A First Look at AWS SAM

Outline

• Comparison to CDK
• Setup
  • Configure AWS CLI
  • Install SAM CLI
• Initialize Project
  • Select a Template
  • Select Package Type
  • Select Language Runtime
  • Select Project Name
  • Select Example Application
• Project Structure
  • SAM Template
  • App Entry Point
  • Test Handler
  • Events
• Deploy to AWS
  • Build Application
  • Configure SAM Deploy
  • Delete Stack

All of this project's code can be found in the First Look monorepo on my GitHub.

Introduction

AWS Serverless Application Model (SAM) is an open-source framework for building serverless applications. It provides shorthand syntax to express functions, APIs, databases, and event source mappings. Much like AWS CDK, SAM transforms and expands its own syntax into CloudFormation syntax during deployment.

Comparison to CDK

• SAM uses a declarative, template-based syntax with JSON or YAML similar to CloudFormation but syntactically simpler
• CDK uses common languages such as JavaScript, TypeScript, or Python to imperatively provision resources similar to Pulumi

SAM CLI is currently used to build locally, test, and package serverless applications defined using AWS CloudFormation or SAM templates. However, there is currently early support for using the SAM CLI to build and test with the CDK.

Setup

Configure AWS CLI

Make sure you have the AWS CLI installed and an AWS account. For general use, aws configure is recommended as the fastest way to set up your AWS CLI installation.

aws configure

When you enter this command, the AWS CLI prompts you for four pieces of information:

• Access key ID
• Secret access key
• AWS Region
• Output format

Go to My Security Credentials to find your Access Key ID, Secret Access Key, and default region. You can leave the output format blank.

AWS Access Key ID: <YOUR_ACCESS_KEY_ID>
AWS Secret Access Key: <YOUR_SECRET_ACCESS_KEY>
Default region name: <YOUR_REGION_NAME>
Default output format [None]: 

Install SAM CLI

There are numerous ways to install the SAM CLI depending on your development environment and operating system. I followed the instructions for installing with Homebrew.

brew tap aws/tap
brew install aws-sam-cli

Check sam version.

sam --version

Output:

SAM CLI, version 1.23.0

Initialize Project

sam init

Output:

SAM CLI now collects telemetry to better understand customer needs.
You can OPT OUT and disable telemetry collection by setting
the environment variable SAM_CLI_TELEMETRY=0 in your shell.

Thanks for your help!
Learn More:
https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-telemetry.html

Select a Template

Which template source would you like to use?
    1 - AWS Quick Start Templates
    2 - Custom Template Location

Select AWS Quick Start Templates.

Select Package Type

What package type would you like to use?
    1 - Zip (artifact is a zip uploaded to S3)  
    2 - Image (artifact is an image uploaded to an ECR image repository)

Select Zip.

Select Language Runtime

Which runtime would you like to use?
    1 - nodejs14.x
    2 - python3.8
    3 - ruby2.7
    4 - go1.x
    5 - java11
    6 - dotnetcore3.1
    7 - nodejs12.x
    8 - nodejs10.x
    9 - python3.7
    10 - python3.6
    11 - python2.7
    12 - ruby2.5
    13 - java8.al2
    14 - java8
    15 - dotnetcore2.1

Select nodejs14.x.

Select Project Name

Project name [sam-app]: ajcwebdev-sam

Give your project a name. I named my project ajcwebdev-sam.

Select Example Application

Cloning app templates from https://github.com/aws/aws-sam-cli-app-templates

AWS quick start application templates:
    1 - Hello World Example
    2 - Step Functions Sample App (Stock Trader)
    3 - Quick Start: From Scratch
    4 - Quick Start: Scheduled Events
    5 - Quick Start: S3
    6 - Quick Start: SNS
    7 - Quick Start: SQS
    8 - Quick Start: Web Backend

Select Hello World Example.

Output:

-----------------------
Generating application:
-----------------------
Name: ajcwebdev-sam
Runtime: nodejs14.x
Dependency Manager: npm
Application Template: hello-world
Output Directory: .

Next steps can be found in the README file at ./ajcwebdev-sam/README.md

Project Structure

├── .gitignore
├── README.md
├── template.yaml
├── events
│   └── event.json
└── hello_world
    ├── .npmignore
    ├── app.js
    ├── package.json
    └── tests
        └── unit
            └── test_handler.js

SAM Template

This example app uses a single Lambda function along with API Gateway.

