Integrate SNS with WebSocket API Gateway

Question

I would like to Publish messages to an SNS topic from a WebSocket API.  I have a REST API working, but need a way to send a response back to the client, so figured I could use WebSocket.   But, I can't figure out how to get the TopicArn and Message parameters into the sns query string.  I have been trying CloudFormation configuration with:

```
RequestParameters:
  integration.request.querystring.Message: $request.body.msg
  integration.request.querystring.TopicArn: $request.body.topic
```
This generates `Invalid mapping expression specified: $request.body.topic`.  Whatever I put on the right hand side triggers similar error.  Is this possible?  Ideally, I want to set the topic to a constant.  Or, do I need to integrate with Lambda, and publish from there?

Answer

If you want to send notifications to the client, you will need to create a WebSockets API. The client will connect to the API, and you will need to record the connection ID for the users, usually, in a DynamoDB table. You do that from a Lambda function integrated with the $connect integration in the API.

When you want to send a message to the client, you will need to find the connection ID for the user, and use the API Gateway's postToConnection API to send the message to the client. If the message is coming from SNS, you will probably invoke a Lambda function that will get the message from SNS and send to API Gateway. You can't configure SNS to send the messages directly to the postToConnection API.

Answer

Hi, you can refer to ServerlessLand example here: https://serverlessland.com/patterns/apigw-websocket-api-sns
This example covers how to use SNS with WebSocket API along with request validation

Integrate SNS with WebSocket API Gateway

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