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Agreement, in the enterprise viewpoint, of a maintenance procedure for
the federation contract; this should include the agreement of testing
and validation procedures and phased plans for implementation, initial
testing, deployment and use. A joint working party from the two orga-
nizations documents agreed procedures, and these are added as a codicil
to the MoU.
The federation should also have an enterprise-level exit strategy, stating
what is to happen if the federation has to be wound up. Periods of notice
for dissolving the federation are agreed, and arrangement for completing
work in progress added. Penalties reflecting lost investment for termi-
nation within two years and lost opportunity for abrupt termination are
added. This is also attached to the MoU.
11.4 Engineering the Federation
Once the federation community has been defined, we know the abstract
view of the information to be exchanged and the structure of the dialogue.
However, we still need to establish the communication technology, the way
links are established, and how the communication formats to be used over
them relate to local usage in the systems run by the individual federation
members. There may be incompatibilities between the way types in the ab-
stract information model are refined to concrete representations, or differences
in local usage, either in the selection of middleware or in the supporting pro-
tocols. There may also be a need to invoke additional management functions
to ensure access or security policies are applied.
What is needed is a way of placing the necessary extra functionality for
access control or data transformation into the communications path. The
engineering channel architecture provides the concept of an interceptor for
precisely this purpose. An interceptor can act as an intermediate system
up to and including the highest level at which there is an incompatibility
between the engineering solutions in use. Messages received on one side of
the interceptor are interpreted using the rules on that side until an element
of the shared abstraction is recognized, and this is then recoded and sent on
its way in the format of the domain on the other side of the interceptor (see
gure 11.2).
The complexity of the conversion to be done by the interceptor depends
on the nature of the incompatibility to be dealt with. If the incompatibility
is at a comparatively low level, affecting only the supporting communication
protocols, the interceptor is reasonably straightforward because it does not
need to be concerned with application-specific aspects of the dialogue, and
can just pass application messages through without interpreting them. This
 
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