title: AWS Lambda: Identifying cold starts
body: <p>Is there a clear way to identify "cold starts"? Either in runtime in the Lambda itself, or via the logs? I know that cold starts are characterized by longer runtimes, which I can actually see, but I'm looking for a clear cut way. 
  I'm using Node.js if that matters.</p>
  
  <p>Update: There are two good answers below, for two use cases:
  - Identifying the cold start as the lambda runs.
  - Identifying the cold start from the CloudWatch log.</p>