Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 122

server will deliver a message to its subscribing client as long as there are no
application or network failures—delivery would fail if some disruption were
to occur during delivery. With guaranteed message delivery, the message
server will deliver a message even if there are application or network fail-
ures. The messaging server will store the message in its persistent store and
then forward the message to its subscribing clients. After the client processes
the message, it sends an acknowledgment to the messaging server and veri-
fies the receipt of the message.
5.6 Request/Reply Messaging Middleware
Most of the asynchronous messaging mechanisms that we have examined so
far follow the fire-and-forget messaging principle. On many occasions, appli-
cations require that request/reply messaging operations be performed. Here,
we can distinguish between two types of request/reply messaging opera-
tions: synchronous request/reply messaging and asynchronous request/
reply messaging operations. Synchronous request/reply messaging is often
necessary when trying to integrate with a Web Service client that blocks and
waits for a synchronous response to return to it. In the asynchronous version
of request/reply messaging, the requestor (sender) expects the reply to arrive
at a later time and continue its work unaffected.
To perform a request/reply operation, the sender must use two channels:
one for the request and one for the reply. The request message needs to contain
reference to the receiver's end point, along with a correlation identifier that
is needed to correlate the request with the response message. The requestor
needs to poll a reply channel for the reply message. Both request/reply mes-
saging modes can be layered on top of message-oriented middleware. Some
Message-oriented middleware systems can further automate this process by
managing the contents of the request/reply message.
5.7 Transaction Processing Monitors
Most messaging systems allow for transactional messaging that entail four
ACID properties, namely, atomicity, consistency, isolation, and durability.
In a synchronous request using distributed objects, the request by the call-
ing object and the response from the receiving object use one process. This
ensures the calling object receives the response to the request in the same
connection to the object-based middleware. In contrast, transactional mes-
saging uses one process for sending the message and a separate process for