AWS Lambda Extensions with Rust

What are Lambda Extensions? <<

AWS Lambda Extensions were introduced a while ago, with the purpose of providing a way to integrate with the Lambda execution lifecycle. They supply a solution for being able to enhance our Lambda functions with our existing monitoring, observability, security, and governance tools. This can be accomplished by developing some code that runs in parallel with our Lambda function and listens to events emitted by the Lambda runtime. These events can be lifecycle events such as Init , Invoke , Shut Down events, telemetry events, or even log events, generated each time the Lambda functions write a message to its standard output.

There is no complex installation or configuration for us to use Lambda Extensions. They are usually deployed as Lambda Layers, which can be easily attached to a Lambda function.

Some use cases for extensions are the following:

We can have 2 types of extensions :

In the upcoming lines, we will discuss how to build external extensions for our functions. External extensions provide higher flexibility compared to internal extensions, but they also come with their caveats.

Why Rust? <<

Extensions can be developed in any language of choice. Since external extensions run in parallel with the actual function, they can be written in an entirely different programming language compared to the one chosen for the lambda. While it's good to have this freedom of choice, AWS documentation recommends using a compiled language. The reasons for this are the following:

Besides these two reasons, a third reason would be the presence of great tooling. While developing extensions on our local machine is still a challenge, we have some great open-source tools and libraries to alleviate some of the hurdles. In this blog post, we will use cargo-lambda to bootstrap and deploy an extension project.

Let's Bootstrap and Deploy an Extension <<

We will use cargo-lambda for our extension. cargo-lambda is an open-source tool, the purpose of which is to help developers to build Lambda Functions and Lambda Extensions. It can generate and bootstrap Rust projects for both lambdas and extensions. It can help to build and compile those projects. This is important since lambdas and extensions need to be cross-compiled to a Linux executable binary. cargo-lambda makes this seamless from both Windows and macOS environments.

The first step would be to create a Rust project for our extension. This can be accomplished with the following cargo-lambda command:

cargo lambda new --extension project-name --logs

Notice that --logs flag at the end of the command. The presence of this flat will make cargo-lambda generate a project with events needed for the Logs API . Another option would be --telemetry flag, which would bootstrap a project with the telemetry API calls . We can choose any of those, the main difference would be the initial, generated Rust code. Since all the required dependencies for the extension are in the same crate (which is added as a dependency), we can just simply transform our project as we wish afterward.

We can build our project using the following command:

cargo lambda build --extension --release

This will build our extension in release mode, targeting x86-64 architecture (even if we are on a Mac M1). If we want to build it for arm64 , we can add the --arm flag in the end.

Now that we have an existing binary built, we would want to deploy this binary to AWS. We can do this using another cargo-lambda command:

cargo lambda deploy --extension

This will deploy our extension to AWS in form of a Lambda Layer. As we've been discussing previously, our extension should be able to run beside any Lambda Runtime. By default, the deploy command will only enable compatibility for provided.al2 runtime (essentially Rust, or any other compiled Lambda function). To enable it for other runtimes such as NodeJS or Python, we can add the --compatible_runtimes flag, for example:

cargo lambda deploy --extension --compatible_runtimes=provided.al2,nodejs16.x,python3.9

A whole list with all the compatible runtimes can be found in the AWS documentation . As a side note, I have to mention that this feature for supporting other runtimes, was implemented by myself for the cargo-lambda project. I hope that other people will find it as useful :)

The last step would be to attach our extension to an existing Lambda function. This can be done from the AWS console by simply attaching a Lambda Layer to a function.

By following these steps, we have created and deployed an extension that does essentially nothing useful. Moving on, we will develop an extension that listens to Lambda log messages and sends them to a Kinesis Firehose stream.

Develop a Log Router Extension for Kinesis Firehose <<

Many organizations employ a log aggregator framework. The reason for this is to be able to have every log message in one place for easier operational and support tasks, debugging, or even legal purposes. By default, Lambda functions are using CloudWatch for logging. To integrate with another log aggregator, extensions are the perfect solution. In fact, many existing log aggregator products are already providing ready-to-use Lambda Extensions. For example, AWS partners such as Datadog, Dynatrace, Honeycomb, Sumo Logic, etc. have their extensions published publicly, some of them having their code open for everybody. A whole list of partners can be found in the AWS docs .

In case we use an internally developed log aggregator, or the product employed does not provide an extension out of the box, we can create one ourselves. In the following lines, we will see how to build an extension that integrates with Kineses Firehose and saves our log messages into an S3 bucket.

In the previous sections, we've already seen how to bootstrap and deploy an extension. To be able to send messages to Kinesis, we can develop the following code:

use aws_sdk_firehose::error::PutRecordError;
use aws_sdk_firehose::model::Record;
use aws_sdk_firehose::output::PutRecordOutput;
use aws_sdk_firehose::types::{Blob, SdkError};
use aws_sdk_firehose::Client;
use lambda_extension::{service_fn, Error, Extension, LambdaLog, LambdaLogRecord, SharedService};
use lazy_static::lazy_static;
use std::env;


// Read the stream name from an environment variable
lazy_static! {
    static ref STREAM: String = env::var(ENV_STREAM_NAME).unwrap_or_else(|e| panic!(
        "Could not read environment variable {}! Reason: {}",
        ENV_STREAM_NAME, e

async fn main() -> Result<(), Error> {
    println!("Loading extension...");
    // Register the handler to our extension
    let logs_processor = SharedService::new(service_fn(handler));



async fn handler(logs: Vec<LambdaLog>) -> Result<(), Error> {
    // Build the Kinesis Firehose client
    let firehose_client = build_firehose_client().await;
    // Listen to all the events emitted when a Lambda Function is logging something. Send these
    // events to a Firehose delivery stream
    for log in logs {
        match log.record {
            LambdaLogRecord::Function(record) | LambdaLogRecord::Extension(record) => {
                put_record(&firehose_client, STREAM.as_str(), &record).await?;
            _ => (),

// Build the Firehose client
async fn build_firehose_client() -> Client {
    let region_provider = RegionProviderChain::default_provider();
    let shared_config = aws_config::from_env().region(region_provider).load().await;
    let client = Client::new(&shared_config);

// Send a message to the Firehose stream
async fn put_record(
    client: &Client,
    stream: &str,
    data: &str,
) -> Result<PutRecordOutput, SdkError<PutRecordError>> {
    let blob = Blob::new(data);


The code itself is pretty self-explanatory. Besides having some initial boilerplate code to register our extension handler, what we are doing is listening to log events and sending those events to a Firehose delivery stream. The stream will batch the incoming events and save them in an S3 bucket.

In terms of IAM permissions, we need to give Firehose write permission to the Lambda Function. We cannot give permissions to a Lambda Layer. Since our code for the extension is running beside the lambda, all the permissions applied to lambda are available for the extensions as well.

Putting these all together <<

Developing and deploying Lambda Extensions can be tedious work as we have seen above. To make our life easier (and also for myself to provide a reproducible example of what I was talking about before), we can write some IaC Terraform code for the whole deployment process.

A working example of a Lambda Function with a Rust Extension can be found on my GitHub page: . It is a Terragrunt project requiring a current version of Terraform (>1.3.0) and Rust (>1.6.3).

References <<

  1. Introducing AWS Lambda Extensions: AWS blogs
  2. Lambda Extension API: AWS docs
  3. Getting the most of AWS Lambda free compute - wrapper scripts:
  4. Sidecar pattern:
  5. cargo-lambda: GitHub
  6. Lambda Logs API: AWS docs
  7. Extension Partners: AWS docs