TL;DR: There is no Datomic-supported way to route lambda fn inputs to locally running ions (but perhaps the serverless plugin or other local AWS simulator could route payloads to your JVM ion... I don't know). The approach Cognitect promotes isn't bad: test your lambda fn ion code as a regular function. It is not obvious how to find all the different "shapes" of input to lambda ions, but it's generally a straightforward translation of the AWS-documented lambda payloads. Then testing/developing becomes Clojure data into a local "plain" function.