On Wed, Jul 10, 2013 at 11:59 AM, Ken Giusti <kgiu...@redhat.com> wrote:
> Hi, > > AMQP 1.0 defines a way to force a sending link to exhaust its credit via a > drain flag in the Flow frame (see the flow control section in the spec). > > Hate to admit my ignorance, but how does one properly drain a link using > the Proton engine api? > > From what I can tell (using the proton-c api here), the application > initiates the drain by calling pn_link_drain() for a given link: > > pn_link_drain( myLink, X ) > > How does the application know when the drain has completed? My guess is > that the drain can be considered complete when the pn_link_credit() call > returns zero (no more credit) - is that correct? > Almost, pn_link_credit() would return the local view of the credit, what we actually need here is to add a pn_link_remote_credit() call which is what you would use. > > Furthermore, it appears that then engine api requires the sending > application acknowledge the drain request by calling pn_link_drained() on > the sending link. But I don't see an engine interface that notifies the > application that a drain has been requested for the link. > > Are we missing a "get drain flag" api call for the sender link? > We could add that certainly, although it's not strictly speaking necessary. We've said to date that the sender should simply always call pn_link_drained() when there are no more messages available and the implementation of the engine will do the appropriate thing depending on the mode of the link. The application itself doesn't need to know/care about the drain flag, simply notify the engine whenever the supply of messages is exhausted. Obviously if there is some burden/overhead/etc associated with the application knowing when the supply of messages is exhausted then that would be good reason to add visibility to the drain flag, but so far it doesn't seem to have been an issue. --Rafael
