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Piotr Mionskowski

How to convert an express app to AWS Lambda?

29 May 2018 aws, cloudformation, lambda, and cloudform

In this post we will see how to convert an existing express application to AWS Lambda. This can help reduce AWS bill even by an order of magnitude. We will also use cloudform to describe the CloudFormation stack.

lambda

An express app

For our example to be complete we need an express application. Let’s use a single index.js file to define it:

const express = require('express');

function apiRoutes(){
    const routes = new express.Router();

    routes.get('/v1/version', (req, res) => res.send({version: '1'}));

    routes.post('/v1/echo', (req, res) => res.send({...req.body}));

    return routes;
}


const app = express()
    .use(express.json())
    .use(apiRoutes());

app.listen(3000, () => console.log(`Listening on 3000`));

The above application has only 2 endpoints:

  • GET /v1/version returns API version
  • POST /v1/echo sends back the request body

We start the application as any other node app with node index.js.

Convert the express app to AWS Lambda

The AWS Lambda Node.js runtime model differs from a simple node fileName.js invocation. For the AWS Lambda to invoke our application code we need to structure it appropriately. Thankfully the aws-serverless-express module adapts express to AWS Lambda Node.js runtime model.

Let’s adapt our index.js file to support aws-serverless-express:

const isInLambda = !!process.env.LAMBDA_TASK_ROOT;
if (isInLambda) {
    const serverlessExpress = require('aws-serverless-express');
    const server = serverlessExpress.createServer(app);
    exports.main = (event, context) => serverlessExpress.proxy(server, event, context)
} else {
    app.listen(3000, () => console.log(`Listening on 3000`));
}

The main function is called by the AWS Lambda Node.js runtime. Note that we’ve used LAMBDA_TASK_ROOT environment variable to detect if the app is running inside AWS Lambda. It is better to split the express application setup and listen call into separate files and use 2 different main files e.g. development.js calling listen(port) and lambda.js using aws-serverless-express. However, this would complicate our example unnecessarily.

Deploy the express app to AWS Lambda

I already showed how we can deploy lambda with cloudform. We will use the previous example as a base:

import cloudform, { Lambda, IAM, Fn, ApiGateway, Refs,  } from 'cloudform';
import { FunctionProperties } from 'cloudform/types/lambda/function';
import { readFileSync } from 'fs';

const
    LambdaExecutionRole = 'LambdaExecutionRole',
    ExpressMain = 'ExpressMain',
    RestApi = 'RestApi',
    RestApiMainResource = 'RestApiMainResource',
    PackageKey = 'PackageKey',
    RestApiDeployment = 'RestApiDeployment',
    RestApiMethod = 'RestApiMethod';

export default cloudform({
    Parameters: {
        PackageKey: {
            Type: 'String',
            Default: 'express-lambda.zip'
        }
    },
    Resources: {
        [ExpressMain]: new Lambda.Function({
            Code: { S3Bucket: 'bright-tmp', S3Key: Fn.Ref(PackageKey) },
            Handler: "index.main",
            Role: Fn.GetAtt(LambdaExecutionRole, "Arn"),
            Runtime: "nodejs6.10"
        }),
        [LambdaExecutionRole]: new IAM.Role({
            AssumeRolePolicyDocument: {
                Statement: [{
                    Effect: "Allow",
                    Principal: { Service: ["lambda.amazonaws.com"] },
                    Action: ["sts:AssumeRole"]
                }]
            },
            ManagedPolicyArns: ["arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole"]
        }),
        PermissionToInvokeLambda: new Lambda.Permission({
            Action: 'lambda:InvokeFunction',
            FunctionName: Fn.GetAtt(ExpressMain, 'Arn'),
            Principal: 'apigateway.amazonaws.com',
            SourceArn: Fn.Join('', [
                'arn:aws:execute-api:', Refs.Region, ':', Refs.AccountId, ':', Fn.Ref(RestApi), '/*']
            )
        })
        ... // Rest Api Gateway see below
    }
})

For the AWS Lambda function source we use a zip file hosted in a S3 Bucket named packages at key specified in the template parameter PackageKey. The package zip file must contain all application source code including node_modules. We also define PermissionToInvokeLambda so that API Gateway can invoke the lambda function.

Add API Gateway to call AWS Lambda

To be able to invoke the AWS Lambda function using HTTP protocol we will use API Gateway. There are multiple ways of setting up the API gateway but we will use an appoach that is simplest in my opinion.

