Databases and Cookies

We're using both a database and cookies to implement the personalization system. Why both? This personalization needs both a short-term and long-term "memory". For example, when people are younger, they tend only to remember what happened 5 minutes ago, but not 5 years ago. Yet as people grow older, they tend to remember what happened 5 years ago, but not 5 minutes ago. Our system cannot have either flaw -- it needs to remember both 5 minutes (heck, 5 seconds) ago, as well as 5 days, months, or years ago.

The database will be our long-term memory. It remembers the visitor's account information permanently, no matter how much time has elapsed between accesses to the account. On the other hand, we run into the problem of HTTP's inherent statelesness. Granted, this is not a problem that makes the nightly news ("tonight with Peter Jennings, the inherent statelesness of HTTP and how it can harm your children"). The problem, in a nutshell, is that the web has no short-term memory whatsoever. As a result, a web application such as our personalization system can't rely on the web server to remember information specific to a user as they move from one web page to another within the site.

We could query the database -- our long-term memory -- every time the user navigates to another page within our site, but this would be inefficient and put a lot of strain on the database. Instead, we need a way to "preserve state" (sort of like marmalade), as they say, during the browsing session, without relying on the database. There are numerous ways to approach this matter, and one popular solution for Apache+mod_perl-based servers is the module Apache::Session. Using this module you can effectively create a global hash, tied to a long-term storage method (filesystem, database), where you can store and access user information from any Perl script running in the mod_perl environment. Apache::Session is well supported and worth investigating, but wasn't appropriate for the site which inspired our particular recipe.

Instead, we've decided to use client-side cookies for short-term memory. A client-side cookie is a small chunk of data that is stored on the visitor's machine. We can store virtually any arbitrary data in a cookie, and the cookie will be associated only with our particular web site. As the visitor moves through our site, the cookie is delivered back to our server, which allows us to read, and thus "remember", the chunk of data during the browsing session.

Cookies can expire when the user closes the browser, or they can persist on disk for longer periods. Our aim, in using cookies for short-term memory, is to use them only during the browser session. We'll be using "session cookies", then, which are simply cookies that are erased once the browser is shut down (or crashes, whichever comes first).

The Perl You Need to Know Part 18: Personalization Methods Part 2: Databases and Cookies
The Perl You Need to Know
The Personalization Database