Dynamic Graphics and Personalization

A continuation of the discussion on creating graphics dynamically on the Web.

Last month, we discussed the different ways a CGI program can create dynamically generated graphics output. That is, we wrote several programs in which the program describes its output not as “text/html”, but as “graphics/gif”.

This month, we will examine some more tools that allow us to create graphics dynamically. However, the graphics will have an additional twist this time, in that they will reflect an individual user's stock portfolio rather than a global set of data values.

As with any non-trivial software project, our first step must be to create a brief specification. In this particular project, we will have two major programs. In the first, the user will be able to create and edit a personal profile, describing the securities he or she owns. The second program will take the information in the user's profile and use it to create a personalized graphic stock portfolio.

Needed Tools

This project brings together a number of tools we have discussed in previous installments of ATF. Nevertheless, it seems like a good idea for us to review them, since we are going to call on so many.

MySQL: MySQL is a small, inexpensive relational database available for Linux and many other operating systems. (See “Resources” for information on where to get it.) In addition to its low price, MySQL is quite fast and efficient, which makes it popular on many web sites. As a relational database, MySQL forces us to store information in one or more tables, in which each row refers to a separate record. As with most relational databases, we communicate with MySQL using SQL, a database query language. We cannot write programs in SQL; rather, we must embed our queries inside of a program written in a full programming language. In our case, that language is Perl.

DBI: While SQL might be a standard query language for communicating with databases, the software and libraries used to speak with those databases vary considerably. To talk to an Oracle server, you need Oracle libraries; to talk to a MySQL server, you need MySQL libraries, and so forth. As a result, the Perl database world was fractured for a long time, with special versions for individual databases.

Now, however, there is a better way: the DBI module standardized the API to relational databases, meaning that programmers moving from one database server to another have to learn only the different nuances of the SQL implementations. Previously, they had to learn a separate Perl API as well, which was frustrating. This was accomplished by separating the database code into two parts, one generic (DBI) and the other specific to each server (DBD). In order to use DBI, you will need to install the generic DBI libraries, and then one or more DBDs appropriate for the products you use.

There is a problem with DBI and the Web, however, which has to do with the way in which database servers were designed. In general, they expect a client program to open a connection, perform many queries, then disconnect. Opening a connection is thus quite slow and inefficient. When a CGI program is a database client, it must open a new database connection for each HTTP transaction. See the mod_perl section immediately below for one solution to this problem, Apache::DBI.

mod_perl for Apache: Web servers traditionally provided custom and dynamic output by invoking external programs, using the CGI standard. An HTTP server would pass information to the CGI program, which would then be expected to send its output to the user's browser. This output generally came in the form of HTML-formatted text, but as we saw last month, it is possible to produce graphics as well.

However, CGI is quite slow; every invocation of a CGI program requires a new process to be created. If you are using Perl, each invocation requires the program to be compiled into Perl's internal format, then executed.

An alternative method is to use mod_perl, a module for the free Apache HTTP server that embeds a fully working version of Perl inside the server. This has several ramifications, one of which being the fact that we can now create custom output without having to rely on external programs.

When using mod_perl, you can take advantage of a module known as Apache::DBI. This module pretends to work the same as DBI, but actually caches database handles ($dbh) across invocations. So even when your program thinks it is opening a new database connection, it is actually reusing a database handle from a previous invocation.

GIFgraph: The GIFgraph set of Perl modules allows us to create charts and graphs on the fly, from within CGI programs or mod_perl modules. We explored a basic use of GIFgraph last month. As its name implies, GIFgraph produces output in GIF format. Last month, we saw how to return GIFs directly to the user's browser. This month, we will instead save the resulting graphs to individual files, to which we will create hyperlinks.

Apache::Session: HTTP, the protocol on which the Web is based, was designed to be lightweight and simple. As part of this consideration, it was also designed to be “stateless”, meaning each transaction is independent. This creates a problem, however, in that you will often want to keep track of which user is which. For instance, in this application, we want to ensure we are tracking the correct user's portfolio. Without state, we cannot track portfolios at all, let alone for multiple users. Apache::Session, as we will see below, allows us to get around this by using a database and HTTP cookies to store one or more pieces of information using a unique identifier.

With the above five technologies, we can create a fairly impressive stock portfolio tracker that allows users to define which securities they own and view their current holdings in graphical format. As presented this month, the application is admittedly a bit crude, but it should show you how easy it is to write such an application and how flexible the above tools can be in creating one.


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