RabbitMQ basic.get and acknowledgement

问题:

I'm invoking:

GetResponse response = channel.basicGet("some.queue", false); // no auto-ack
....
channel.basicAck(deliveryTag, ...);

However, when I invoke basicGet, the messages in the queue stay in "Ready", rather than in "Unacknowledged". I want them to be in unacknowledged, so that I can either basic.ack them (thus discarding them from the queue), or basic.nack them

回答1:

I'm doing the following to mimic Delaying the ack:

At consumption time

1. Get(consume) the message form the initial Queue.
2. Create a "PendingAck_123456" Queue.
   123456 is a unique id of the message.
   Set the following properties
   • x-message-ttl (to requeue after timeout)
   • x-expires (to make sure the temp queue will be deleted)
   • x-dead-letter-exchange and x-deal-letter-routing-key to requeue to the initial Queue upon TTL expiration.
3. Publish the message Pending ack to this "PendingAck_123456" Queue
4. Ack the message to delete it from the initial queue

At Acknowledge time

1. Calculate Queue Name from Message Id and Get from the "PendingAck_123456" Queue
2. Acknowledge it (no need to call .getBody() ).
   That'll delete it from this pending queue, preventing the TTL to requeue it

Remarks

• A Queue for only 1 message.. Is that an issue if there are a lot of such Queues ?
• A requeued message will be sent at the queue input side.. not at the queue output (as would do a real ack).. There is an impact on the messages order.
• Message is copied by the application to the Pending Queue.. This is an additional step that may have impacts on the overall performance.
• To mimic a Nack/Reject, you you may want to Copy the message to the Initial Queue, and Ack it from the PendingAck queue. By default, the TTL would do it (later).

回答2:

When doing ack immediately after the get it works fine. However, in my case, they were separated by a request. And spring's template closes the channel and connection on each execution. So there are three options:

• keep one channel and connection open throughout the whole lifetime of the application
• have some kind of conversation-scope (or worst-case: use the session) to store the same channel and reuse it.
• use one channel per request, acknowledge receipt immediately, and store the messages in memory.

In the former two cases you can't do it with spring's RabbitTemplate