Congratulations, it’s a lambda

Your first AWS Lambda function exposed via an API Gateway REST interface

AWS Lambda with API Gateway

The purpose of this article is to help you setup a simple AWS lambda function and expose it as a public REST service via the API Gateway. We hope to show you how to do the following:

• Create the lambda function
• Create a test for it
• Create an API Gateway
• Wire up a REST call to the lambda function with mapped parameters

Requirements

• AWS account

Create Lambda Function

• Login to AWS console, search Lambda, and click resulting item, Lambda
• Click Create function, Author from scratch
• Enter baby-lambda as name, Node.js 8.10 as Runtime
• Select Create Role, enter babyLambdaRole as name, use Simple microservice permissions template

Boilerplate Test

At this point, you should have a boilerplate hello world lambda function.

• Click the Test button in the upper right of your screen
• Enter babyLambdaEvent into the name field
• Leave the payload alone, it doesn’t matter at this point
• Click Create
• You should now have a babyLambdaEvent in the dropdown, click Test again
• Check the details of the response and see the hardcoded, Hello from Lambda! response

Initial baby-lambda.js

So far we have setup a simple boilerplate hello world lambda function. Let's go ahead and add our first pass of the baby-lambda.js code. Enter the code below into the Function Code section

exports.handler = async (event) => {
    const months = 23;
    const lbs = 100;
    let message = "not a baby";
    if(months < 24) {
        message = lbs > 30?"it's a big baby":"it's a baby"
    }
    const response = {
        statusCode: 200,
        body: JSON.stringify({months, lbs, message}),
    };
    return response;
};

Setup AWS API Gateway

The lambda function above is simply a function living in the aws lambda environment but we have not wired it up to receive RESTful requests and pass external parameters to it. In order to expose this lambda function we will create an api gateway and connect an http action to the lambda entrypoint.

Create

• Search API Gateway and click resulting item, API Gateway
• Click Get Started
• Select REST as protocol, Select New API, Enter babyLambdaAPI as the name, Enter a description
• Click Create API

Configure

• Select Actions => Create Method, Select GET action, Click checkmark
• Select Lambda Function as Integration Type, Enter baby-lambda as Lambda Function
• Click Save
• Click OK on the Add Permission to Lambda Function

Handle Variables

• Select the Integration Request cell in the GET - Method Execution detail
• Select when there are no templates defined in the Mapping Templates => Request Body Passthrough
• Select Add mapping template, enter application/json, click the checkmark

Add the json template below into the application/json template

{
  "lbs": "$input.params('lbs')",
  "months": "$input.params('months')"
}

Deploy API Gateway

• Select Actions => Deploy API
• Select [New Stage] for Deployment Stage => Enter test for name
• Click the Invoke URL and you should see a similar response to the Test you created

Update the baby-lambda.js function

Our lambda function has hardcoded variables and now we’d like to pass through the lbs and months from the api gateway to the lambda function itself.

Update Code

• Search for Lambda and select the baby-lambda function
• Change the code in the function to use the event.lbs and event.months

exports.handler = async (event) => {
    const months = event.months;
    const lbs = event.lbs;
    if( !months || !lbs) {
      return {
        statusCode: 400,
        body: JSON.stringify({error: "You must give a weight in lbs and age in months"}),
      }
    }
    let message = "not a baby";
    if(months < 24) {
        message = lbs > 30?"it's a big baby":"it's a baby";
    }
    const response = {
        statusCode: 200,
        body: JSON.stringify({months, lbs, message}),
    };
    return response;
};

Update Test(s)

Select the babyLambdaEvent, Select configure test events, select the babyLambdaEvent and change its content to

{
  "lbs": 10,
  "months": 19
}

• Create another event named bigBabyEvent and change the lbs to 40
• Create another event named errorEvent and make it read {}
• Select each event and run a Test for it and verify the output is correct

Verify the API Gateway hits the Lambda function

• Search API Gateway and click resulting item, API Gateway
• Click babyLambdaAPI, Stages, test, GET, Invoke URL
• You should get {"statusCode":400,"body":"{\"error\":\"You must give a weight in lbs and age in months\"}"}
• Add ?lbs=9&months=36 to the Invoke URL and you should get - {"statusCode":200,"body":"{\"months\":\"36\",\"lbs\":\"9\",\"message\":\"not a baby\"}"}

Summary

Congratulations! You just created a simple serverless lambda function that is accessible via the api gateway. These functions are a useful tool for services development when you have bitesized functionality that can exist outside of a heavy enterprise service. Typically a lambda service would interact with a persistence mechanism or another service oriented peer. That however, is left as an exercise to the reader.