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LLRT (Low Latency Runtime) is an experimental, lightweight JavaScript runtime designed to address the growing demand for fast and efficient Serverless applications.




LLRT (Low Latency Runtime) is a lightweight JavaScript runtime designed to address the growing demand for fast and efficient Serverless applications. LLRT offers up to over 10x faster startup and up to 2x overall lower cost compared to other JavaScript runtimes running on AWS Lambda

It's built in Rust, utilizing QuickJS as JavaScript engine, ensuring efficient memory usage and swift startup.


LLRT is an experimental package. It is subject to change and intended only for evaluation purposes.

LLRT - DynamoDB Put, ARM, 128MB: DynamoDB Put LLRT

Node.js 20 - DynamoDB Put, ARM, 128MB: DynamoDB Put Node20

HTTP benchmarks measured in round trip time for a cold start (why?)

Configure Lambda functions to use LLRT

Download the last LLRT release from

Option 1: Custom runtime (recommended)

Choose Custom Runtime on Amazon Linux 2023 and package the LLRT bootstrap binary together with your JS code.

Option 2: Use a layer

Choose Custom Runtime on Amazon Linux 2023, upload or as a layer and add to your function

Option 3: Package LLRT in a container image

See our AWS SAM example or:

FROM --platform=arm64 busybox
WORKDIR /var/task/
COPY app.mjs ./
ADD /usr/bin/llrt
RUN chmod +x /usr/bin/llrt

ENV LAMBDA_HANDLER "app.handler"

CMD [ "llrt" ]

That's it 🎉


Even though LLRT supports ES2020 it's NOT a drop in replacement for Node.js. Consult Compatibility matrix and API for more details. All dependencies should be bundled for a browser platform and mark included @aws-sdk packages as external.

Option 4: AWS SAM

The following example project sets up a lambda instrumented with a layer containing the llrt runtime.

Option 5: AWS CDK

You can use cdk-lambda-llrt construct library to deploy LLRT Lambda functions with AWS CDK.

import { LlrtFunction } from "cdk-lambda-llrt";

const handler = new LlrtFunction(this, "Handler", {
  entry: "lambda/index.ts",

See Construct Hub and its examples for more details.

Testing & ensuring compatibility

The best way to ensure your code is compatible with LLRT is to write tests and execute them using the built-in test runner. The test runner currently supports Jest/Chai assertions. There are two main types of tests you can create:

Unit Tests

  • Useful for validating specific modules and functions in isolation
  • Allow focused testing of individual components

End-to-End (E2E) Tests

  • Validate overall compatibility with AWS SDK and WinterCG compliance
  • Test the integration between all components
  • Confirm expected behavior from end-user perspective

For more information about the E2E Tests and how to run them, see here.

Test runner

Test runner uses a lightweight Jest-like API and supports Jest/Chai assertions. For examples on how to implement tests for LLRT see the /tests folder of this repository.

To run tests, execute the llrt test command. LLRT scans the current directory and sub-directories for files that ends with *.test.js or *.test.mjs. You can also provide a specific test directory to scan by using the llrt test -d <directory> option.

The test runner also has support for filters. Using filters is as simple as adding additional command line arguments, i.e: llrt test crypto will only run tests that match the filename containing crypto.

Compatibility matrix


LLRT only support a fraction of the Node.js APIs. It is NOT a drop in replacement for Node.js, nor will it ever be. Below is a high level overview of partially supported APIs and modules. For more details consult the API documentation

Node.js LLRT ⚠️
buffer ✔︎ ✔︎️
streams ✔︎ ✔︎*
child_process ✔︎ ✔︎⏱
net:sockets ✔︎ ✔︎⏱
net:server ✔︎ ✔︎
tls ✔︎ ✘⏱
fetch ✔︎ ✔︎
http ✔︎ ✘⏱**
https ✔︎ ✘⏱**
fs/promises ✔︎ ✔︎
fs ✔︎ ✘⏱
path ✔︎ ✔︎
timers ✔︎ ✔︎
uuid ✔︎ ✔︎
crypto ✔︎ ✔︎
process ✔︎ ✔︎
encoding ✔︎ ✔︎
console ✔︎ ✔︎
events ✔︎ ✔︎
ESM ✔︎ ✔︎
CJS ✔︎ ✔︎
async/await ✔︎ ✔︎
Other modules ✔︎

⚠️ = partially supported in LLRT ⏱ = planned partial support * = Not native ** = Use fetch instead

Using node_modules (dependencies) with LLRT

Since LLRT is meant for performance critical application it's not recommended to deploy node_modules without bundling, minification and tree-shaking.

LLRT can work with any bundler of your choice. Below are some configurations for popular bundlers:


esbuild index.js --platform=node --target=es2020 --format=esm --bundle --minify --external:@aws-sdk --external:@smithy --external:uuid


import resolve from "@rollup/plugin-node-resolve";
import commonjs from "@rollup/plugin-commonjs";
import terser from "@rollup/plugin-terser";

export default {
  input: "index.js",
  output: {
    file: "dist/bundle.js",
    format: "esm",
    sourcemap: true,
    target: "es2020",
  plugins: [resolve(), commonjs(), terser()],
  external: ["@aws-sdk", "@smithy", "uuid"],


import TerserPlugin from "terser-webpack-plugin";
import nodeExternals from "webpack-node-externals";

export default {
  entry: "./index.js",
  output: {
    path: "dist",
    filename: "bundle.js",
    libraryTarget: "module",
  target: "web",
  mode: "production",
  resolve: {
    extensions: [".js"],
  externals: [nodeExternals(), "@aws-sdk", "@smithy", "uuid"],
  optimization: {
    minimize: true,
    minimizer: [
      new TerserPlugin({
        terserOptions: {
          ecma: 2020,

