How can I detect cold starts in AWS Lambda?

Question

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.

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.

Answer 1

If you add some initialization code to the top of your NodeJS script, you will be able to tell in the code that it is a cold start, and you will then be able to log that if you want to see it in the logs. For example:

var coldStart = true;
console.log("This line of code exists outside the handler, and only executes on a cold start");


exports.myHandler = function(event, context, callback) {
  if (coldStart) {
    console.log("First time the handler was called since this function was deployed in this container");
  }
  coldStart = false;

   ...
   
  callback(...);
}

---

Update:

If you only care about seeing cold starts in the logs, Lambda now logs an extra "Init Duration" value in CloudWatch Logs for cold starts.