[QUESTION] Message ordering in a fanout exchange

[Dominik Enzinger]

Lady- and Gentlerabbits,

considering the following template, is every consumer of each dedicated queue receiving messages in the exact same order as the are published?

Important Notes

• Exchange A is a fanout exchange
  
• Application A will send each message in order with Publisher Confirms and Batch Publishing, see Code Snippet A
  
• Each applications has its own dedicated queue and will be consumed by only one consumer.
• RabbitMQ version is >= 3.7.5

Code Snippet A

$channel = $this->connection->channel();

$channel->set_ack_handler(
    function (AMQPMessage $AMQPMessage) {
        $this->acknowledge($AMQPMessage); // This will delete the message from the message buffer
    }
);

$channel->confirm_select();

// Ensure exchange exists
$channel->exchange_declare($this->exchange, '', true);

$i = 0;
foreach ($this->messageBuffer->findUnsentMessages() as $message) { // This will iterate through every message from the buffer in order
    $AMQPMessage = $this->converter->toAMQPMessage($message);

    $channel->batch_basic_publish($AMQPMessage, $this->exchange);

    if ($i % self::BATCH_SIZE === 0) {
        $channel->publish_batch();
    }

    $i++;
}

$channel->publish_batch();

$channel->wait_for_pending_acks();

$channel->close();

Thanks in advance,

Dominik.

[Michael Klishin]

Exchanges do not store messages. They process them in the order they were published

on the channel.

Queues then store and deliver messages in the FIFO manner. Requeueing [1]

can complicate this but RabbitMQ will try to requeue deliveries into the original position

if possible.

HTH.

1. http://www.rabbitmq.com/confirms.html

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MK

Staff Software Engineer, Pivotal/RabbitMQ