mstn/unhandled-exceptions-in-aws-lambdas

some experiments

JavaScript

How to handle AWS Lambda errors

According to [4] an AWS Lambda can fail in three cases

Unhandled exception. For example, explicit uncaught throw, TypeError or ReferenceError.
Timeouts.
Out of memory.

Retry on error

"Depending on the event source, AWS Lambda might retry the failed Lambda function. For example, if Kinesis is the event source, AWS Lambda retries the failed invocation until the Lambda function succeeds or the records in the stream expire." [1]

Asynchronous invocation – Lambda retries function errors twice [3].
We should pay attention if some side effects are executed inside the lambda. Field context.awsRequestId [4] is what we are interested in if the lambda is not idempotent. More strategies in [4] (in memory data store, step functions).

Unhandle exceptions

Good pratices

try-catch in entrypoint lambda
How to read the thrown error

How to log timeouts and out of memory

Timeout and out of memory errors are logged in Cloudwatch, but sometimes we want to make them more visible.

Timeout errors

They cannot be captured with try-catch, but you can use getRemainingTimeInMillis as explained in [5].
Even if that technique can generate false positive, I think it is valuable since timeouts are hard to observe.

Out of memory

Some technique similar to [5] using memoryLimitInMB and nodejshttps://nodejs.org/api/process.html#process_process_memoryusage should be fine.

Scenarios

we return { statusCode: number, body: string } since it is what it is expected by API Gateway. Otherwise, we get {"message": "Internal server error"}. Other ways to invoke lambdas might require different return values.
Runtime nodejs12.x (latest January 2020)

curl /functions/1

try-catch everything/force TypeError
Return {}
CloudWatch INFO ERROR {}
Lost info about error

curl /functions/2

Don't try-catch/force TypeError
Return {"message": "Internal server error"}
CloudWatch ERROR Invoke Error {"errorType":"TypeError","errorMessage":"Cannot read property 'someProperty' of undefined"...
We get a clear description of the error

curl /functions/3

try-catch everything/force TypeError
Return {}
CloudWatch INFO ERROR {}
Lost info about error

curl /functions/4

try-catch everything/explicit throw
Return {}
CloudWatch INFO ERROR {}
Lost info about error

curl /functions/5

try-catch everything/explicit throw
Return {}
CloudWatch INFO ERROR {}
Lost info about error

curl /functions/6

a global try-catch swallows error info
Use this trick to print or return error info

curl /function/7

CloudWatch Task timed out after 1.00 seconds

References