Information Technology Reference
In-Depth Information
the properties required for the interaction; this is usually done based on the
transparencies requested and the actual deployment details, since you need
to know whether the objects involved are on the same system or on different
ones, and if so what sort of communications are available." Nigel thought for
a moment. \But that may change if the placement of the objects changes."
\Sure, you need to know what changes can be foreseen. If an object is made
mobile, for example, communication can change as it moves from using local
access to interaction across a rewall." \So you have to plan for the worst?"
\Up to a point. But you may want to x some conguration aspects in the
design."
\I don't understand that, Alex," said Trudy, \don't you want to keep your
options open?" \Not always. Think about cases where you want to express
parts of the computational design using fine-grain data objects; a configuration
of these objects may be generated by a factory object and may then be kept
together throughout its life | a bit like a database table row. You know that
these objects can always interact locally, wherever they are, because you have
decided individual members of the configuration should not migrate away
from the rest. The group form a unit, and are created together, migrated
together, and checkpointed together. In cases like this, you can optimize the
communications support."
\And how does this aect the engineering specication?" \Well, Claire,
the engineering language offers a number of concepts relating to such group-
ings. A group of objects that share the same lifecycle is called a cluster, and
the whole group can be manipulated by talking to a special cluster manager
object. There are other useful grouping concepts | the capsule for relating
objects with similar fault management and the node for relating objects shar-
ing resources at the lowest level." \And the engineering language concepts let
you express these as architectural requirements to be mapped onto whatever
platforms you decide to use?" \Yes, Nigel, that's more or less how it's done."
They all thought about this while they finished their meal. At the end,
Nigel pushed his chair back. \Well, this has turned out to be a very instructive
lunch." Alex grinned. \Both tasty and protable; oh, the joys of bookable
time!"
5.1
What Is the Engineering Viewpoint For?
The goal of the engineering viewpoint is to identify and specify the sup-
porting mechanisms for distributed interactions between objects. The focus
is on specifying how distribution works | how objects are distributed over
nodes, and how the structures of the nodes, and of the channels linking the
nodes, are going to be modelled. It also defines common functions needed
to support the required distribution transparencies. Clearly, this viewpoint is
