Sommerville-Software-Engineering-10ed

17.3 ■ Architectural patterns for distributed systems 507
Comp 1
Comp 2
Comp 3
Comp 4
Common
services
Common
services
Common
services
Common
services
Communication middleware
Figure 17.12 A
distributed component
architecture
Client Client Client Client Client
have to decide what services should be included in each layer. In practice, however, it
is not always clear whether a service is a data management service, an application
service, or a database service. Designers must also plan for scalability and so provide
some means for servers to be replicated as more clients are added to the system.
A more general approach to distributed system design is to design the system as a
set of services, without attempting to allocate these services to layers in the system.
Each service, or group of related services, can be implemented using a separate object
or component. In a distributed component architecture (Figure 17.12), the system is
organized as a set of interacting components as I discussed in Chapter 16. These components
provide an interface to a set of services that they provide. Other components
call on these services through middleware, using remote procedure or method calls.
Distributed component systems are reliant on middleware. This manages component
interactions, reconciles differences between types of the parameters passed between
components, and provides a set of common services that application components can
use. The CORBA standard (Orfali, Harkey, and Edwards 1997) defined middleware
for distributed component systems, but CORBA implementations have never been
widely adopted. Enterprises preferred to use proprietary software such as Enterprise
Java Beans (EJB) or .NET.
Using a distributed component model for implementing distributed systems has a
number of benefits:
1. It allows the system designer to delay decisions on where and how services
should be provided. Service-providing components may execute on any node of
the network. There is no need to decide in advance whether a service is part of a
data management layer, an application layer, or a user interface layer.
2. It is a very open-system architecture that allows new resources to be added as
required. New system services can be added easily without major disruption to
the existing system.
3. The system is flexible and scalable. New objects or replicated objects can be added
as the load on the system increases, without disrupting other parts of the system.