Interactive graphical user interfaces are beyond the scope of this
book. There are several handy graphic frameworks, including Tkinter and
GTK that help you write graphical user interfaces. However, GUI programs
are still started from the command line, so this section is relevant for
those kinds of programs.
There are many design patterns for our application programs. We
can identify a number of features that distinguish different kinds of
programs. We can create a taxonomy of program designs based on how we
interact with them. We could also create a taxonomy based on the
program's internal structure or its interfaces.
We can look at a program based on the type of interaction that it
has with a person. There's a spectrum of interaction.
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A program can be started from the command line and have no
further interaction with the human user. We can call these batch
programs because they usually process a batch of individual
transactions. We can also call them command-line programs because
our only interaction is at the command prompt. A large number of
data analysis and business-oriented programs work with batches of
data. Additionally, we can describe servers as being similar to
batch programs. This is a focus for this chapter.
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A program can have very sophisticated interaction with the
human user. The interaction may be character-oriented, or it can
have a graphic user interface (GUI) and be started by
double-clicking an icon. What's important is that the user drives
the processing, not the batch of data. Typically, a program with
rich user interaction will be a client of one or more services.
These programs are beyond the scope of this book.
We can also look at programs based on their structure and how the
interact with other programs.
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Some programs stand alone. They have an executable file which
starts things off, and perhaps includes some libraries. Often a
client program is a stand-alone program that runs on someone's
desktop. This is a focus for this chapter.
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Some programs plug into a larger and more sophisticated
frameworks. The framework is, essentially, a closely related
collection of libraries and interfaces. Most web applications are
built as programs which plug into a web server framework. There is a
tremendous amount of very common processing in handling a web
transaction. There's little value in repeating this programming, so
we inherit it from the framework.
We can distinguish programs in how they interact with other
programs to create a larger system. We'll turn to this topic in the next
chapter, Chapter 36, Programs: Clients, Servers, the Internet and the World Wide
Web
.
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Some programs are clients. They rely on services provided by
other programs. The service it relies on might be a web server or a
database server. In some cases, the client program has rich user
interaction and stands alone.
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Some programs are servers. They provide services to other
programs. The service might be domain names, time, or any of a
myriad of services that are an essential part of Linux and other
operating systems.
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Some programs are both servers and clients of other services.
Most servers have no interaction; they are command-line programs
which are clients of other command-line programs. A web server
typically has plug in web applications which use database servers. A
database server may make use of other services within an operating
system.
Some Subspecies. Stand-alone, command-line programs have a number of design
patterns. Some programs are filters that read
an input file, perform an extract or a calculation and produce a
result file that is derived from the input. Programs can be
compilers, performing extremely complex
transformations from one or more input files to create an output file.
Programs can be interpreters, where statements
in a language are read and processed. Some programs, like the Unix
awk utility, combine filtering and
interpreting.
Stand-alone, iteractive programs allow a user to create and
manipulate data objects. Interactive programs often have sophisticated
graphics. All games are interactive progams. The software often
characterized as the office suite, including word
processors, spread sheets, graphics programs, schedule managers and
contact managers are interactive programs.
Some programs are clients of services. For
example, browsers take user queries, fetch and format data, and present
the data to the user. An FTP client program may display contents of an
FTP server, accepting user commands through a graphical user interface
(GUI) and transferring files. An IMAP client program may display
mailboxes on a mail server, accepting commands and transferring or
displaying mail messages.
Many programs combine interaction with being a client of one or
more services. Most browsers, like Firefox, are clients for servers
which use a number of protocols, including HTTP, POP3, IMAP4, FTP, NNTP,
and GOPHER. Besides being a client, a browser also provides graphics,
handling numerous MIME data types for different kinds of images and
sounds.
Yet another common type of program is a
server. These programs are also interactive, but
they interact with client programs, not a person through a GUI. An HTTP
server like Apache, for instance, responds to browser requests for web
pages. An FTP server responds to FTP client requests for file transfers.
A server is often a kind of batch program, since it is left running for
indefinite periods of time, and has no user interaction.
