AWS Lambda: Identifying cold starts

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

As an update, AWS now provide visible info on cold starts in the form of "Init Duration" , inside the Report section of a Cloudwatch Log. The calls that do not suffer from a cold start will not contains this information in the log

Duration: 1866.19 ms Billed Duration: 1900 ms Memory Size: 512 MB Max Memory Used: 163 MB Init Duration: 2172.14 ms

Answer 2

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(...);
}

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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.