How to deploy Lambda functions in Rust

Learning how to use Lambda functions written in Rust is an extremely useful skill for devs today. AWS Lambda provides an on-demand computing service without needing to deploy or maintain a long-running server, removing the need to install and upgrade an operating system or patch security vulnerabilities.

In this article, we’ll learn how to create and deploy a Lambda function written in Rust. All of the code for this project is available on my GitHub for your convenience.

Jump Ahead:

• Cold starts and Rust
• Getting started
  • Generate boilerplate with the new command
  • Cargo manifest file
• Start a development server with the watchcommand
  • Include additional information in our project
  • Send a request with the invoke command
• Deploy the project to production
  • Bundle function artifacts with the buildcommand
  • Create IAM role
  • Upload to AWS with the deploy command
  • Alternative deployment options

Cold starts and Rust

One issue that has persisted with Lambda over the years has been the cold start problem.

A function cannot spin up instantly, as it requires usually a couple of seconds or longer to return a response. Different programming languages have different performance characteristics; some languages like Java will have much longer cold start times because of their dependencies, like the JVM.

Rust on the other hand has extremely small cold starts and has become a programming language of choice for developers looking for significant performance gains by avoiding larger cold starts.

Let’s jump in.

Getting started

To begin, we’ll need to install cargo with rustup and cargo-lambda. I will be using Homebrew because I’m on a Mac, but you can take a look at the previous two links for installation instructions on your own machine.

curl https://sh.rustup.rs -sSf | sh
brew tap cargo-lambda/cargo-lambda
brew install cargo-lambda

Generate boilerplate with the new command

First, let’s create a new Rust package with the new command. If you include the --http flag, it will automatically generate a project compatible with Lambda function URLs.

cargo lambda new --http rustrocket
cd rustrocket

This generates a basic skeleton to start writing AWS Lambda functions with Rust. Our project structure contains the following files:

.
├── Cargo.toml
└── src
    └── main.rs

main.rs under the src directory holds our Lambda code written in Rust.

We’ll take a look at that code in a moment, but let’s first take a look at the only other file, Cargo.toml.

Cargo manifest file

Cargo.toml is the Rust manifest file and includes a configuration file written in TOML for your package.

# Cargo.toml

[package]
  name = "rustrocket"
  version = "0.1.0"
  edition = "2021"

[dependencies]
  lambda_http = "0.6.1"
  lambda_runtime = "0.6.1"
  tokio = { version = "1", features = ["macros"] }
  tracing = { version = "0.1", features = ["log"] }
  tracing-subscriber = { version = "0.3", default-features = false, features = ["fmt"] }

• The first section in all Cargo.toml files defines a package. The name and version fields are the only required pieces of information — an extensive list of additional fields can be found in the official packagedocumentation
• The second section specifies our dependencies, along with the version to install

We’ll only need to add one extra dependency (serde) for the example we’ll create in this article.

Serde is a framework for serializing and deserializing Rust data structures. The current version is 1.0.145 as of the time of writing his article, but you can install the latest version with the following command:

cargo add serde

Now our [dependencies] includes serde = "1.0.145".

Start a development server with the watch command

Now, let’s boot up a development server with the watchsubcommand to emulate interactions with the AWS Lambda control plane.

cargo lambda watch

Since the emulator server includes support for Lambda function URLs out of the box, you can open localhost:9000/lambda-url/rustrocket to invoke your function and view it in your browser.

Open main.rs in the src directory to see the code for this function.

// src/main.rs

use lambda_http::{run, service_fn, Body, Error, Request, RequestExt, Response};

async fn function_handler(event: Request) -> Result<Response<Body>, Error> {
    let resp = Response::builder()
        .status(200)
        .header("content-type", "text/html")
        .body("Hello AWS Lambda HTTP request".into())
        .map_err(Box::new)?;
    Ok(resp)
}

#[tokio::main]
async fn main() -> Result<(), Error> {
    tracing_subscriber::fmt()
        .with_max_level(tracing::Level::INFO)
        .with_target(false)
        .without_time()
        .init();
    run(service_fn(function_handler)).await
}

Now, we’ll make a change to the text in the body by including a new message. Since the header is already set to text/html, we can wrap our message in HTML tags.

Our terminal has also displayed a few warnings, which can be fixed with the following changes:

• Rename event to _event
• Remove the unused RequestExt from imports

// src/main.rs

use lambda_http::{run, service_fn, Body, Error, Request, Response};

async fn function_handler(_event: Request) -> Result<Response<Body>, Error> {
    let resp = Response::builder()
        .status(200)
        .header("content-type", "text/html")
        .body("<h2>Hello from LogRocket</h2>".into())
        .map_err(Box::new)?;
    Ok(resp)
}

The main() function will be unaltered. Return to your browser to see the change:

Include additional information in our project

Right now, we have a single function that can be invoked to receive a response, but this Lambda is not able to take specific input from a user and our project cannot be used to deploy and invoke multiple functions.

To do this, we’ll create a new file, hello.rs, inside a new directory called, bin.

mkdir src/bin
echo > src/bin/hello.rs

The bin directory gives us the ability to include multiple function handlers within a single project and invoke them individually.