These are defined in template.yaml which includes a SAM template for specifying your application's AWS resources.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  ajcwebdev-sam

  Sample SAM Template for ajcwebdev-sam

Globals:
  Function:
    Timeout: 3

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: hello-world/
      Handler: app.lambdaHandler
      Runtime: nodejs14.x
      Events:
        HelloWorld:
          Type: Api
          Properties:
            Path: /hello
            Method: get

Outputs:
  HelloWorldApi:
    Description: "API Gateway endpoint URL for Prod stage for Hello World function"
    Value: !Sub "https://${ServerlessRestApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/hello/"
  HelloWorldFunction:
    Description: "Hello World Lambda Function ARN"
    Value: !GetAtt HelloWorldFunction.Arn
  HelloWorldFunctionIamRole:
    Description: "Implicit IAM Role created for Hello World function"
    Value: !GetAtt HelloWorldFunctionRole.Arn

App Entry Point

The hello-world directory contains code for the application's Lambda function inside app.js and a package.json file for the necessary dependencies and scripts needed for our build process.

// hello-world/app.js

let response

/**
 *
 * @param {Object} event - API Gateway Lambda Proxy Input Format
 * @param {Object} context
 * @returns {Object} object - API Gateway Lambda Proxy Output Format
 * 
 */
exports.lambdaHandler = async (event, context) => {
  try {
    response = {
      'statusCode': 200,
      'body': JSON.stringify({
        message: 'hello world',
      })
    }
  } catch (err) {
    console.log(err)
    return err
  }
  return response
}

You specify dependencies in a manifest file that you include in your application. Since our example is using Node.js functions, our manifest file is package.json. This file is required for sam build.

{
  "name": "hello_world",
  "version": "1.0.0",
  "description": "hello world sample for NodeJS",
  "main": "app.js",
  "repository": "https://github.com/awslabs/aws-sam-cli/tree/develop/samcli/local/init/templates/cookiecutter-aws-sam-hello-nodejs",
  "author": "SAM CLI",
  "license": "MIT",
  "dependencies": {
    "axios": "^0.21.1"
  },
  "scripts": {
    "test": "mocha tests/unit/"
  },
  "devDependencies": {
    "chai": "^4.2.0",
    "mocha": "^8.2.1"
  }
}

Test Handler

The hello-world directory also contains a tests directory with a unit directory for unit tests and a test-handler.js file.

// hello-world/tests/unit/test-handler.js

'use strict'

const app = require('../../app.js')
const chai = require('chai')
const expect = chai.expect
var event, context

describe('Tests index', function () {
  it('verifies successful response', async () => {
    const result = await app.lambdaHandler(event, context)

    expect(result).to.be.an('object')
    expect(result.statusCode).to.equal(200)
    expect(result.body).to.be.an('string')

    let response = JSON.parse(result.body)

    expect(response).to.be.an('object')
    expect(response.message).to.be.equal("hello world")
  })
})

Events

The events directory contains invocation events. There are a lot of events in event.json, so we'll take a look at each key and its corresponding value.

{
  "body": "{\"message\": \"hello world\"}",
  "resource": "/{proxy+}",
  "path": "/path/to/resource",
  "httpMethod": "POST",
  "isBase64Encoded": false,
  "queryStringParameters": {
    "foo": "bar"
  },
  "pathParameters": {
    "proxy": "/path/to/resource"
  },
  "stageVariables": {
    "baz": "qux"
  },
  "headers": {...},
  "requestContext": {...}
}

Event	Definition
body	HTTP body containing data associated with the request (like content of an HTML form), or the document associated with a response.
resource	Sets a proxy in front of the resource.
path	Sets a path for the proxy to send the request.
httpMethod	The HTTP POST method sends data to the server. The type of the body of the request is indicated by the Content-Type header.
isBase64Encoded	Base encoding
queryStringParameters	Query string parameters
pathParameters	Proxies the request to our resources.
stageVariables	Variables for staging

headers includes a bunch of headers. I definitely know what all of these do. Totally.

{
  ...
  "headers": {
    "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
    "Accept-Encoding": "gzip, deflate, sdch",
    "Accept-Language": "en-US,en;q=0.8",
    "Cache-Control": "max-age=0",
    "CloudFront-Forwarded-Proto": "https",
    "CloudFront-Is-Desktop-Viewer": "true",
    "CloudFront-Is-Mobile-Viewer": "false",
    "CloudFront-Is-SmartTV-Viewer": "false",
    "CloudFront-Is-Tablet-Viewer": "false",
    "CloudFront-Viewer-Country": "US",
    "Host": "1234567890.execute-api.us-east-1.amazonaws.com",
    "Upgrade-Insecure-Requests": "1",
    "User-Agent": "Custom User Agent String",
    "Via": "1.1 08f323deadbeefa7af34d5feb414ce27.cloudfront.net (CloudFront)",
    "X-Amz-Cf-Id": "cDehVQoZnx43VYQb9j2-nvCh-9z396Uhbp027Y2JvkCPNLmGJHqlaA==",
    "X-Forwarded-For": "127.0.0.1, 127.0.0.2",
    "X-Forwarded-Port": "443",
    "X-Forwarded-Proto": "https"
  },
  "requestContext": {...}
}

The requestContext object is a map of key-value pairs. In each pair, the key is the name of a $context variable property, and the value is the value of that property.

