Sommerville-Software-Engineering-10ed

472 Chapter 16 ■ Component-based software engineering
Support services
Component
management
Concurrency
Transaction
management
Persistence
Resource
management
Security
Platform services
Figure 16.5 Middleware
services defined in a
component model
Addressing
Interface
definition
Exception
management
Component
communications
For components that are executable routines rather than external services, the
component model defines the services to be provided by the middleware that
supports the executing components. Weinreich and Sametinger use the analogy of an
operating system to explain component models. An operating system provides a set
of generic services that can be used by applications. A component model implementation
provides comparable shared services for components. Figure 16.5 shows some
of the services that may be provided by an implementation of a component model.
The services provided by a component model implementation fall into two
categories:
1. Platform services, which enable components to communicate and interoperate
in a distributed environment. These are the fundamental services that must be
available in all component-based systems.
2. Support services, which are common services that many different components
are likely to require. For example, many components require authentication to
ensure that the user of component services is authorized. It makes sense to
provide a standard set of middleware services for use by all components. This
reduces the costs of component development, and potential component incompatibilities
can be avoided.
Middleware implements the common component services and provides interfaces to
them. To make use of the services provided by a component model infrastructure, you
can think of the components as being deployed in a “container.” A container is an implementation
of the support services plus a definition of the interfaces that a component
must provide to integrate it with the container. Conceptually, when you add a component
to the container, the component can access the support services and the container
can access the component interfaces. When in use, the component interfaces themselves
are not accessed directly by other components. They are accessed through a container
interface that invokes code to access the interface of the embedded component.
Containers are large and complex and, when you deploy a component in a container,
you get access to all middleware services. However, simple components may