How do I fail a specific SQS message in a batch from a Lambda?

Question

I have a Lambda with an SQS trigger. When it gets hit, a batch of records from SQS comes in (usually about 10 at a time, I think). If I return a failed status code from the handler, all 10 messages will be retried. If I return a success code, they'll all be removed from the queue. What if 1 out of those 10 messages failed and I want to retry just that one?

exports.handler = async (event) => {

    for(const e of event.Records){
        try {
            let body = JSON.parse(e.body);
            // do things
        }
        catch(e){
            // one message failed, i want it to be retried
        }        
    }

    // returning this causes ALL messages in 
    // this batch to be removed from the queue
    return {
        statusCode: 200,
        body: 'Finished.'
    };
};

Do I have to manually re-add that ones message back to the queue? Or can I return a status from my handler that indicates that one message failed and should be retried?

Answer 1

Yes you have to manually re-add the failed messages back to the queue.

What I suggest doing is setting up a fail count, so that if all messages failed you can simply return a failed status for all messages, otherwise if the fail count is < 10 then you can individually send back the failed messages to the queue.

Answer 2

You've to programmatically delete each message from after processing it successfully.

So you can have a flag set to true if anyone of the messages failed and depending upon it you can raise error after processing all the messages in a batch so successful messages will be deleted and other messages will be reprocessed based on retry policies.

So as per the below logic only failed and unprocessed messages will get retried.

import boto3

sqs = boto3.client("sqs")

def handler(event, context):
    for message in event['records']:
        queue_url = "form queue url recommended to set it as env variable"
        message_body = message["body"]
        print("do some processing :)")
        message_receipt_handle = message["receiptHandle"]
        sqs.delete_message(
            QueueUrl=queue_url,
            ReceiptHandle=message_receipt_handle
        )

there is also another way to save successfully processed message id into a variable and perform batch delete operation based on message id

response = client.delete_message_batch(
    QueueUrl='string',
    Entries=[
        {
            'Id': 'string',
            'ReceiptHandle': 'string'
        },
    ]
)