Lambda function does not retry the sqs message processing in case the Java code throws a runtimeException

I have a lambda function written in java that listens to sqs events and tries to do some kind of processing on those sqs messages.

As per lambda documentation, if lambda code throws a runtimeException then, lambda would retry the same message twice before it sends it back to the queue. However, I don't see that behavior. I only see it processing the message just once.

Here is a snippet from the relevant lambda code in this case.

@Override
    public Boolean handleRequest(SQSEvent sqsEvent, Context context) {

          try{
               ........some queue message processing.....
          }
          catch(Exception ex){
               throw new RuntimeException("exception occurred");
          }

}

Is this not good enough for lambda to retry the message 2 more times? I did check the cloudwatch to see what lambda logs and it just has logs from the very first processing only and not the retries.

Can someone tell me what did I miss here because of which it does not work as expected.

Solution

You are missing the throws clause in handleRequest. If you don't have that then, lambda would just swallow the exception

public Boolean handleRequest(SQSEvent sqsEvent, Context context) throws RuntimeException

Other than that, what Thales Munussi has told you about synchronous polling is absolutely right. When you hook sqs with lambda, lambda polls sqs which keeps an open connection between the two hence making it a synchronous connection As per aws documentation, lambda doesn’t retry in such synchronous cases. Setting up a dlq and retires in sqs itself is your best recourse

Keep in mind that lambda would send the message back to the queue after a runtime exception is thrown in your java code Based on the redrive setting in your sqs, the sqs will generate the same event based on redrive number.

Once lambda fails to process successfully for redrive number of times, message is sent to the DLQ from the main queue