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is shown in figure 5.2. The nested components are typically instantiated in
sequence as the system is initialized.
At first, the node is configured by its nucleus. The nucleus then allocates
resources to a capsule and its capsule manager. The capsule manager then
creates any necessary clusters with their cluster managers. When doing this,
the capsule manager allocates resources to the cluster manager. Finally, BEOs
are instantiated, either inside a cluster or directly in a capsule, to perform the
tasks specified for the computational objects they represent.
When a BEO's work is done, its resources are returned to its direct man-
ager. If this is a cluster, the cluster manager may decide to terminate and,
when all its work is done, a capsule may terminate, releasing its resources
back to the nucleus for reallocation so that the system returns to the initial
state in which there is just a node and a nucleus.
5.4 Channel Architecture
The second main set of concepts in the engineering language is concerned
with defining a channel architecture that represents the communication in-
frastructure, which allows engineering objects to interact.
The basic element is the channel , which is the engineering equivalent of a
computational binding. A channel consists of stubs, binders, protocol objects
and interceptors and links communicating basic engineering objects, generally
residing in different nodes. Figure 5.3 shows an example of the channel that
connects the GUI2User and UserOps BEOs of the PhoneMob application.
Normally there is no need to specify these elements in detail because they
are provided by the underlying middleware platform (some of which are them-
selves standardized [13]). However, there are occasions in which we want to
model them functionally, in order to specify some of a channel's properties,
or to express requirements on it | such as performance requirements on the
channels or security constraints on the protocol objects or interceptors.
All channels involve the same functions, using specialized engineering ob-
jects to implement the required functionality in an ordered manner. However,
different specific communication architectures may each organize or interleave
these functions in their own way.
Stubs transform or monitor information in the channel. This includes,
for example, the marshalling and unmarshalling of message elements,
the translation of local interfaces into interoperable interface references,
or provision of message content encryption. Stubs are the elements that
enable access transparency in the communication between two objects
written in different languages (such as C++ and Smalltalk). The client
object talks to its local stub, which is in charge of translating the request
