title: Anyone has experience with triggering step function with S3 event?
body: <p>I'm learning about step function and specifically, I'm trying to figure out how to trigger state machine execution with S3 event. I was reading this post: <a href="https://docs.aws.amazon.com/step-functions/latest/dg/tutorial-cloudwatch-events-s3.html" rel="noreferrer">https://docs.aws.amazon.com/step-functions/latest/dg/tutorial-cloudwatch-events-s3.html</a>. This documentation does give a rough guide of how to configure stuff but I'm still unclear about the following questions:</p>
  
  <ol>
  <li><p>What does the state machine input mean? So this documentation doesn't explain too much about what does each field in the input mean. Does anyone know where to find the documentation for it? For example, what's the id field in this input?</p></li>
  <li><p>How does a Java lambda retrieve useful information from the input? I saw documentation about how to manipulate input that was predefined in the state machine schema (cloudformation or Amazon Statemachine Lamguage) but not for the input that was auto-generated by a s3 event.</p></li>
  </ol>
  
  <p>Does anyone ever built similar functionality before using state machine + s3 event? Any thoughts would be appreciated.</p>