How to build an API using AWS Lambda and API Gateway (Written with Go)

Feb 27, 2021

Introduction

Overview

In this article I will outline the steps to build an API using AWS Lambda and API Gateway using my favorite language at the moment, Go.

Why

Sometimes you just want to perform a simple operation that is accessible through a RESTful API and don’t want to have manage/pay for a server.

Now the steps

Pre-requisite

Install Go. Instructions here.
Sign up for an AWS account here.

Sample Lambda function

Go to amazon’s documentation to see some sample code. Otherwise, you can use mine.

// main.go
package main

import (
	"github.com/aws/aws-lambda-go/lambda"
)

type Request struct {
	Message string `json:"message"`
}

type Response struct {
	Message string `json:"message"`
}

func HandleLambdaEvent(r Request) (Response, error) {
	return Response{Message: r.Message}, nil
}

func main() {
	lambda.Start(HandleLambdaEvent)
}

This is a basic function that returns what’s passed in the message key of the Request in the message key of the Response body.

First we import github.com/aws/aws-lambda-go/lambda package.

We can get this Go package by running this in the terminal:

$ go get github.com/aws/aws-lambda-go/lambda

Next we define the struct that serves as the type that holds the request’s body.

The name for the struct is completely arbitrary.

For this example we have Request. In the AWS documentation above, they use MyEvent.

Build app

Run:

$ GOOS=linux go build main.go

This will generate a main application.

Next we’ll need to zip the app.

$ zip function.zip main

The name of the zip file is arbitrary.

Create Lambda function

Go to AWS Lambda

Click Create function.

Name your function. I choose createPost as the name because the configuration that we are going to build is very similar to how an API endpoint would look like if we are to create a Post record. More on that later.

Select Go 1.x.

You can set other things like VPC etc but for this exercise we can just use the default settings.

Click Create function.

Next upload the zip file that you just created earlier.

Once you’re done, click Edit on Runtime settings.

Change the Handler to main. This tells AWS which function to invoke in the main package. Click Save.

Now let’s publish the function. Click Publish new version and Publish for the dialog after that.

API Gateway

Create a new API

Name your API and click create API.

Create Resource

With the / highlighted, click on Actions and select Create Resource.

What is a Resource

A resource typically refers to a collection. You can name it whatever you want.

The resource path (according to best practice) would usually be a noun.

In this example, we would have the resource name identical to the endpoint path.

example-domain-name.com/posts

You can check Enable API Gateway CORS or not. There are some optimizations but nothing worth mentioning for this exercise.

Create a method

A method here simply refers to a HTTP method.

Have /posts highlighted and click Actions then select Create Method.

Select POST. With a POST method on /posts endpoint, this resembles an endpoint that creates a record on a traditional backend API application. We are not creating a record here but this API design language should be familiar to RESTful conventions.

Select Lambda Function and key in the name of the lambda function that we just created. Click Save. A dialog would pop up to inform you about permissions, click Ok.