Lambda Live Debugger is an indispensable tool for debugging AWS Lambda functions from your computer, even though they are deployed in the cloud. It supports Lambdas written in JavaScript or TypeScript. It is free and open source.

This tool offers similar functionality to SST and Serverless Framework v4, with the addition of an Observability Mode.

It supports the following frameworks:

• AWS CDK v2
• Serverless Framework v3 (SLS) and osls fork
• AWS Serverless Application Model (SAM)
• Terraform
• Any other framework or setup by implementing a simple function in TypeScript
• ... (Need support for another framework? Let me know!)

Why?

Serverless is amazing and solves many issues with traditional systems. However, writing code for Lambda functions can be challenging. The cycle of writing, deploying, running, fixing, and redeploying is time-consuming and tedious. You could use tools to run Lambda locally or use unit/integration tests; those approaches often don't replicate the actual environment closely enough.

How It Works

Lambda Live Debugger connects to your deployed Lambda, routes requests to your computer, and sends responses back to the deployed Lambda. This allows you to debug locally, but the system behaves as if the code is running in the cloud with the same permissions. In case of code changes, you do not have to redeploy. The code is reloaded automatically without deploying or even restarting the debugger.

The tool attaches Lambda Extensions (via a Layer), intercepts, and relays calls to AWS IoT. AWS IoT transfers messages between your Lambda and the local machine. If the Lambda is written in TypeScript, it's transpiled to JavaScript. The code is executed via the Node Worker Thread.

AWS keys generated on the cloud for Lambda are transferred to the local environment, so the code has the same permissions as it would executed on the cloud. There could be a difference in packaging, mainly regarding static files, which are probably in different locations. You can use additional environment variables to adjust the code:

• IS_LOCAL = true = Lambda is executed locally
• LOCAL_PROJECT_DIR = directory of the project

Infrastructure Changes

Lambda Live Debugger makes the following changes to your AWS infrastructure:

• Deploys the Lambda Layer
• Attaches the Layer to each Lambda you're debugging
• Adds a policy to the Lambda Role for AWS IoT access

In case you do not want to debug all functions and add the Layer to them, you can limit to the ones you need via the function parameter.

The tool generates temporary files in the .lldebugger folder, which can be deleted after debugging. The wizard can add .lldebugger to .gitignore for you.

Help and Feedback

The tool has been thoroughly tested across many scenarios. It is flexible and can be adjusted to fit almost any setup. However, projects and TypeScript settings can be configured in numerous ways. If you need help or encounter a bug, please let me know by opening a GitHub Issue. This will not only help you but also others, because I can improve the tool and the documentation.

If the tool is working well for you, please fill out the general FEEDBACK FORM. This helps me understand the interest in the tool and encourages further development.

You can also contact me via LinkedIn or email [email protected]. Please check out my blog: www.serverlesslife.com and follow me on X (Twitter).

Reporting an Issue

• Make sure the bug hasn't already been reported. If you find that it has been, add a "+1" comment so that I know there are multiple users struggling with the same issue. If possible, add some additional info.
• Enable verbose logging and provide the full log.
• Specify the exact framework version (CDK, SLS, SAM ...) and the exact Lambda Live Debugger version.
• Describe your setup in detail, or better yet, provide a sample project.

Getting Started

Installation

Install globally:

npm install lambda-live-debugger -g

(On Linux and Mac: sudo npm install lambda-live-debugger -g)

or locally

npm install lambda-live-debugger

(If installed locally, you must prefix all commands with npx, if they are not part of scripts in package.json.)

Running the Debugger

With default profile, region, and other default settings:

lld

You probably need to tweak some settings. You can do it via CLI parameters or, even better, run the wizard:

lld -w

The configuration is saved to lldebugger.config.ts.

CLI Parameters

 -V, --version                   output the version number
 -r, --remove [option]           Remove Lambda Live Debugger infrastructure. Options: 'keep-layer' (default), 'remove-all'. The latest also removes the Lambda Layer
 -w, --wizard                    Program interactively asks for each parameter and saves it to lldebugger.config.ts
 -v, --verbose                   Verbose logging
 -c, --context <context>         AWS CDK context (default: [])
 -s, --stage <stage>             Serverless Framework stage
 -f, --function <function name>  Filter by function name. You can use * as a wildcard
 -m, --subfolder <subfolder>     Monorepo subfolder
 -o, --observable                Observability mode
 -i --interval <interval>        Observability mode interval (default: "3000")
 --config-env <evironment>       SAM environment
 --sam-config-file <file>        SAM configuration file
 --sam-template-file <file>      SAM template file
 --profile <profile>             AWS profile to use
 --region <region>               AWS region to use
 --role <role>                   AWS role to use
 --framework <framework>         Framework to use (cdk, sls, sam, terraform)
 --gitignore                     Add .lldebugger to .gitignore
 -h, --help                      display help for command

Configuration File

Example lldebugger.config.ts:

import { type LldConfigTs } from 'lambda-live-debugger';

export default {
  framework: 'cdk',
  context: ['environment=development'],
  region: 'eu-central-1',
  observable: false,
  verbose: false,
  //getLambdas: async (foundLambdas) => {
  //  you can customize the list of Lambdas here or create your own
  //  return foundLambdas;
  //},
} satisfies LldConfigTs;

The settings are the same as for the CLI parameters.

