Do you have any resources on testing a Lambda? When I was fooling around with it, the only thing I ran into was that AWS Sam-client or whatever. Thing looked like an absolute nightmare to get up and running.
Your handler should just accept the JSON value and the lambda context and then convert the JSON to whatever plain old object your code needs to do to process it and call your domain logic.
AWS has samples of what the JSON looks like for the different types of events. You can see the samples by just creating a new lambda in the console, click on test and then see the different types of events.
You can also log the JSON you receive and use that to setup your test harness. I don’t mean an official AAA type of unit test, it can be as simple as having a console app that calls your lambda function and passes in the JSON.
For instance in Python, you can wrapped your test harness in an
if __name__ == "__main__":
This is the same method that a lot of people use to test API controllers without using something like Postman.
So you tried it? I don't remember it being hard to set up, at least compared to a DB. Or, you can use the underlying docker images (open source, https://github.com/lambci/lambci) to run your Lambdas in. SAM provides some nice abstractions, e.g. API gateway "emulation", posting JSON to the function(s), or providing an AWS SDK-compatible interface for invoking the functions via e.g. boto3. This way you can run the same integration tests you would run in prod against a local testing version.
The good news; you should be able to accomplish most testing locally, in memory. The bad news; your test code is probably going to be much larger and your going to have to be very aware of the data model representing the interface to the lambda and you're going to have to test the different possible states of that data.
I'll typically write all the core functionality first, test it, then write the lambda_handler function.