How Do Web Sites Use Cookies?

Cookies evolved because they solve a big problem for the people who implement Web sites. In the broadest sense, a cookie allows a site to store state information on your machine. This information lets a Web site remember what state your browser is in. An ID is one simple piece of state information -- if an ID exists on your machine, the site knows that you have visited before. The state is, "Your browser has visited the site at least one time," and the site knows your ID from that visit.

Web sites use cookies in many different ways. Here are some of the most common examples:

• Sites can accurately determine how many people actually visit the site. It turns out that because of proxy servers, caching, concentrators and so on, the only way for a site to accurately count visitors is to set a cookie with a unique ID for each visitor. Using cookies, sites can determine:
  • How many visitors arrive
  • How many are new versus repeat visitors
  • How often a visitor has visited
    
    

  The way the site does this is by using a database. The first time a visitor arrives, the site creates a new ID in the database and sends the ID as a cookie. The next time the user comes back, the site can increment a counter associated with that ID in the database and know how many times that visitor returns.
  

• Sites can store user preferences so that the site can look different for each visitor (often referred to as customization). For example, if you visit msn.com, it offers you the ability to "change content/layout/color." It also allows you to enter your zip code and get customized weather information. When you enter your zip code, the following name-value pair gets added to MSN's cookie file:

  WEAT CC=NC%5FRaleigh%2DDurham®ION= www.msn.com/

  Since I live in Raleigh, N.C., this makes sense.

  Most sites seem to store preferences like this in the site's database and store nothing but an ID as a cookie, but storing the actual values in name-value pairs is another way to do it (we'll discuss later why this approach has lost favor).

• E-commerce sites can implement things like shopping carts and "quick checkout" options. The cookie contains an ID and lets the site keep track of you as you add different things to your cart. Each item you add to your shopping cart is stored in the site's database along with your ID value. When you check out, the site knows what is in your cart by retrieving all of your selections from the database. It would be impossible to implement a convenient shopping mechanism without cookies or something like them.

In all of these examples, note that what the database is able to store is things you have selected from the site, pages you have viewed from the site, information you have given to the site in online forms, etc. All of the information is stored in the site's database, and in most cases, a cookie containing your unique ID is all that is stored on your computer.

Problems with Cookies

Cookies are not a perfect state mechanism, but they certainly make a lot of things possible that would be impossible otherwise. Here are several of the things that make cookies imperfect.

• People often share machines - Any machine that is used in a public area, and many machines used in an office environment or at home, are shared by multiple people. Let's say that you use a public machine (in a library, for example) to purchase something from an online store. The store will leave a cookie on the machine, and someone could later try to purchase something from the store using your account. Stores usually post large warnings about this problem, and that is why. Even so, mistakes can happen. For example, I had once used my wife's machine to purchase something from Amazon. Later, she visited Amazon and clicked the "one-click" button, not realizing that it really does allow the purchase of a book in exactly one click.

  On something like a Windows NT machine or a UNIX machine that uses accounts properly, this is not a problem. The accounts separate all of the users' cookies. Accounts are much more relaxed in other operating systems, and it is a problem.
  
  If you try the example above on a public machine, and if other people using the machine have visited HowStuffWorks, then the history URL may show a very long list of files.
  

• Cookies get erased - If you have a problem with your browser and call tech support, probably the first thing that tech support will ask you to do is to erase all of the temporary Internet files on your machine.
  
  When you do that, you lose all of your cookie files. Now when you visit a site again, that site will think you are a new user and assign you a new cookie. This tends to skew the site's record of new versus return visitors, and it also can make it hard for you to recover previously stored preferences. This is why sites ask you to register in some cases -- if you register with a user name and a password, you can log in, even if you lose your cookie file, and restore your preferences. If preference values are stored directly on the machine (as in the MSN weather example above), then recovery is impossible. That is why many sites now store all user information in a central database and store only an ID value on the user's machine.
  
  

  If you erase your cookie file for HowStuffWorks and then revisit the history URL in the previous section, you will find that HowStuffWorks has no history for you. The site has to create a new ID and cookie file for you, and that new ID has no data stored against it in the database.allows you to reset your history list whenever you like.)
  

• Multiple machines - People often use more than one machine during the day. For example, I have a machine in the office, a machine at home and a laptop for the road. Unless the site is specifically engineered to solve the problem, I will have three unique cookie files on all three machines. Any site that I visit from all three machines will track me as three separate users. It can be annoying to set preferences three times. Again, a site that allows registration and stores preferences centrally may make it easy for me to have the same account on three machines, but the site developers must plan for this when designing the site.
  
  

  If you visit the history URL demonstrated in the previous section from one machine and then try it again from another, you will find that your history lists are different. This is because the server created two IDs for you, one on each machine.

There are probably not any easy solutions to these problems, except asking users to register and storing everything in a central database.

When you register with the HowStuffWorks registration system, the problem is solved in the following way: The site remembers your cookie value and stores it with your registration information. If you take the time to log in from any other machine (or a machine that has lost its cookie files), then the server will modify the cookie file on that machine to contain the ID associated with your registration information. You can therefore have multiple machines with the same ID value.