Debugging

You might want to configure your development tool for debugging. The wizard automatically configures for VsCode in .vscode/launch.json. Here is an example:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Lambda Live Debugger",
      "type": "node",
      "request": "launch",
      "runtimeExecutable": "${workspaceFolder}/node_modules/.bin/lld",
      "runtimeArgs": [],
      "console": "integratedTerminal",
      "skipFiles": ["<node_internals>/**"],
      "env": {}
    }
  ]
}

Now, you have to press F5 or press Run -> Start Debugging, and you can set breakpoints, step through lines of code, inspect variables... For more information on how to debug in VSCode, please refer to this link.

For other tools, please send documentation to include here. WebStorm instructions are especially needed.

Removing

When you no longer want to debug and want Lambda to execute the code deployed to the cloud, you need to remove the Lambda Live Debugger:

lld -r

This detaches the Layer from your Lambdas and removes the IoT permission policy. It will not remove the Layer because others might use it.

To also remove the Layer:

lld -r=all

Development Process

Since you deploy code to a real AWS account, it's best to have a dedicated environment only for yourself. It could be your personal environment or an environment created for a feature. That is common practice when developing serverless systems. If that's not feasible due to organizational or technical reasons, use Observability Mode.

Observability Mode

In Observability Mode, Lambda Live Debugger intercepts requests and sends them to your computer without waiting for a response. The Lambda continues as usual. The response from your machine is ignored. This mode can be used in the development, testing, or even, if you are adventurous, production environment. It samples requests every 3 seconds by default (configurable with an interval setting) to avoid overloading the system.

Monorepo Setup

Set the subfolder parameter if your framework is in a subfolder.

Frameworks

AWS CDK v2

Use the context parameter to pass context to your CDK code. This is a common way to pass variables to your code, most often the environment name.

Serverless Framework v3 (SLS)

Use the stage parameter to pass the stage/environment name.

AWS Serverless Application Model (SAM)

Use the config-env parameter to pass the stage/environment name.

Terraform

Only the basic setup is supported. Check the test case.

I am not a Terraform developer, so I only know the basics. Please provide a sample project so I can build better support.

Custom Setup

Configuration file lldebugger.config.ts enables you to modify the list of Lambdas, change the code path, esBuild configuration, or provide your own list of Lambdas, thereby support any framework. For this to work, install Lambda Live Debugger locally in the project.

getLambdas: async (foundLambdas, config) => {
  //you can customize the list of Lambdas here or create your own
  return foundLambdas;
};

Filter list of functions:

getLambdas: async (foundLambdas, config) => {
  return foundLambdas?.filter((l) => l.functionName.includes('myfunction'));
};

Modify code path:
For example, when the framework only has a list of JavaScript files, but you transpiled them from TypeScript with your own solution.

getLambdas: async (foundLambdas, config) => {
  if (foundLambdas) {
    for (const lambda of foundLambdas) {
      lambda.codePath = lambda.codePath
        .replace('/dist/', '/src/')
        .replace('.js', '.ts');
    }
  }
  return foundLambdas;
};

Modify esBuild configuration:

import { type EsBuildOptions, type LldConfigTs } from "lambda-live-debugger";

export default {
  ...
  getLambdas: async (foundLambdas, config) => {
    if (foundLambdas) {
      for (const lambda of foundLambdas) {
        lambda.esBuildOptions = {
          target: "node14",
        };
      }
    }
    return foundLambdas;
  }
} satisfies LldConfigTs;

Provide your own list of Lambdas and support any framework:

export default {
  //framework: <-- you do not need this line
  ...
  getLambdas: async (foundLambdas, config) => {
    return [
      {
          // function name as deployed on AWS
          functionName: "mystack-myfunction",
          codePath: "/src/myLambda.ts",
      },
    ];
  },
} satisfies LldConfigTs;

Modify existing framework support:

import { CdkFramework, type LldConfigBase, type LldConfigTs } from "lambda-live-debugger";

class MyCdkFramework extends CdkFramework {
  override getCdkContext(cdkConfigPath: string, config: LldConfigBase) {
    // your implementation
  }
}

export default {
  ...
  getLambdas: async (foundLambdas, config) => {
    return new MyCdkFramework().getLambdas(config);
  }
} satisfies LldConfigTs;s

Known Issues

Check the GitHub issues.

Missing Features

For the current list of missing features, please refer to the GitHub issues page. The most significant missing features at the moment are MFA authentication and additional Terraform configurations.

If you have a new feature idea, please create and issue.

Authors

• Marko (ServerlessLife)
• ⭐ Your name here for big code contributions.

Contributors

(alphabetical)

• Kristian Dreher
• Roger Chi
• Sebastian Bille
• ⭐ Your name here for notable code or documentation contributions or sample projects submitted with a bug report that resulted in tool improvement.

Disclaimer

Lambda Live Debugger is provided "as is," without warranty of any kind, expressed or implied. Use it at your own risk, and be mindful of potential impacts on performance, security, and costs when using it in your AWS environment.