The last chapter concluded with a discussion of the downsides of deploying applications on the Web. But now that I’ve told you all the reasons you might not want to design systems for the Web, I’m going to completely contradict myself and present a system that cries out for a web-based implementation. This chapter presents the PyErrata web site, a Python program that lets arbitrary people on arbitrary machines submit book comments and bug reports (usually called errata) over the Web, using just a web browser.
PyErrata is in some ways simpler than the PyMailCgi case study presented in the previous chapter. From a user’s perspective, PyErrata is more hierarchical than linear: user interactions are shorter and spawn fewer pages. There is also little state retention in web pages themselves in PyErrata; URL parameters pass state in only one isolated case, and no hidden form fields are generated.
On the other hand, PyErrata introduces an entirely new dimension: persistent data storage. State (error and comment reports) is stored permanently by this system on the server, either in flat pickle files or a shelve-based database. Both raise the specter of concurrent updates, since any number of users out in cyberspace may be accessing the site at the same time.
Before you ponder too long over the seeming paradox of a book that comes with its own bug-reporting system, I should provide a little background. Over the last five years, I’ve been fortunate ...