AWS API-Gateway communicating to SNS

Question

I am building an API which will be serviced by Lambda functions but I need these to be asynchronous so rather than connecting the API-Gateway directly to the Lambda function I'm using the "AWS Service Proxy" to publish SNS messages and then have the Lambda function subscribe to the relevant SNS topic so it receives delivery of the requests. Here's a picture which illustrates the flow:

I have tested both the Lambda function in isolation as well pub/sub messaging between SNS and Lambda but I am struggling with the API-Gateway to SNS handoff. Documentation is quite light but what I am assuming right now is that the following attributes must be sent in the POST request:

1. Action: the API-Gateway offers to set this in the UI and I have put in the Publish action which is the appropriate SNS action

2. Message: the body of the POST message should be a JSON document. It would be passed by the web client and proxied through the gateway to SNS.

3. TopicArn: indicates the SNS topic that we're publishing to. In my design this would be a static value/endpoint so I'd prefer that the web-client not have to pass this too but if it were easier to do this that would be fine too.

I have tried lots of things but am just stuck. Would love to find a good code example somewhere but any help at all would be appreciated.

---

Wanted to add a little more context on my current attempt:

I have tried publishing my API and using Postman to try and get a valid response. Here's the postman screens(one for header vars, one for JSON body):

This results in the following error message:

{
   "Error": {
     "Code": "InvalidParameter",
     "Message": "Invalid parameter: TopicArn or TargetArn Reason: no value for required parameter",
     "Type": "Sender"
  },
  "RequestId": "b33b7700-e8a3-58f7-8ebe-39e4e62b02d0"
}

the error seems to indicate that the TopicArn parameter is not being sent to SNS but I have included the following in API-Gateway:

Answer 1

I'm from the Api Gateway team.

I believe there are a few formats for the HTTP request to the Publish API, but here's the one I used first:

AWS Region us-west-2

AWS Service sns

AWS Subdomain

HTTP method POST

Action Publish

== query strings ==

Subject 'foo'
Message 'bar'
TopicArn 'arn:aws:sns:us-west-2:xxxxxxxxxxxx:test-api'

This worked for me to publish a message.

Let me know if you have further troubles.

Jack