It's important to note that a single runtime execution may serve several Lambda function invocations if they arrive close together - this is called a warm start. As opposed to a cold start where AWS Lambda has to spin up the container and start the runtime. For functions that need to always respond with a very low latency, some users even schedule dummy invocations (i.e. with a specific Cloudwatch event, etc.) to keep the runtime running and ensure warm starts.

Lambda Runtime API

The AWS Lambda Runtime API currently contains only four endpoints:

  • Initalization error - for if the initialization steps fail (i.e. doing one-time initializations for global resources, etc.) prior to calling the handler function: POST /runtime/init/error
  • Next invocation - an endpoint from which to GET the invocation event and some metadata (AWS Request ID): GET /runtime/invocation/next
  • Invocation response - an endpoint to POST the successful response of the handler function: POST /runtime/invocation/${AwsRequestId}/response
  • Invocation error - an endpoint to POST the error message of the handler function, if it fails: POST /runtime/invocation/${AwsRequestId}/error

The base URL for each API call is http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/ - note we do not need TLS.

We need to extract the AWS Request ID from the headers returned in the next invocation response. The body of the response is the JSON for the triggering event itself (note that the aws_lambda_events crate has templates for deserializing the events from AWS services).

Runtime library

Using all of the above, we can implement a custom runtime in a very small amount of code:

pub fn lambda(handler: fn(&str) -> std::result::Result<String, String>) {
    // Could initialise one-time resources here
    // If initialisation error, POST to /runtime/init/error

    let aws_lambda_runtime_api = std::env::var("AWS_LAMBDA_RUNTIME_API").unwrap();
    // Loop getting new invocation events and passing to handler
    loop {
        let invocation = ureq::get(&format!(

	// Extract AWS Request ID for in order to post responses
        let request_id = invocation

	// Run the handler function and get the output (note above we
	// specified this will be a Result<String, String> )
        let response = handler(invocation.into_string().unwrap().as_str());

        match response {
	    // If succeeded post output to response
            Ok(res) => {
                let _resp = ureq::post(&format!(
                    aws_lambda_runtime_api, request_id

            Err(err) => {
                // If invocation error, POST to /runtime/invocation/AwsRequestId/error
                let _resp = ureq::post(&format!(
                    aws_lambda_runtime_api, request_id

In this case we just call lambda() with a function that takes a &str (which will be the event JSON), and returns a Result<String, String>.

Note that in practice you would want to support different response and error types (perhaps anything that implements Display), and possibly also asynchronous handler functions, etc. - this is what the lambda_runtime crate does.

This code is available on Github in my micro_lambda repository.

If you want to follow along, the tutorial on publishing a custom runtime is also very useful - covering an example of an AWS Lambda custom runtime in bash.

Example Lambda function

Using the above library crate, a Lambda function might look like:

use micro_lambda::lambda;

fn main() {
    // Put one-time initialisations (loggers, etc.)
    // before calling lambda()

fn handler(event: &str) -> std::result::Result<String, String> {
    println!("{}", event);
    if event.contains("fail") {
        return Err("ERROR".to_string());


We can then upload this to AWS Lambda, and verify that it works for a successful test case:

Success test case

And for an error test case:

Error test case

Note when deploying, you will likely want to use static linking by using the musl target to avoid issues with glibc version mismatches:

$ cargo build --release --target=x86_64-unknown-linux-musl
$ cp ./target/x86_64-unknown-linux-musl/release/bootstrap ./
$ strip --strip-all ./bootstrap
$ zip bootstrap.zip bootstrap

If you do use dynamic linking with glibc, then consider setting the Lambda function to use Amazon Linux 2 (AL2) in the AWS Lambda Console, so the container will have a newer version of glibc at least.

lambda_runtime and #[lambda]

Now we can understand the comment about the #[lambda] macro in the aforementioned Github issue:

Due to how cold/warm starts work on Lambda, the naive approach of setting up resources (such as loggers) within a #[lambda]-decorated block will cause the logger initializer to panic in a warm start.

The #[lambda] macro allows you to write simple code like:

async fn main(event: Value, _: Context) -> Result<Value, Error> {

But it is important to note that in this case the function being wrapped is treated as the handler function (here is the relevant code). That is, any initialisation should take place before the handler function is called, so that it is only run once even if there are multiple invocations before the runtime is terminated (this is a "warm start").

This causes problems when new users see the above example, and put logger intialisation code, etc. in there directly, which will then fail in the cases of warm starts - which might appear as sporadic and confusing failures to the developer.

This can be fixed by separating out the handler invocation, just like we did in our earlier code. In this case it could be:

async fn main() -> Result<(), Error> {
    // Put any one-time initialisation code up here
    // Before lambda::run is called!
    let func = handler_fn(func);

async fn func(event: Value, _: Context) -> Result<Value, Error> {


I hope it's clear now how the #[lambda] macro can cause confusion, and how to avoid this issue if you write your own Lambda functions.

This might seem a bit overkill for covering such a simple issue, but as Richard Feynman said:

What I cannot create, I do not understand.

If you look at the the micro_lambda repository, you'll see I tried to reduce the binary size by some means. I'd originally wanted to write this as a #[no_std] crate but the requirement to make HTTP requests made this too difficult, considering the networking required. If you do know how to handle this well please create an issue or PR on the repo!

Another possible extension of this would be to support the AWS Lambda Extensions API.