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From Kai Hudalla <sophokles...@gmail.com>
Subject Re: [proton-j] handling of link-credit
Date Wed, 01 Mar 2017 15:29:41 GMT
On Wed, 2017-03-01 at 15:07 +0000, Robbie Gemmell wrote:
> On 1 March 2017 at 14:31, Kai Hudalla <sophokles.kh@gmail.com> wrote:
> > Hi,
> > 
> > we are working on the Eclipse Hono project where we use vertx-proton and
> > (implicitly) proton-j under the hood for exchanging large amounts of messages
> > using AMQP 1.0.
> > 
> > During our tests I stumbled across the way that proton-j seems to handle
> > link-
> > credit being exchanged via FLOWs.
> > My understanding is the following:
> > 
> > Assuming that we have a link established between a Receiver (r) and a Sender
> > (s)
> > with a current link-credit of 4 and a delivery count of 20 on both sides.
> > 
> > When invoking r.flow(6), the given credit (6) is _added_ to the receiver's
> > current credit resulting in r.getCredit() returning 10.
> > 
> > When the FLOW is then flushed to the sender, the sender seems to _add_ the
> > link-
> > credit from the FLOW to its already existing credit. Analogously, this
> > results in
> > s.getCredit() now returning 10.
> 
> Correct. In proton the flow(credits) method grants additional credits.
> 
> > 
> > With this approach, it doesn't seem to be possible to stop the sender from
> > sending messages. The only thing a receiver can do is to wait until the
> > sender
> > has used up all its credit (which may be a lot given that with the current
> > approach the sender's credit can pile up substantially).
> > 
> > Or am I mistaken?
> 
> Granting credits is essentially giving the sender ability to send that
> many messages. If you are controlling credit, any 'pile up' of credit
> at the sender is one that you create yourself - if you dont want the
> sender to be able to send more than a certain number of messages at a
> given time, dont allow more than that number of credits to be
> outstanding at the time.

So, still, there is no way to stop a sender from sending once it has credit and
wants to send messages, right? My understanding of the AMQP 1.0 spec is that it
should be possible to do so by flowing a link-credit of 0 to the sender.

> 
> One thing you may not be aware of is you can drain the link of
> existing credit, which essentially asks the sender to send what they
> can (something they typically will have already done if they have
> credit and anything to send) then spontaneously advance the delivery
> count to the extent needed to use up all outstanding credit.
> 

I am aware of this but it also doesn't help with stopping a sender that is
already sending, does it?

My understanding of all of this is that a Receiver should not grant much credit
to a Sender (because it cannot stop the sender once it has started sending). 
Then, when a message is coming in, the receiver should check whether there is
credit left for the sender (using Receiver.getCredit() ?) and if not it should
grant some more credit using Receiver.flow(small number) again.

Does that make sense?


> > Also, I cannot find the code where the link credit is actually updated from a
> > FLOW received from a Receiver. Can you point me to the right class?
> > 
> 
> TransportSender is probably what you want.
> 
> > Regards,
> > Kai
> > 
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