I've worked with a couple of clients who use standby queues. This is supposed to solve problems, but I think it really creates more.
The problem is the same as described in The Message Consumer Rollback Pattern: A consumer successfully receives a valid message but cannot process it because a resource it needs is down.
We know the message is valid because it passes the message validator.
Options: Throw away the message, retry, put it on an invalid message queue. All bad.
If only there were someplace to queue up these messages that cannot be processed, so that they can be processed later.
Therefore, use a standby queue. When a message cannot be processed, move it to the standby queue.
This seems great. The message is out of the way but not lost. Processing can continue on other messages on the queue.
But wait: If some of the messages fail, won't they all fail? The queue should be a datatype channel, so the messages should all be doing the same sort of thing, thus all requiring the unavailable resource; so why keep reading from it? Let's say it's not a datatype channel, that some of the messages need database A and others need database B. Then you need two standby queues, one for messages that need database A and one for B messages; so that when one database is available again, you'll know which messages to retry. With two standby queues, A and B, you've now created datatype channels, so you should have just done that in the first place.
Anyway, once you get the messages on the standby queue, how do you get them off again? You don't want to do it until the resource is available again, but how do you know when that is? When it is, what do you do with the messages? Move them from the standby queue back to the input queue? How?
Standby queues also make transactions very difficult. When you move a message from one queue to another, you need to do so in a single transaction so that the message cannot be lost. But if the transaction tries to use the resource and fails (which is the whole premise of this (anti)pattern), then the transaction is shot, so it can't be used to commit the message onto the standby channel. So separate transactions need to be used to read the message off the input queue and update the resource, which is a lousy transaction model that can easily duplicate or loose data.
So standby queues are bad. I wouldn't use them.
What should you do instead? Use The Message Consumer Rollback Pattern. If the message is valid, but you can't process it right now, don't bother putting it on a dead letter queue or an invalid message queue or a standby queue, just roll back the transaction and the message will roll back onto the input queue. It's the best way to put things back the way they were and wait until the resource is available again.