RabbitMQ, delivery tag value and message order after reconnect

Question

I use pika for python to communicate with RabbitMQ. I have 6 threads, which consume and acknowledge messages from the same queue. I use different connections(and channels) for each thread. So i have a few questions very close to each other:

1. If connection to rabbit will close in 1 of the thread, and i will make reconnect, delivery tag value will reset and after reconnect it will start from 0?

2. After reconnect i will receive same unacknowledged messages in the same order for each thread or it will start distribute them again between all threads or it will start from reconnect point?

It is important in my app, because there is delay between message receiving and acknowledgement, and i want to avoid duplicates on the next process steps.

Answer 1

1. Delivery tag are channel-specific and server assigned. See details in AMQP spec, section 1.1. AMQP­defined Domains or RabbitMQ's documentation for delivery-tag. RabbitMQ initial value for delivery-tag is 1,

   Zero is reserved for client use, meaning "all messages so far received".

2. With multiple consumers on a single queue there are no guarantee that consumers will get messages in the same order they was queued. See RabbitMQ's Broker Semantics, "Message ordering guarantees" paragraph for details:

   Section 4.7 of the AMQP 0-9-1 core specification explains the conditions under which ordering is guaranteed: messages published in one channel, passing through one exchange and one queue and one outgoing channel will be received in the same order that they were sent. RabbitMQ offers stronger guarantees since release 2.7.0.

   Messages can be returned to the queue using AMQP methods that feature a requeue parameter (basic.recover, basic.reject and basic.nack), or due to a channel closing while holding unacknowledged messages. Any of these scenarios caused messages to be requeued at the back of the queue for RabbitMQ releases earlier than 2.7.0. From RabbitMQ release 2.7.0, messages are always held in the queue in publication order, even in the presence of requeueing or channel closure.

   With release 2.7.0 and later it is still possible for individual consumers to observe messages out of order if the queue has multiple subscribers. This is due to the actions of other subscribers who may requeue messages. From the perspective of the queue the messages are always held in the publication order.

   Also, see this answer for "RabbitMQ - Message order of delivery" question.