Amazon Web Services introduced Serverless Application Model, or SAM, a couple of months ago. It defines a simplified syntax for expressing serverless resources. SAM extends AWS CloudFormation to add support for API Gateway, AWS Lambda, and Amazon DynamoDB. This blog will show how to create a simple microservice using SAM. Of course, we’ll use Couchbase instead of DynamoDB!

This blog will also use the basic concepts explained in Microservice using AWS API Gateway, AWS Lambda, and Couchbase. SAM will show the ease with which the entire stack for microservice can be deployed and managed.

As a refresher, here are key components in the architecture:

• Client could be curl, AWS CLI/Console, Postman client, or any other tool/API that can invoke a REST endpoint.
• AWS API Gateway is used to provision APIs. The top level resource is available at path /books. HTTP GET and POST methods are published for the resource.
• Each API triggers a Lambda function. Two Lambda functions are created, a book-list function for listing all the books available and a book-create function to create a new book.
• Couchbase is used as a persistence store in EC2. All the JSON documents are stored and retrieved from this database.

Serverless Application Model (SAM) Template

An AWS CloudFormation template with serverless resources conforming to the AWS SAM model is referred to as a SAM file or template. It is deployed as a CloudFormation stack.

Let’s take a look at our SAM template:

This template is available at github.com/arun-gupta/serverless/blob/master/aws/microservice/template.yml.

AWSTemplateFormatVersion : '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: Microservice using API Gateway, Lambda and Couchbase
Resources:
  MicroserviceGetAllGateway:
    Type: AWS::Serverless::Function
    Properties:
      Handler: org.sample.serverless.aws.couchbase.gateway.BucketGetAll
      Runtime: java8
      CodeUri: s3://serverless-microservice/microservice-http-endpoint-1.0-SNAPSHOT.jar
      Timeout: 30
      MemorySize: 1024
      Environment:
        Variables:
          COUCHBASE_HOST: ec2-35-163-21-104.us-west-2.compute.amazonaws.com
      Role: arn:aws:iam::598307997273:role/microserviceRole
      Events:
        GetResource:
          Type: Api
          Properties:
            Path: /books
            Method: get
  MicroservicePostGateway:
    Type: AWS::Serverless::Function
    Properties:
      Handler: org.sample.serverless.aws.couchbase.gateway.BucketPost
      Runtime: java8
      CodeUri: s3://serverless-microservice/microservice-http-endpoint-1.0-SNAPSHOT.jar
      Timeout: 30
      MemorySize: 1024
      Environment:
        Variables:
          COUCHBASE_HOST: ec2-35-163-21-104.us-west-2.compute.amazonaws.com
      Role: arn:aws:iam::598307997273:role/microserviceRole
      Events:
        GetResource:
          Type: Api
          Properties:
            Path: /books
            Method: post

The SAM template specification provides complete details about contents in the template. The key parts of the template are:

• Defines two resources, both of Lambda Function type identified by AWS::Serverless::Function attribute. The name of the Lambda function is defined by Resources.<resource>.
• The class for each handler is defined by the value of Resources.<resource>.Properties.Handler attribute.
• Java 8 runtime is used to run the function defined by the Resources.<resource>.Properties.Runtime attribute.
• Code for the class is uploaded to an S3 bucket, in our case to s3://serverless-microservice/microservice-http-endpoint-1.0-SNAPSHOT.jar
• Resources.<resource>.Properties.Environment.Variables.COUCHBASE_HOST attribute value defines the host where Couchbase is running. This can be easily deployed on EC2 as explained at Setup Couchbase.
• Each Lambda function is triggered by an API. It is deployed using AWS API Gateway. The path is defined by Events.GetResource.Properties.Path. HTTP method is defined using the Events.GetResource.Properties.Method attribute.

Java Application

The Java application that contains the Lambda functions is at github.com/arun-gupta/serverless/tree/master/aws/microservice/microservice-http-endpoint.

