Heh, as I just replied to the comment you made elsewhere -
"Yeah, RabbitMQ opted for the 2nd because of that. BQ in that case is whatever queue implementation was selected; if you look at both the lqueue.erl and priority_queue.erl, you see that they implemented len() by pulling apart a tuple and taking a value verbatim; no further operations. As such, I'd assume every queue type they support has its own internal wrappers to maintain a length."
EDIT: Just to explain the Erlang a bit, what is happening is the #q record that is being pattern matched on in the function you mention has a background_queue field. This field is an atom, corresponding to the module the queue implementation lives in. So 'lqueue' or 'priority_queue', or whatever else fits the interface (behavior in Erlang terms). So it actually ends up calling lqueue:len(BQS) or priority_queue:len(BQS), or similar. If you look at the implementations of those, you'll see they've wrapped up the actual queue into a tuple, bundling along a length, such that they can simply return the length, rather than recalculate it every time. So it looks like in the intervening 4-5 years, they -have- fixed it.
"Yeah, RabbitMQ opted for the 2nd because of that. BQ in that case is whatever queue implementation was selected; if you look at both the lqueue.erl and priority_queue.erl, you see that they implemented len() by pulling apart a tuple and taking a value verbatim; no further operations. As such, I'd assume every queue type they support has its own internal wrappers to maintain a length."
EDIT: Just to explain the Erlang a bit, what is happening is the #q record that is being pattern matched on in the function you mention has a background_queue field. This field is an atom, corresponding to the module the queue implementation lives in. So 'lqueue' or 'priority_queue', or whatever else fits the interface (behavior in Erlang terms). So it actually ends up calling lqueue:len(BQS) or priority_queue:len(BQS), or similar. If you look at the implementations of those, you'll see they've wrapped up the actual queue into a tuple, bundling along a length, such that they can simply return the length, rather than recalculate it every time. So it looks like in the intervening 4-5 years, they -have- fixed it.