title: Amazon SQS message multi-delivery
body: <p>I understand that to bring vast scalability and reliability, SQS does extensive parallelization of resources. It uses redundant servers for even small queues and even the messages posted to the queues are stored redundantly as multiple copies. These are the factors which prevent it from exactly-once-delivery like in RabbitMQ. I have seen even deleted messages being delivered.</p>
  
  <p>The implications for the developers is that they need to be prepared for multiple delivery of messages. Amazon claims it not to be a problem, but it it is, then the developer must use some synchronization construct like a database-transaction-lock or dynamo-db conditional write. both of these reduce scalability.</p>
  
  <h2>Question is,</h2>
  
  <p>In light of the duplicate delivery problem, how the <strong>message-invisible-period</strong> feature holds? The message is not guaranteed to be invisible. If the developer has to make own arrangements for synchronization, what benefit is of the invisibility-period. I have seen messages re-delivered even when they were supposed to be invisible.</p>
  
  <h2>Edit</h2>
  
  <p>here i include some references</p>
  
  <ol>
  <li><a href="https://stackoverflow.com/q/13484845/1043824">What is a good practice to achieve the &quot;Exactly-once delivery&quot; behavior with Amazon SQS?</a></li>
  <li><a href="http://aws.amazon.com/sqs/faqs/#How_many_times_will_I_receive_each_message" rel="nofollow noreferrer">http://aws.amazon.com/sqs/faqs/#How_many_times_will_I_receive_each_message</a></li>
  <li><a href="http://aws.amazon.com/sqs/faqs/#How_does_Amazon_SQS_allow_multiple_readers_to_access_the_same_message_queue_without_losing_messages_or_processing_them_many_times" rel="nofollow noreferrer">http://aws.amazon.com/sqs/faqs/#How_does_Amazon_SQS_allow_multiple_readers_to_access_the_same_message_queue_without_losing_messages_or_processing_them_many_times</a></li>
  <li><a href="http://aws.amazon.com/sqs/faqs/#Can_a_deleted_message_be_received_again" rel="nofollow noreferrer">http://aws.amazon.com/sqs/faqs/#Can_a_deleted_message_be_received_again</a></li>
  </ol>