Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 112

5.3 Asynchronous Middleware
In an environment where multiple applications and Web Services need to
interact with each other, it is not practical to expect that each application
knows the signature characteristics of every other application's methods.
Instead, the intricacies of the service interface should not necessarily be
known to all interacting applications. Asynchronous communication pro-
motes a loosely coupled environment in which an application does not need
to know the intimate details of how to reach and interface with other appli-
cations. Each participant in a multistep business process flow need only be
concerned with ensuring that it can send a message to the messaging system.
In general, asynchronous communication is often the preferred solution for
EAI and cross-enterprise computing, especially when applications want to
transfer data between internal enterprise information systems, for example,
databases and ERP packages, or between their systems and those of their
partners.
5.3.1 Store and Forward Messaging
With the store and forward queuing mechanism, messages are exchanged
through a queue, which is the destination to which senders send messages
and a source from which receivers receive messages. Messages are placed on
a virtual channel called a message queue by a sending application and are
retrieved by the receiving application as needed—the queue is a container
that can keep hold of a message until the recipient collects it. The message
queue is independent of both the sender and receiver applications and acts
as a buffer between the communicating applications. The physical location
of the queue or the physical details of the host platform are immaterial, all
that is required is that an application is in some way registered or connected
to the message queue subsystem. This provides a useful form of abstrac-
tion that enables physical implementations to be changed on either platform,
without affecting the rest of the implementation.
The store and forward queuing mechanism is typical of a many-to-one mes-
saging paradigm where multiple applications can send messages to a single
application. The same application can be sender, receiver, or both sender and
receiver. Message queuing provides a highly reliable, although not always
timely, means of ensuring that application operations are completed.
5.3.2 Publish/Subscribe Messaging
Publish/subscribe messaging is a slightly more scalable form of messaging
when compared to the store and forward mechanism. With this type of
asynchronous communication, the application that produces information
publishes it and all other applications that need this type of information