The Lambda function that is triggered by the HTTP GET method as shown:

public class BucketGetAll implements RequestHandler<GatewayRequest, GatewayResponse> {

    @Override
    public GatewayResponse handleRequest(GatewayRequest request, Context context) {
        try {
            N1qlQuery query = N1qlQuery
                    .simple(select("*")
                            .from(i(CouchbaseUtil.getBucketName()))
                            .limit(10));

            String result = CouchbaseUtil.getBucket().query(query).allRows().toString();

            return new GatewayResponse(200, result, GatewayResponse.HEADERS_JSON);
        } catch (ConfigurationException e) {
            return new GatewayResponse(400, e.getMessage(), GatewayResponse.HEADERS_TEXT);
        }
    }
}

A little bit of explanation:

• Each Lambda function needs to implement the interface com.amazonaws.services.lambda.runtime.RequestHandler.
• API Gateway and Lambda integration require a specific input format and output format. These formats are defined as GatewayRequest and GatewayResponse classes.
• Function logic uses Couchbase Java SDK to query the Couchbase database. A N1QL query is used to query the database. The results and exception are then wrapped in GatewayRequest and GatewayResponse.

The Lambda function triggered by HTTP POST method is pretty straightforward as well:

public class BucketPost implements RequestHandler<GatewayRequest, GatewayResponse> {

    @Override
    public GatewayResponse handleRequest(GatewayRequest request, Context context) {

        try {
            JsonDocument document = CouchbaseUtil.getBucket().upsert(Book.fromStringToJson(request.getBody()));
            return new GatewayResponse(200, document.content().toString(), GatewayResponse.HEADERS_JSON);
        } catch (Exception ex) {
            return new GatewayResponse(400, ex.getMessage(), GatewayResponse.HEADERS_TEXT);
        }
    }
}

A bit of explanation:

• Incoming request payload is retrieved from GatewayRequest
• Document inserted in Couchbase is returned as response.
• Like the previous method, the function logic uses Couchbase Java SDK to query the Couchbase database. The results and exception are then wrapped in GatewayRequest and GatewayResponse.

Build the Java application as:

mvn -f microservice-http-endpoint/pom.xml clean package

Upload Lambda Function to S3

The SAM template reads the code from an S3 bucket. Let’s create an S3 bucket:

aws s3 mb s3://serverless-microservice --region us-west-2

us-west-2 region is one of the supported regions for API Gateway. S3 bucket names are globally unique but their location is region specific.

Upload the code to S3 bucket:

aws s3 cp microservice-http-endpoint/target/microservice-http-endpoint-1.0-SNAPSHOT.jar s3://serverless-microservice/microservice-http-endpoint-1.0-SNAPSHOT.jar

The code is now uploaded to the S3 bucket. The SAM template is ready to be deployed!

Deploy SAM Template

Deploy the SAM template:

aws cloudformation deploy \
  --template-file template.yml \ 
  --stack-name microservice-gateway \ 
  --region us-west-2

It shows the output:

Waiting for changeset to be created.. 
Waiting for stack create/update to complete 
Successfully created/updated stack - microservice-gateway

This one command deploys Lambda functions and REST Resource/APIs that trigger these Lambda functions.

Invoke the Microservice

API Gateway publishes a REST API that can be invoked by curl, wget, AWS CLI/Console, Postman or any other app that can call a REST API. This blog will use AWS Console to show the interaction.

API Gateway home at us-west-2.console.aws.amazon.com/apigateway/home?region=us-west-2#/apis shows:

Click on the API to see all the APIs in this resource:

Click on POST to see the default page for POST method execution:

Click on Test to test the API:

Add the payload in Request Body and click on Test to invoke the API. The results are shown as below:

Now click on GET to see the default execution page:

Click on Test to test the API:

No request body is needed, just click on Test the invoke the API. The results are as shown:

Output from the Couchbase database is shown in the Response Body.