Software Engineering for Students A Programming Approach

160 Chapter 12 ■ Design patterns
Figure 12.5 Layers
user interface layer
business logic layer
database access layer
database level stores and retrieves documents. The middle layer processes commands,
such as deleting text.
The strength of the Layers pattern is that it provides a large-scale structure for certain
types of software. It also means that each layer of the software can be modified
without affecting the other layers. For example, in the word processor, we can alter the
way that commands are invoked (the presentation layer), for example, by adding
menus, without changing the database access layer.
The word processor is a single-machine application. However, many current
applications – such as an ATM – are network-based or internet-based. The simplest
network-based architecture is the client-server architecture. This consists of two layers –
the client software running on the client machine and the server software running on the
server. The classic example is a web browser and a web server. The client passes a request
to the server; the server responds.
A classic example of the Layers pattern is the structure of the software for internet
communication, called the TCP/IP stack. Every computer that uses the internet has
this software. A typical application program makes requests on the transport layer that
supports the TCP protocol. This in turn makes requests of the internet layer that supports
the IP protocol. This in turn calls the physical layer which actually carries out
data transfers. Thus the software consists of four levels, including the application level.
Another version of the Layers pattern, with four levels, describes many networked
solutions, Figure 12.6. Again, each layer uses the layer below it. The communication
between presentation and server layer is carried out according to a chosen network protocol,
such as HTTP.
In designing a distributed system, there are choices about which layer does what. In
a thin client system, the client software does very little, while the bulk of the work is
carried out on the server. In a fat client architecture, the business logic layer migrates
to the client machine. The decision about where to perform processing depends on
such issues as performance, maintenance and security.
Another benefit of the Layers pattern is facilitating security. The boundaries between
layers serve as well-defined interfaces where security checks can be carried out. For
example encryption can be used between the client and the server. Finally, Layers provide
for scalability. A server acting as a proxy (see pattern above) can delegate its task
to other servers as the load increases.