According to the docs, a timeout should be in the Errors metric. I observed weird behaviour with the count (e.g. having an error count of 0.5). Hence I made a CloudWatch alarm for the errors count > 0 (not >= 1).
You could also do something with the REPORT message or with
Task timed out after 25.00 seconds
which can be found in the Cloudwatch logs.
You can have CloudWatch trigger an alarm when a certain message shows up in the logs. I can't seem to find any official documentation on this, but you create a "Metric Filter" in CloudWatch Logs, and then you can create an alarm from from that. This blog post seems to describe the process well.
I've created an alarm in CloudWatch for a Lambda metric of type "Duration" and selected the Statistic of "Maximum" to alert me when the execution duration is greater/equal 30000 (= 30 seconds) for a Lambda function configured with a timeout of 30 seconds.
If the duration of a single execution ("maximum" of the period) exceeds the timeout time, you will be notified. It is working fine for me.
You can check within a function how many milliseconds left and if your function is about to timeout send a notification from there.
From the docs:
context.getRemainingTimeInMillis()
Returns the approximate remaining execution time (before timeout occurs) of the Lambda function that is currently executing. The timeout is one of the Lambda function configuration. When the timeout reaches, AWS Lambda terminates your Lambda function.
You can use this method to check the remaining time during your function execution and take appropriate corrective action at run time.
