Styjun

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.

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

The documentation says it retries two more times if the invocation is asynchronous. SQS is a poll-based system. Lambda will poll the Queue and all of its invocations will be synchronous.

For poll-based AWS services (Amazon Kinesis, Amazon DynamoDB, Amazon
Simple Queue Service), AWS Lambda polls the stream or message queue
and invokes your Lambda function synchronously.

What you can do is configure a DLQ on your source SQS queue in case your message fails so you can either analyse it further or process the message again based on the logic you have configured.

EDIT

The OP is not able to see the messages in the DLQ for somehow. I have attached images to show it works.

Lambda sqs-test is triggered by a new message in SQS queue sqs-test

These are the Queues (sqs-test-dlq is configured as a DLQ for sqs-test).

This is the code for the Lambda function:

This is the configuration for sqs-test

And this is the redrive policy

After the messages failed in the Lambda function, they were successfully sent to the configured DLQ:

You must be missing some basic configuration because it works seamlessly as the images above show.