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TL;DR - it stops very abruptly so make sure you monitor the Lambda Errors CloudWatch metrics.
I'd heard a rumour that when a Lambda ran out of RAM, it didn't log a message or metric, which didn't sound right, so I thought I'd try it out and find out.
I wrote a Lambda which had a RAM limit of 128MB and then purposefully exceeded it and deployed it. [0]
Then, I executed it via the AWS CLI.
aws lambda invoke \
--invocation-type RequestResponse \
--function-name oom-dev-memory \
--region eu-west-2 \
--log-type Tail \
--payload "{}" \
output.txt
Next, I checked the Lambda dashboard and I could clearly see the failed invocation, showing that a CloudWatch metric was available.
The first time I ran the Lambda, I'd forgotten to increase the maximum execution time from the Serverless Framework's 6 seconds. Even though it had used up a little more than the 128MB of RAM I'd allocated, it was clearly logged as a timeout.
Once I increased that number, it was clear in the log entries that `Process exited before completing request` is the log entry associated with running out of RAM, while `Task timed out after 6.00 seconds` is written after a timeout.
START RequestId: 16f8d3a2-26fc-11e8-bca6-73dbc61c39e7 Version: $LATEST I'm about to use up a lot of RAM... 1MB consumed 2MB consumed 3MB consumed 4MB consumed 5MB consumed 6MB consumed 7MB consumed 8MB consumed 9MB consumed 10MB consumed 11MB consumed 12MB consumed 13MB consumed 14MB consumed 15MB consumed 16MB consumed 17MB consumed 18MB consumed 19MB consumed 20MB consumed 21MB consumed 22MB consumed 23MB consumed 24MB consumed 25MB consumed 26MB consumed 27MB consumed 28MB consumed 29MB consumed 30MB consumed 31MB consumed 32MB consumed 33MB consumed 34MB consumed 35MB consumed 2018/03/13 20:21:44 unexpected EOF 2018/03/13 20:21:44 unexpected EOF END RequestId: 16f8d3a2-26fc-11e8-bca6-73dbc61c39e7 REPORT RequestId: 16f8d3a2-26fc-11e8-bca6-73dbc61c39e7 Duration: 24792.85 ms Billed Duration: 24800 ms Memory Size: 128 MB Max Memory Used: 129 MB RequestId: 16f8d3a2-26fc-11e8-bca6-73dbc61c39e7 Process exited before completing request
If the difference between the failure reasons is important, it would be easy enough to write a CloudWatch log extractor to be able to see the difference between out-of-memory and timeout execution failures.
The `2018/03/13 20:31:51 unexpected EOF` wasn't written by my Lambda, it looks like it's part of the Go Lambda runtime, since it uses the default console log format for Go's `log` package.
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