Why does API Gateway (Test) not wait for async lambda? - node.js

Question

I am writing an async lambda on AWS using Node.js (14.x). This lambda is being called by an API Gateway (REST API, POST method, CORS enabled).
To allow the API Gateway to call an async lambda, I added the following HTTP Header to its Integration Request: Name = X-Amz-Invocation-Type and Mapped from (value) = 'Event' as specified here.
When I run Test, I get the following output:
Execution log for request BLAH
Fri Mar 26 07:30:34 UTC 2021 : Starting execution for request: BLAH
Fri Mar 26 07:30:34 UTC 2021 : HTTP Method: POST, Resource Path: BLAH
Fri Mar 26 07:30:34 UTC 2021 : Method request path: {}
Fri Mar 26 07:30:34 UTC 2021 : Method request query string: {}
Fri Mar 26 07:30:34 UTC 2021 : Method request headers: {}
Fri Mar 26 07:30:34 UTC 2021 : Method request body before transformations: {
"id": "BLAH",
"recaptcha": "BLAH"
}
Fri Mar 26 07:30:34 UTC 2021 : Request validation succeeded for content type application/json
Fri Mar 26 07:30:34 UTC 2021 : Endpoint request URI: https://lambda.BLAH.amazonaws.com/2015-03-31/functions/arn:aws:lambda:BLAH:function:BLAH/invocations
Fri Mar 26 07:30:34 UTC 2021 : Endpoint request headers: {X-Amz-Date=BLAH, x-amzn-apigateway-api-id=BLAH, Accept=application/json, User-Agent=AmazonAPIGateway_BLAH, Host=lambda.BLAH.amazonaws.com, X-Amz-Content-Sha256=BLAH, X-Amzn-Trace-Id=Root=BLAH, x-amzn-lambda-integration-tag=BLAH, Authorization=*****BLAH*****, X-Amz-Source-Arn=arn:aws:execute-api:BLAH/test-invoke-stage/POST/BLAH, X-Amz-Invocation-Type=Event, X-Amz-Security-Token=BLAH [TRUNCATED]
Fri Mar 26 07:30:34 UTC 2021 : Endpoint request body after transformations: {
"id": "BLAH",
"recaptcha": "BLAH"
}
Fri Mar 26 07:30:34 UTC 2021 : Sending request to https://lambda.BLAH.amazonaws.com/2015-03-31/functions/arn:aws:lambda:BLAH:function:BLAH/invocations
Fri Mar 26 07:30:34 UTC 2021 : Received response. Status: 202, Integration latency: 34 ms
Fri Mar 26 07:30:34 UTC 2021 : Endpoint response headers: {Date=Fri, 26 Mar 2021 07:30:34 GMT, Content-Length=0, Connection=keep-alive, x-amzn-RequestId=BLAH, x-amzn-Remapped-Content-Length=0, X-Amzn-Trace-Id=root=BLAH;sampled=0}
Fri Mar 26 07:30:34 UTC 2021 : Endpoint response body before transformations:
Fri Mar 26 07:30:34 UTC 2021 : Method response body after transformations:
Fri Mar 26 07:30:34 UTC 2021 : Method response headers: {X-Amzn-Trace-Id=Root=BLAH;Sampled=0, Access-Control-Allow-Origin=*, Content-Type=application/json}
Fri Mar 26 07:30:34 UTC 2021 : Successfully completed execution
Note that the response gives Status: 202 and that there is no response body. No matter what code I put in my lambda, this seems to be the case. Here are a couple basic examples that I've tried:
// Load the AWS SDK for Node.js and set the region.
var AWS = require('aws-sdk');
AWS.config.update({region: 'BLAH'});
// Main function, responds to AWS API Gateway.
exports.handler = async function (event, context) {
var response = {
httpStatus : 200,
message : "Success"
};
return JSON.stringify(response);
};
and
// Load the AWS SDK for Node.js and set the region.
var AWS = require('aws-sdk');
AWS.config.update({region: 'BLAH'});
// Main function, responds to AWS API Gateway.
exports.handler = async function (event, context) {
var promise = new Promise(function(resolve, reject) {
var response = {
httpStatus : 200,
message : "Success"
};
resolve(JSON.stringify(response));
});
return promise;
};
I made sure to deploy the API and Lambda. How can I get a non-202 status (i.e. get the API Gateway to wait on the async lambda)?

Answer 1

This is working as intended. The second sentence in AWS docs on "Asynchronous invocation" says:
When you invoke a function asynchronously, you don't wait for a response from the function code.
If you want a response with a result, asynchronous invocation will not work.