I am trying to connect a Slack slash-command to an AWS Lambda (through the AWS API Gateway).
I was able to adapt the Hello, World blueprint (written in Python) in the Lambda tutorial section. I set up an API gateway as a trigger, then had Slack POST the slash-command to the API endpoint. I had to manually decode the base64 body, then use parse_qs to convert the query string-like POST body into a dictionary, where I could then access it just fine.
My real Lambda code is currently in Java, but I've started with the java-basic sample app from the Developers Guide. I built the app, uploaded the jar, and confirmed the correct handler was being called. I set up another API gateway and pointed a different Slack slash-command at the new endpoin. It fails.
The log looks like:
java.lang.RuntimeException: An error occurred during JSON parsing
Caused by: java.io.UncheckedIOException: com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_OBJECT token
at [Source: (ByteArrayInputStream); line: 1, column: 1]
I'm guessing this means Slack is sending the same query string-like argument in the POST body, and for some reason, something is expecting JSON input, not just text. I don't understand why the Python handler was okay with building up an event dictionary and making the Lambda decode the POST body, whereas the Java handler is never getting called, but is dying before that when attempting to deserialize the POST body, thinking it's JSON.
Both Lambdas and API gateways were set up with the usual defaults, AFAICT.