Serverless: AWS HTTP Gateway — 502 Bad Gateway

In my continued work with serverless and AWS Lambda, I ran into a problem when trying to call an HTTP gateway.

My project looked like this:

serverless.yaml:

service: http-gateway

frameworkVersion: ">=1.2.0 <2.0.0"

provider:
  name: aws
  runtime: python3.6
  timeout: 180

functions:
  no-op:
      name: NoOp
      handler: handler.noop
      events:
        - http: POST noOp

handler.py:

def noop(event, context):
    return "hello"

Let’s deploy to AWS:

$ serverless  deploy
Serverless: Packaging service...
Serverless: Excluding development dependencies...
Serverless: Uploading CloudFormation file to S3...
Serverless: Uploading artifacts...
Serverless: Uploading service .zip file to S3 (179 B)...
Serverless: Validating template...
Serverless: Updating Stack...
Serverless: Checking Stack update progress...
..............
Serverless: Stack update finished...
Service Information
service: http-gateway
stage: dev
region: us-east-1
api keys:
  None
endpoints:
  POST - https://29nb5rmmd0.execute-api.us-east-1.amazonaws.com/dev/noOp
functions:
  no-op: http-gateway-dev-no-op

And now we’ll try and call it using cURL:

$ curl -X POST https://29nb5rmmd0.execute-api.us-east-1.amazonaws.com/dev/noOp
{"message": "Internal server error"}

That didn’t work so well, what do the logs have to say?

$ serverless  logs --function no-op
START RequestId: 64ab69b0-7d8f-11e7-9db5-13b228cd4cb6 Version: $LATEST
END RequestId: 64ab69b0-7d8f-11e7-9db5-13b228cd4cb6
REPORT RequestId: 64ab69b0-7d8f-11e7-9db5-13b228cd4cb6Duration: 0.27 msBilled Duration: 100 ms Memory Size: 1024 MBMax Memory Used: 21 MB

So the function is completely fine. It turns out I’m not very good at reading the manual and should have been returning a map instead of a string:

API Gateway expects to see a JSON map with keys “body”, “headers”, and “statusCode”.

Let’s update our handler function and re-deploy.

def noop(event, context):
    return {
        "body": "hello",
        "headers": {},
        "statusCode": 200
    }

Now we’re ready to try the endpoint again:

$ curl -X POST https://29nb5rmmd0.execute-api.us-east-1.amazonaws.com/dev/noOp
hello

Much better!