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6.3 Architectural patterns 161

Name Client–server

Description In a client–server architecture, the functionality of the system is organized into services,
with each service delivered from a separate server. Clients are users of these services and
access servers to make use of them.
Example Figure 6.11 is an example of a film and video/DVD library organized as a client–server
system.

When used Used when data in a shared database has to be accessed from a range of locations.
Because servers can be replicated, may also be used when the load on a system is
variable.

Advantages The principal advantage of this model is that servers can be distributed across a network.
General functionality (e.g., a printing service) can be available to all clients and does not
need to be implemented by all services.
Disadvantages Each service is a single point of failure so susceptible to denial of service attacks or
server failure. Performance may be unpredictable because it depends on the network
as well as the system. May be management problems if servers are owned by different
organizations.

A system that follows the client–server pattern is organized as a set of services
Figure 6.10 The
client–server pattern and associated servers, and clients that access and use the services. The major com-
ponents of this model are:

1. A set of servers that offer services to other components. Examples of servers
include print servers that offer printing services, file servers that offer file man-
agement services, and a compile server, which offers programming language
compilation services.
2. A set of clients that call on the services offered by servers. There will normally
be several instances of a client program executing concurrently on different
computers.

3. A network that allows the clients to access these services. Most client–server
systems are implemented as distributed systems, connected using Internet
protocols.

Client–server architectures are usually thought of as distributed systems architec-
tures but the logical model of independent services running on separate servers can
be implemented on a single computer. Again, an important benefit is separation and
independence. Services and servers can be changed without affecting other parts of
the system.
Clients may have to know the names of the available servers and the services that
they provide. However, servers do not need to know the identity of clients or how
many clients are accessing their services. Clients access the services provided by a
server through remote procedure calls using a request-reply protocol such as the http