On Mon, Apr 22, 2013 at 09:31:09AM -0400, Rafael Schloming wrote: > To understand this, it's helpful to think in terms of which state is > associated with the transport layer and which is associated with the > application layer. It also helps to think in terms of communication > endpoints, i.e. an entity capturing the relevant state for a given > communication layer. The link construct exists at the transport layer, and > at the transport layer, link establishment never fails. An attach is always > answered by another attach, and paired set of link endpoints (both local > and remote) are always created. The source and target serve as a binding > between the application level communication endpoint and the transport > layer endpoint. The reason this can be null is because there isn't always > an application level endpoint. > > > As a concrete example you might consider sending an attach from a client to > a broker and naming a queue that does not exist: > > Client -------- ATTACH(target="no-such-queue") ---> Broker > > The broker then responds with an attach where the target is null: > > Client <------- ATTACH(target=null) --------------- Broker > > The client then compares the target he asked for ("non-existent-queue") > with the target he got (null) and can indicate an error to the user. > > This might seem like an odd way to fail link establishment, however it is > really a special case of a more general negotiation pattern, for example > the same pattern might be used to do a kind of redirect: > > Client -------- ATTACH(target="old-queue") -------> Broker > Client <------- ATTACH(target="new-queue") -------- Broker > > There are also other properties of sources and targets that can be > negotiated in this way, e.g. distrubtion mode, filters, general > capabilities and/or properties. So the general pattern is link > establishment always succeeds at the transport layer, however at the > application layer you might not get exactly what you asked for, and if the > remote target/source is indicated as null, then you didn't get anything at > all.
Ah, I see how it's useful. It just seemed odd without the concrete examples. Thanks! -- Darryl L. Pierce, Sr. Software Engineer @ Red Hat, Inc. Delivering value year after year. Red Hat ranks #1 in value among software vendors. http://www.redhat.com/promo/vendor/
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