Using AWS SDK (v3) with LLRT

LLRT includes many AWS SDK clients and utils as part of the runtime, built into the executable. These SDK Clients have been specifically fine-tuned to offer best performance while not compromising on compatibility. LLRT replaces some JavaScript dependencies used by the AWS SDK by native ones such as Hash calculations and XML parsing. V3 SDK packages not included in the list below have to be bundled with your source code while marking the following packages as external:

Bundled AWS SDK packages


LLRT currently does not support returning streams from SDK responses. Use response.Body.transformToString(); or response.Body.transformToByteArray(); as shown below.

const response = await client.send(command);
// or 'transformToByteArray()'
const str = await response.Body.transformToString();

Running TypeScript with LLRT

Same principle as dependencies applies when using TypeScript. TypeScript must be bundled and transpiled into ES2020 JavaScript.


LLRT will not support running TypeScript without transpilation. This is by design for performance reasons. Transpiling requires CPU and memory that adds latency and cost during execution. This can be avoided if done ahead of time during deployment.


What justifies the introduction of another JavaScript runtime in light of existing options such as Node.js, Bun & Deno?

Node.js, Bun, and Deno represent highly proficient JavaScript runtimes. However, they are designed with general-purpose applications in mind. These runtimes were not specifically tailored for the demands of a Serverless environment, characterized by short-lived runtime instances. They each depend on a (Just-In-Time compiler (JIT) for dynamic code compilation and optimization during execution. While JIT compilation offers substantial long-term performance advantages, it carries a computational and memory overhead.

In contrast, LLRT distinguishes itself by not incorporating a JIT compiler, a strategic decision that yields two significant advantages:

A) JIT compilation is a notably sophisticated technological component, introducing increased system complexity and contributing substantially to the runtime's overall size.

B) Without the JIT overhead, LLRT conserves both CPU and memory resources that can be more efficiently allocated to code execution tasks, thereby reducing application startup times.


There are many cases where LLRT shows notable performance drawbacks compared with JIT-powered runtimes, such as large data processing, Monte Carlo simulations or performing tasks with hundreds of thousands or millions of iterations. LLRT is most effective when applied to smaller Serverless functions dedicated to tasks such as data transformation, real time processing, AWS service integrations, authorization, validation etc. It is designed to complement existing components rather than serve as a comprehensive replacement for everything. Notably, given its supported APIs are based on Node.js specification, transitioning back to alternative solutions requires minimal code adjustments.

Building from source

Clone code and cd to directory

git clone --recursive
cd llrt

Install git submodules if you've not cloned the repository with --recursive

git submodule update --init

Install rust

curl --proto '=https' --tlsv1.2 -sSf | bash -s -- -y
source "$HOME/.cargo/env"

Install dependencies

# MacOS
brew install zig make cmake zstd node corepack

# Ubuntu
sudo apt -y install make zstd
sudo snap install zig --classic --beta

# Windows WSL2
sudo apt -y install cmake g++ gcc make zip zstd
sudo snap install zig --classic --beta

# Windows WSL2 (If Node.js is not yet installed)
sudo curl -o- | bash
nvm install --lts

Install Node.js packages

corepack enable

Install generate libs and setup rust targets & toolchains

make stdlib && make libs


If these commands exit with an error that says can't cd to zstd/lib, you've not cloned this repository recursively. Run git submodule update --init to download the submodules and run the commands above again.

Build release for Lambda

make release-arm64
# or for x86-64, use
make release-x64

Optionally build for your local machine (Mac or Linux)

make release

You should now have a or You can manually upload this as a Lambda layer or use it via your Infrastructure-as-code pipeline

Running Lambda emulator

Please note that in order to run the example you will need:

  • Valid AWS credentials via a ~/.aws/credentials or via environment variables.
export AWS_REGION=us-east-1
  • A DynamoDB table (with id as the partition key) on us-east-1
  • The dynamodb:PutItem IAM permission on this table. You can use this policy (don't forget to modify <YOUR_ACCOUNT_ID>):
  "Version": "2012-10-17",
  "Statement": [
      "Sid": "putItem",
      "Effect": "Allow",
      "Action": "dynamodb:PutItem",
      "Resource": "arn:aws:dynamodb:us-east-1:<YOUR_ACCOUNT_ID>:table/quickjs-table"

Start the lambda-server.js in a separate terminal

node lambda-server.js

Then run llrt:

make run

Benchmark Methodology

Although Init Duration reported by Lambda is commonly used to understand cold start impact on overall request latency, this metric does not include the time needed to copy code into the Lambda sandbox.

The technical definition of Init Duration (source):

For the first request served, the amount of time it took the runtime to load the function and run code outside of the handler method.

Measuring round-trip request duration provides a more complete picture of user facing cold-start latency.

Lambda invocation results (λ-labeled row) report the sum total of Init Duration + Function Duration.


See CONTRIBUTING for more information.


This library is licensed under the Apache-2.0 License. See the LICENSE file.


LLRT (Low Latency Runtime) is an experimental, lightweight JavaScript runtime designed to address the growing demand for fast and efficient Serverless applications.



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