aleph appears to acquire from a pool here : https://github.com/ztellman/aleph/blob/master/src/aleph/flow.clj#L78 , which is using the callback mechanism to fulfil a Deferred ... presumably the callback mechanism is to allow for the case where a pool has already reached its maximum size and .acquire requests are then queued

without having traced the code all the way, that's what appears to be happening here : https://github.com/ztellman/dirigiste/blob/master/src/io/aleph/dirigiste/Pool.java#L409

i need to look at the aleph client some more... i've had a bug recently where the aleph client appears to stop working, although everything else carries on working fine... my guess was that aleph was leaking connections causing a terminal pause here: https://github.com/ztellman/aleph/blob/master/src/aleph/http.clj#L261 , but i've not managed to duplicate the bug in a test environment yet