[RestApi]: new ApiGateway.RestApi({ Name: "Express API" }),
[RestApiMainResource]: new ApiGateway.Resource({
    RestApiId: Fn.Ref(RestApi),
    ParentId: Fn.GetAtt(RestApi, 'RootResourceId'),
    PathPart: "{proxy+}",
}),
[RestApiMethod]: new ApiGateway.Method({
    HttpMethod: 'ANY',
    ResourceId: Fn.Ref(RestApiMainResource),
    RestApiId: Fn.Ref(RestApi),
    AuthorizationType: 'NONE',
    Integration: {
        Type: "AWS_PROXY",
        IntegrationHttpMethod: "POST",
        Uri: Fn.Sub("arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${ExpressMain.Arn}/invocations", {})
    }
}),
[RestApiDeployment]: new ApiGateway.Deployment({
    RestApiId: Fn.Ref(RestApi),
    StageName: 'test'
})

We first define a RestApi which is a collection of resources with various configuration options describing the API. Next is the RestApiMainResource which depicts a single REST resource. However, in our case we use wildcard PathPart to match all paths. This way we can have a single resource encompassing all API endpoints. For the RestApiMainResource we also need to define a single RestApiMethod. Note that the HttpMethod is set to ANY which matches all verbs. The Integration specifies that we would like to proxy requests to the ExpressMain AWS Lambda function. Last but not least we define a RestApiDeployment so that we have an URL to call the API.

It is handy to define Outputs in our template so that we can easily access the API url:

Outputs: {
    RestApiUrl: {
        Value: Fn.Join('', [Fn.Ref(RestApi), '.execute-api.', Refs.Region, '.amazonaws.com/test'])
    }
}

Test API Gateway calling AWS Lambda

With our cloudform template deployed with a single command:

aws cloudformation update-stack \
  --stack-name lambda-example \
  --capabilities CAPABILITY_IAM \
  --template-body file://<(./node_modules/.bin/cloudform aws-template.ts)

We can also fetch the API url:

API_URL=$(aws cloudformation describe-stacks \
  --stack-name lambda-example \
  --query 'Stacks[0].Outputs[?OutputKey==`RestApiUrl`].OutputValue' \
  --output text)

Finally, with the help of httpie we can test our API:

> http https://${API_URL}/v1/version

HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 15
Content-Type: application/json; charset=utf-8
Date: Mon, 28 May 2018 19:58:13 GMT
Via: 1.1 XXXXXXXXXXXXX.cloudfront.net (CloudFront)
X-Amz-Cf-Id: XXXXXXXXXXXXX==
X-Amzn-Trace-Id: Root=1-5b0c5f54-806e190d437c7d31a4a0d4ba
X-Cache: Miss from cloudfront
etag: W/"f-sHigu4BMVa0IJ0LR3NDJ5y8l4sc"
x-amz-apigw-id: HnPVQH4yjoEF8-w=
x-amzn-Remapped-connection: close
x-amzn-Remapped-content-length: 15
x-amzn-Remapped-date: Mon, 28 May 2018 19:58:13 GMT
x-amzn-RequestId: 73eff8a6-62b1-11e8-907c-79117668835e
x-powered-by: Express

{
    "version": "1"
}

And a POST to the echo endpoint:

http https://${API_URL}/v1/echo message="Hello 👋 My name is Piotr"

HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 41
Content-Type: application/json; charset=utf-8
Date: Mon, 28 May 2018 20:00:31 GMT
Via: 1.1 XXXXXXXXXXXXX.cloudfront.net (CloudFront)
X-Amz-Cf-Id: XXXXXXXXXXXXX==
X-Amzn-Trace-Id: Root=1-5b0c5fdf-802178cceb5160684ef2cc34
X-Cache: Miss from cloudfront
etag: W/"29-SKqhJThIfjmVId6IIeTilD7Mkk0"
x-amz-apigw-id: HnPq5HeJjoEFuRQ=
x-amzn-Remapped-connection: close
x-amzn-Remapped-content-length: 41
x-amzn-Remapped-date: Mon, 28 May 2018 20:00:31 GMT
x-amzn-RequestId: c67b41c0-62b1-11e8-a23f-c7cbebde15f2
x-powered-by: Express

{
    "message": "Hello 👋 My name is Piotr"
}

Reduced infrastructure cost

With the above setup we no longer have to pay for an always running EC2 instance. Our monthly bill depends on the number of requests that are executed against the exposed API. More than that, we get much better scalability characteristics, especially when we have nonlinear request rates.