How to debug Lambda functions with Visual Studio Code

In this example we will look at how to debug AWS Lambda functions with Visual Studio Code (VS Code) using SST.

SST allows you to build and test Lambda functions locally using Live Lambda Development. This means that you can attach breakpoints and inspect your Lambda functions locally, even if they are invoked remotely.

Here is a video of it in action.

Let’s look at how.

Requirements

Node.js 16 or later
We’ll be using TypeScript in this example but you can use regular JavaScript as well
An AWS account with the AWS CLI configured locally

Create an SST app

Change indicator Let’s start by creating an SST app.

$ npx create-sst@latest --template=base/example vscode
$ cd vscode
$ npm install

By default, our app will be deployed to the us-east-1 AWS region. This can be changed in the sst.config.ts in your project root.

import { SSTConfig } from "sst";

export default {
  config(_input) {
    return {
      name: "vscode",
      region: "us-east-1",
    };
  },
} satisfies SSTConfig;

Project layout

An SST app is made up of two parts.

stacks/ — App Infrastructure

The code that describes the infrastructure of your serverless app is placed in the stacks/ directory of your project. SST uses AWS CDK, to create the infrastructure.
packages/functions/ — App Code

The code that’s run when your API is invoked is placed in the packages/functions/ directory of your project.

Setting up our API

For this example we’ll be testing using a simple API endpoint.

Change indicator Replace your stacks/ExampleStack.ts with the following.

import { Api, StackContext } from "sst/constructs";

export function ExampleStack({ stack }: StackContext) {
  // Create the HTTP API
  const api = new Api(stack, "Api", {
    routes: {
      "GET /": "packages/functions/src/lambda.handler",
    },
  });

  // Show the API endpoint in the output
  stack.addOutputs({
    ApiEndpoint: api.url,
  });
}

Adding function code

Our functions are stored in the packages/functions/ directory. In this case, we’ll have a simple Lambda function that’s printing out the time the request was made.

Change indicator Create a packages/functions/src/lambda.ts with.

import { APIGatewayProxyEventV2, APIGatewayProxyHandlerV2 } from "aws-lambda";

export const handler: APIGatewayProxyHandlerV2 = async (
  event: APIGatewayProxyEventV2
) => {
  const message = `The time in Lambda is ${event.requestContext.time}.`;
  return {
    statusCode: 200,
    headers: { "Content-Type": "text/plain" },
    body: `Hello, World! ${message}`,
  };
};

Adding VS Code Launch Configurations

To allow VS Code to set breakpoints and debug our Lambda functions we’ll add it to our Launch Configurations. The starter template should already come with this but you can also add it manually by doing the following.

Change indicator Create a .vscode/launch.json file with.

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug SST Dev",
      "type": "node",
      "request": "launch",
      "runtimeExecutable": "${workspaceRoot}/node_modules/.bin/sst",
      "runtimeArgs": ["start", "--increase-timeout"],
      "console": "integratedTerminal",
      "skipFiles": ["<node_internals>/**"]
    }
  ]
}

Extending Lambda function timeouts

Since we are going to set breakpoints in our Lambda functions, it makes sense to increase the timeouts.

SST has an --increase-timeout option that increases the function timeouts in your app to the maximum 15 minutes. We are using this option in our launch.json.

"runtimeArgs": ["start", "--increase-timeout"],

Note that, this doesn’t increase the timeout of an API. Since those cannot be increased for more than 30 seconds. But you can continue debugging the Lambda function, even after the API request times out.

Starting your dev environment

Now if you open up your project in VS Code, you can set a breakpoint in your packages/functions/src/lambda.ts.

Next, head over to the Run And Debug tab > select the above configured Debug SST Dev, and hit Play.

Set Lambda function breakpoint in VS Code

The first time you start the Live Lambda Development environment, it’ll take a couple of minutes to do the following:

It’ll bootstrap your AWS environment to use CDK.
Deploy a debug stack to power the Live Lambda Development environment.
Deploy your app, but replace the functions in the packages/functions/ directory with ones that connect to your local client.
Start up a local client.

Once complete, you should see something like this.

===============
 Deploying app
===============

Preparing your SST app
Transpiling source
Linting source
Deploying stacks
dev-vscode-ExampleStack: deploying...

 ✅  dev-vscode-ExampleStack


Stack dev-vscode-ExampleStack
  Status: deployed
  Outputs:
    ApiEndpoint: https://siyp617yh1.execute-api.us-east-1.amazonaws.com

The ApiEndpoint is the API we just created. Now if you head over to that endpoint in your browser, you’ll notice that you’ll hit the breakpoint.

Hitting a breakpoint in a Lambda function in VS Code

Making changes

An advantage of using the Live Lambda Development environment is that you can make changes without having to redeploy them.

Change indicator Replace packages/functions/src/lambda.ts with the following.

import { APIGatewayProxyEventV2, APIGatewayProxyHandlerV2 } from "aws-lambda";

export const handler: APIGatewayProxyHandlerV2 = async (
  event: APIGatewayProxyEventV2
) => {
  return {
    statusCode: 200,
    headers: { "Content-Type": "text/plain" },
    body: `Hello, World! Your request was received at ${event.requestContext.time}.`,
  };
};

Now if you head back to the endpoint.

https://siyp617yh1.execute-api.us-east-1.amazonaws.com/

You should see the new message being printed out.

Deploying your API

Now that our API is tested and ready to go. Let’s go ahead and deploy it for our users.

However, we are going to deploy your API again. But to a different environment, called prod. This allows us to separate our environments, so when we are working in our local stage, it doesn’t break the API for our users.

Change indicator Run the following in your terminal.

$ npx sst deploy --stage prod

A note on these environments. SST is simply deploying the same app twice using two different stage names. It prefixes the resources with the stage names to ensure that they don’t thrash.

Cleaning up

Finally, you can remove the resources created in this example using the following command.

$ npx sst remove

And to remove the prod environment.

$ npx sst remove --stage prod

Conclusion

And that’s it! You’ve got a brand new serverless API. A local development environment, to test and make changes. And you can use Visual Studio Code to debug and set breakpoints in your Lambda functions. Check out the repo below for the code we used in this example. And leave a comment if you have any questions!