The hub-and-spoke paradigm is based on a centralized hub and a number of

The centralized enterprise application integration middleware represents the
hub, and the applications to be integrated are reflected by the spokes. The
applications interact with each other via the centralized enterprise application
integration hub.
It is an important feature of hub-and-spoke architectures that the sender of
a message need not encode the receiver of the message. Instead, each message
is sent to the enterprise application integration hub. The hub is configured in
such a way that the message structure and content can be used to automatically
detect the receiver or receivers of a message.
The advantage of these centralized middleware architectures is that the
number of connections can be reduced. No longer are connections in the order
of N � N required to connect N application systems. Since each application
system is attached to the centralized hub, N interfaces will suffice. Using these
interfaces, the specific relationships between the applications can be reflected
in the configuration of the middleware.
The centralized hub provides
adapters that hide the heterogeneity of the application systems from
each other. Each application system requires the development of a dedicated
adapter to attach to the hub.
Depending on the complexity of these systems�and the availability of
generic adapters provided by the enterprise application integration vendor�
the development of the adapter might consume considerable resources. When
the adapters are in place and the hub is configured, the applications can
interact with each other in an integrated manner.
On a technical level, message brokers can be used to realize a hub-andspoke
enterprise application integration system. Message brokers are software
systems that allow a user to define rules for communication between applications.
Therefore, the burden of implementing�and changing�communication
structures is taken away from applications. By defining in a declarative way
how communication between applications takes place, implementation is redeemed
by declaration, i.e., by the declaration of the communication structures.
Response to change is improved, because the sender is not required to
implement these changes locally. These changes can be specified in a declarative
way in the central hub, rather than by coding in the applications.
The hub uses rules to manage the dependencies between the applications.
Based on these rules, the hub can use information on the identity of the sender,
the message type, and the message content to decide on which message queues
to relay a message received. Besides relaying messages to recipients, message
brokers also transform messages to realize data mapping between the applications,
so that data heterogeneity issues can be handled appropriately.
Adapters of application systems are used to perform these message transformations.
As shown in Figure 2.8, each application is linked to the message broker,
reflected by the directed arcs from the applications to the message broker, in
particular, to the rule evaluation component of the message broker. On receipt
of a message, the message broker evaluates the rules and inserts the message
into the queues of the recipients.
The queues are used for guaranteed delivery of messages. Note that any
change in the communication is handled through the message broker: by establishing
new rules or by adapting existing rules, these changes can be realized.
There is no implementation effort required for realizing these changes; just a
modification of the declarative rules.
Publish/subscribe is a mechanism to link applications to message brokers.
The idea is that applications can subscribe to certain messages or types of
messages. Applications can also publish messages. The information received
by publish and subscribe are used by the enterprise application integration
hub to realize the relaying of messages. it also shows that at a technical
level enterprise application integration with a message broker relies on adapters
that are used for transforming data and protocols between senders
and receivers.