We also have to add a bin section at the bottom of our Cargo.toml file containing the name of our new handler, like so:

# Cargo.toml

[[bin]]
  name = "hello"

Include the following code to transform the event payload into a Response that is passed with Ok() to the main() function, like this:

// src/bin/hello.rs

use lambda_runtime::{service_fn, Error, LambdaEvent};
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
struct Request {
    command: String,
}

#[derive(Serialize)]
struct Response {
    req_id: String,
    msg: String,
}

pub(crate) async fn my_handler(event: LambdaEvent<Request>) -> Result<Response, Error> {
    let command = event.payload.command;
    let resp = Response {
        req_id: event.context.request_id,
        msg: format!("{}", command),
    };
    Ok(resp)
}

#[tokio::main]
async fn main() -> Result<(), Error> {
    tracing_subscriber::fmt()
        .with_max_level(tracing::Level::INFO)
        .without_time()
        .init();
    let func = service_fn(my_handler);
    lambda_runtime::run(func).await?;
    Ok(())
}

Send a request with the invoke command

Send requests to the control plane emulator with the invoke subcommand.

cargo lambda invoke hello \
  --data-ascii '{"command": "hello from logrocket"}'

Your terminal will respond back with this:

{
  "req_id":"64d4b99a-1775-41d2-afc4-fbdb36c4502c",
  "msg":"hello from logrocket"
}

Deploy the project to production

Up to this point, we’ve only been running our Lambda handler locally on our own machines. To get this handler on AWS, we’ll need to build and deploy the project’s artifacts.

Bundle function artifacts with the build command

Compile your function natively with the buildsubcommand. This produces artifacts that can be uploaded to AWS Lambda.

cargo lambda build

Create IAM role

AWS IAM (identity and access management) is a service for creating, applying, and managing roles and permissions on AWS resources.

Its complexity and scope of features has motivated teams such as Amplify to develop a large resource of tools around IAM. This includes libraries and SDKs with new abstractions built for the express purpose of simplifying the developer experience around working with services like IAM and Cognito.

In this example, we only need to create a single, read-only role, which we’ll put in a file called rust-role.json.

echo > rust-role.json

We will use the AWS CLI and send a JSON definition of the following role. Include the following code in rust-role.json:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Principal": {
      "Service": "lambda.amazonaws.com"
    },
    "Action": "sts:AssumeRole"
    }
  ]
}

To do this, first install the AWS CLI, which provides an extensive list of commands for working with IAM. The only command we’ll need is the create-role command, along with two options:

• Include the IAM policy for the --assume-role-policy-document option
• Include the name rust-role for the --role-name option

aws iam create-role \
  --role-name rust-role \
  --assume-role-policy-document file://rust-role.json

This will output the following JSON file:

{
  "Role": {
    "Path": "/",
    "RoleName": "rust-role2",
    "RoleId": "AROARZ5VR5ZCOYN4Z7TLJ",
    "Arn": "arn:aws:iam::124397940292:role/rust-role",
    "CreateDate": "2022-09-15T22:15:24+00:00",
    "AssumeRolePolicyDocument": {
      "Version": "2012-10-17",
      "Statement": [
        {
          "Effect": "Allow",
          "Principal": {
            "Service": "lambda.amazonaws.com"
          },
          "Action": "sts:AssumeRole"
        }
      ]
    }
  }
}

Copy the value for Arn, which in my case is arn:aws:iam::124397940292:role/rust-role, and include it in the next command for the --iam-role flag.

Upload to AWS with the deploy command

Upload your functions to AWS Lambda with the deploy subcommand. Cargo Lambda will try to create an execution role with Lambda’s default service role policy AWSLambdaBasicExecutionRole.

If these commands fail and display permissions errors, then you will need to include the AWS IAM role from the previous section by adding --iam-role FULL_ROLE_ARN.

cargo lambda deploy --enable-function-url rustrocket
cargo lambda deploy hello

If everything worked, you will get an Arn and URL for the rustrocket function and an Arn for the hello function, as shown in the image below.

🔍 function arn: arn:aws:lambda:us-east-1:124397940292:function:rustrocket
🔗 function url: https://bxzpdr7e3cvanutvreecyvlvfu0grvsk.lambda-url.us-east-1.on.aws/

Open the function URL to see your function:

For the hello handler, we’ll use the invoke command again, but this time with a --remote flag for remote functions.

cargo lambda invoke \
  --remote \
  --data-ascii '{"command": "hello from logrocket"}' \
  arn:aws:lambda:us-east-1:124397940292:function:hello

Alternative deployment options

Up to this point, we’ve only used Cargo Lambda to work with our Lambda functions, but there are at least a dozen different ways to deploy functions to AWS Lambda!

These numerous methods vary widely in approach, but can be roughly categorized into one of three groups:

1. Other open source projects like Cargo Lambda maintained by individuals or collections of people, potentially AWS employees themselves or AWS consultants working with clients looking to build on AWS
2. Tools built by companies who wish to compete with AWS by offering a nicer, more streamlined developer experience
3. Services created and driven by teams internally at AWS who are building new products that seek to improve the lives of existing AWS developers while also bringing in newer and less experienced engineers

For instructions on using tools from the second and third category, see the section called Deploying the Binary to AWS Lambda on the aws-lambda-rust-runtime GitHub repository.

Final thoughts

The future of Rust is bright at AWS. They are heavily investing in the team and foundation supporting the project. There are also various AWS services that are now starting to incorporate Rust, including:

• Firecracker (the underlying infrastructure for Lambda itself)
• Bottlerocket
• EC2

To learn more about running Rust on AWS in general, check out the AWS SDK for Rust. To go deeper into using Rust on Lambda, visit the Rust Runtime repository on GitHub.

I hope this article served as a useful introduction on how to run Rust on Lambda, please leave your comments below on your own experiences!