Go(lang) and AWS Lambda: Trigger a Lambda Function Through an API Gateway

Remove unwanted HTML tags from a URL in your Go application

Photo by Jackson So on Unsplash

Here, we write an application with Go that receives a URL and removes unwanted HTML tags to make it readable. Then we deploy it to AWS Lambda and configure the AWS API Gateway to interact with our Lambda function.

Prerequisites

Please install and configure the following packages on your system.

• Install AWS CLI
• Install AWS SAM CLI
• Install Go

What we learn

• How to write an AWS Lambda function in Go
• Build and deploy a Lambda function
• Trigger the Lambda function by using API Gateway

Go readability API

To make this tutorial a little useful, I decided to have an API to receives a URL and returns a readable version of its content, exactly the task that has been done by Pocket.

Before starting to write code, please install the following Go packages:

$ go get github.com/go-shiori/go-readability
$ go get github.com/aws/aws-lambda-go/events
$ go get github.com/aws/aws-lambda-go/lambda

The go-readability converts an article to a readable version by removing unwanted HTML tags. The aws-lambda-go/lambda is responsible to execute the handler function by passing required parameters to it, and send the returned response back to AWS Lambda API. And aws-lambda-go/events helps us to access the request’s body and send a proper response back.

After installing the packages, create directory and call it go-readable :

$ mkdir go-readable && cd go-readable
$ go mod readability

create a file main.go and add the following content in it:

The main function calls the lambda.Start function provided by AWS SDK, it receives a handler function, execute it, and send the response back to AWS Runtime API.

The handler function receives two parameters, ctx which gives some information about the function, and request, which contains information about the request. The handler function can have one of the followings signatures:

func ()
func () error
func (TIn) error
func () (TOut, error)
func (context.Context) error
func (context.Context, TIn) error
func (context.Context) (TOut, error)
func (context.Context, TIn) (TOut, error)

As you can see this function is really simple, it receives the request body and after convert it to the Event struct, it passes the Url to the readability and attach the output to the response.

Build and deploy the Lambda function

In order to trigger this function, we are going to create an API endpoint by using AWS API Gateway. In the root directory of your project, create a file template.yaml and add the following content in it:

Under Resources there is a list of our AWS services that we want to be created by AWS SAM CLI. In this case we only have one resource which is the Lambda function. The Lambda function resource have some properties:

• CodeUrl indicates that where is the root directory of your project. In our case, . means current directory.
• Handler is the name of the function that Lambda Runtime API will look for and execute.
• Runtime is set to go1.x that means we are using Go Runtime.
• Policies adds one or more IAM policies to your Lambda function.
• Events specifies that this function can be triggered by the following events, in our case we want to trigger our function by an API request.
• The Properties under Events indicates the Path of our API which is our endpoint and must be a POST request, and it doesn’t need any authentication.

After creating this template, run sam build and sam deploy -g and answer the questions:

It’ll create a file in your root directory called samconfig.toml and stores all the provided information in it. The next time, you only need to run sam deploy in order to deploy your function. It also creates a CloudFormation stack, upload your code to AWS Lambda, and create an API resource in API Gateway.

Deploy our API

Now it’s time to deploy our API. Open the AWS Management Console and go to API Gateway service. You should see something like this:

Click on go-readable-lambda-app :

As you can see, we have one resource in the left sidebar that only accepts POST requests. On the right side of the page, you can see the process flow of our API. It receives the request, pass it to LAMBDA_PROXY, then triggers our Lambda function, and then it receives the response from our Lambda function and after doing some integration, it sends the response back. In order to have access to this end point, we need to deploy it. Click on the Action button on top of the page, select Deploy API

and then select a stage to deploy your API:

After deploying, you will get a URL to your API.

Let’s test our API:

$ curl --header "Content-Type: application/json" \
  --request POST \
  --data '{"url":"https://smoqadam.me/posts/books-i-loved-most-in-2021/"}' https://763w7zdo8g.execute-api.eu-central-1.amazonaws.com/Prod/readable

If you have done everything correctly, you will see a readable result of the given URL.

That’s it, please let me know if you have any feedback or questions.