Using API Gateway to publish SNS topics / multiple lambda function with API Gateway

Question

Right now my requirement is, whenever I get data through API, I have to save it into 2-3 different places (for example, into my own DB, into some BI service and also sometimes into a logging DB).

I don't know if it's possible to bind a single resource and single method into multiple lambda functions or so. So, my alternate approach was, as I already know how to trigger multiple lambda functions by subscribing to SNS topic, I thought maybe if I can somehow publish to SNS topic from the API Gateway, the rest will be easy. My current thinking is something below:

But the problem is, I am not able to publish to SNS topic from the API Gateway. I am getting errors like TopicArn or TargetArn Reason: no value for required parameter.

My approach is, create a normal SNS topic. Then, create a special role policy like below:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "StmtXXXXXXXXXXX",
            "Effect": "Allow",
            "Action": [
                "sns:Publish",
                "sns:Subscribe",
                "sns:Unsubscribe"
            ],
            "Resource": [
                "SNS-TOPIC-ARN"
            ]
        }
    ]
}

Then creating a API with POST/GET method (I tried both) and added SNS topic as AWS Service Proxy and the Role as Execution role.

So why did you decide to have the SNS topic hit the API Gateway which will call your lambda functions instead of just triggering a function? Wouldn't that just add unnecessary costs? — arjabbar
Do you think another lambda function will be cheaper than SNS topic?? — William Francis Gomes

Big Rich Big Rich · Accepted Answer · 2018-10-15T14:21:13

The following resource(s) go some way to detailing direct API-Gateway-to-SNS integration, without a Lambda definition:

Article: Async API with API Gateway and SNS

Code/template example: Profit4Cloud(NL) API-to-SNS Example code @Bitbucket

Basically, the a x-amazon-apigateway-integration Swagger/OpenAPI 'extension' is configured within the gateway's API description. Notice that the 'ExampleTopic' SNS artifact is referenced within the gateway extension (see integration.request.querystring.TopicArn).

AWSTemplateFormatVersion: "2010-09-09"
Transform: "AWS::Serverless-2016-10-31"

Resources:
  ExampleTopic:
    Type: "AWS::SNS::Topic"
    Properties:
      TopicName: !Sub "${AWS::StackName}-example-topic"

  ExampleAPI:
    Type: "AWS::Serverless::Api"
    Properties:
      StageName: "prod"
      DefinitionBody:
        swagger: "2.0"
        info:
          title: !Sub "${AWS::StackName}-api"
        paths:
          /example-path:
            post:
              responses:
                "202":
                  description: Accepted
              x-amazon-apigateway-integration:
                type: "aws"
                httpMethod: "POST"
                uri: !Sub "arn:aws:apigateway:${AWS::Region}:sns:action/Publish"
                credentials: !GetAtt ExampleTopicAPIRole.Arn
                requestParameters:
                  integration.request.querystring.Message: "method.request.body"
                  integration.request.querystring.TopicArn: !Sub "'${ExampleTopic}'"
                responses:
                  default:
                    statusCode: 202


  ExampleTopicAPIRole:
    Type: "AWS::IAM::Role"
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: "Allow"
            Principal:
              Service: "apigateway.amazonaws.com"
            Action:
              - "sts:AssumeRole"
      Policies:
        - PolicyName: !Sub "${AWS::StackName}-example-topic-policy"
          PolicyDocument:
            Version: "2012-10-17"
            Statement:
              - Action: "sns:Publish"
                Effect: "Allow"
                Resource: !Ref ExampleTopic
      ManagedPolicyArns:
        - "arn:aws:iam::aws:policy/service-role/AmazonAPIGatewayPushToCloudWatchLogs"

  ExampleLambda:
    Type: 'AWS::Serverless::Function'
    Properties:
      Handler: index.handler
      Runtime: nodejs6.10
      CodeUri: ./app
      Events:
        SNSMessage:
          Type: SNS
          Properties:
            Topic: !Ref ExampleTopic

Using API Gateway to publish SNS topics / multiple lambda function with API Gateway

2 Answers