{
  ...
  "headers": {...},
  "requestContext": {
    "accountId": "123456789012",
    "resourceId": "123456",
    "stage": "prod",
    "requestId": "c6af9ac6-7b61-11e6-9a41-93e8deadbeef",
    "requestTime": "09/Apr/2015:12:34:56 +0000",
    "requestTimeEpoch": 1428582896000,
    "identity": {
      "cognitoIdentityPoolId": null,
      "accountId": null,
      "cognitoIdentityId": null,
      "caller": null,
      "accessKey": null,
      "sourceIp": "127.0.0.1",
      "cognitoAuthenticationType": null,
      "cognitoAuthenticationProvider": null,
      "userArn": null,
      "userAgent": "Custom User Agent String",
      "user": null
    },
    "path": "/prod/path/to/resource",
    "resourcePath": "/{proxy+}",
    "httpMethod": "POST",
    "apiId": "1234567890",
    "protocol": "HTTP/1.1"
  }
}

Deploy to AWS

Build Application

The sam build command will build your serverless application and prepare it for subsequent steps in your workflow, like locally testing the application or deploying it to AWS.

sam build

Output:

Building

codeuri: /Users/ajcwebdev/projects/ajcwebdev-sam/hello-world
runtime: nodejs14.x
metadata: {}
functions: ['HelloWorldFunction']

Running NodejsNpmBuilder:NpmPack
Running NodejsNpmBuilder:CopyNpmrc
Running NodejsNpmBuilder:CopySource
Running NodejsNpmBuilder:NpmInstall
Running NodejsNpmBuilder:CleanUpNpmrc

Build Succeeded

Built Artifacts  : .aws-sam/build
Built Template   : .aws-sam/build/template.yaml

Commands you can use next
=========================
[*] Invoke Function: sam local invoke
[*] Deploy: sam deploy --guided

This command builds any dependencies that your application has, and copies your application source code to folders under .aws-sam/build to be zipped and uploaded to Lambda.

Configure SAM Deploy

sam deploy --guided

Looking for config file [samconfig.toml] :  Not found

Setting default arguments for 'sam deploy'

Stack Name [sam-app]: ajcwebdev-sam
AWS Region [us-west-1]: us-west-1
Confirm changes before deploy [y/N]: N
Allow SAM CLI IAM role creation [Y/n]: Y

For the question, "HelloWorldFunction may not have authorization defined," AWS SAM is informing you that the sample application configures an API Gateway API without authorization. When you deploy the sample application, AWS SAM creates a publicly available URL.

HelloWorldFunction may not have authorization defined
Is this okay? [y/N]: Y
Save arguments to configuration file [Y/n]: Y
SAM configuration file [samconfig.toml]: samconfig.toml
SAM configuration environment [default]: default

Looking for resources needed for deployment.
Found!

  Managed S3 bucket: aws-sam-cli-managed-default-samclisourcebucket-1234
  A different default S3 bucket can be set in samconfig.toml

Saved arguments to config file
Running 'sam deploy' for future deployments will use the parameters saved above.
The above parameters can be changed by modifying samconfig.toml
Learn more about samconfig.toml syntax at 
https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-cli-config.html

Deploying with following values:

Stack name                   : ajcwebdev-sam
Region                       : us-west-1
Confirm changeset            : False
Deployment s3 bucket         : aws-sam-cli-managed-default-samclisourcebucket-1234
Capabilities                 : ["CAPABILITY_IAM"]
Parameter overrides          : {}
Signing Profiles             : {}

CloudFormation outputs from deployed stack:

Key                 HelloWorldFunctionIamRole                                                                       
Description         Implicit IAM Role created for Hello World function                                              
Value               arn:aws:iam::1234:role/ajcwebdev-sam-HelloWorldFunctionRole-1234               

Key                 HelloWorldApi                                                                                   
Description         API Gateway endpoint URL for Prod stage for Hello World function                                
Value               https://y4mpitesn5.execute-api.us-west-1.amazonaws.com/Prod/hello/                              

Key                 HelloWorldFunction                                                                              
Description         Hello World Lambda Function ARN                                                                 
Value               arn:aws:lambda:us-west-1:1234:function:ajcwebdev-sam-HelloWorldFunction-1234   

Successfully created/updated stack - ajcwebdev-sam in us-west-1

Copy the URL contained in the Value for HelloWorldApi and send a request with your API tool of choice such as cURL, Postman, or Insomnia.

You can also just use a good ol' fashion web browser.

Delete Stack

Since this is a simple hello world application with an unsecured API endpoint, you should consider tearing the project down unless you intend on adding addition security features. To delete the sample application that you created:

aws cloudformation delete-stack --stack-name ajcwebdev-sam

If you send another request to the endpoint you will receive a 500 error message.