Tuesday, 3 January 2017

Debugging AWS Lambda functions locally using VS Code and lambda-local

I've just started using AWS Lambda with node.js. I was able to develop these locally using the lambda-local npm package, e.g. with node.js installed (via brew) and lambda-local installed (using npm) then the following "hello, world" example is run as follows:


'use strict';

console.log('Loading function');

exports.handler = (event, context, callback) => {
    console.log('Received event:', JSON.stringify(event, null, 2));
    console.log('value1 =', event.key1);
    console.log('value2 =', event.key2);
    console.log('value3 =', event.key3);
    callback(null, event.key1);  // Echo back the first key value
    //callback('Something went wrong');



module.exports =
"key1": "hello",
"key2": "lambda",
"key3": "node"


/usr/local/bin/lambda-local -l hellolambda.js -e default event.js

Loading function
info: Logs
info: ------
info: START RequestId: d683128b-ac14-93c3-b2c1-5541f3bb3fda
Received event: {
  "key1": "hello",
  "key2": "lambda",
  "key3": "node"
value1 = hello
value2 = lambda
value3 = node
info: END
info: Message
info: ------
info: hello
info: -----

info: lambda-local successfully complete.

Rather than use bash and vi (I'm running on MacOS) I wanted to use some sort of IDE. VS Code seemed ideal as it's free and it also has builtin node.js debugging. Using it for editing is very simple, just open the folder containing the source. In this case ~/tmp/hellolambda

However, switching to the debugging section and creating the default launch configuration where VS Code will launch node with the specify file as the program doesn't do much good.

This is because when running a lambda locally using local-lambda the program that node needs to run is the local lambda environment that local-lambda creates and for it to launch the lambda function.

This can be simply configured by specifying the local-lambda script as the program (it's a node script) and then passing the lambda script and the event data as arguments using the args key (which isn't included when using the VS Code option to add a configuration). The original example above can be launched using the following configuration.

In the output window at the bottom the results of executing the lambda are shown. Breakpoints can be set and hit.

It's important that each command line argument, i.e. the option and the value are specified separately. Even though '-l' and its value are a pair they are separate command line arguments (2 in total) where "-l ${workspaceRoot}/hellolambda.js" is a single argument.

NOTE: The lambdas I'm writing are also using the AWS DynamoDB. Using a local instance of DynamoDB along with installing the AWS SDK via npm I've been able to successfully invoke local lambdas that have used the local instance of the DB.