Monthly Archives: March 2013

Web 101: Cookies Make the Web Better

As we learned in the last post, web sites use cookies to remember who you are. Why do you want web sites to do this?

Let’s say you want to do some online shopping. You go to a web site, look at a few items, and put one into your shopping cart. If the web site couldn’t tell who you are from page to page, as soon as you click on “check out” your shopping cart would disappear. You would never be able to make a purchase, because there would be no way* to make the web site know that the person who asked for the shopping cart page is the same person who added the item to the cart.

Perhaps you are buying anything today, but you are logging in to Facebook instead. Cookies are how Facebook matches you up to the person who logged in. This lets Facebook show you your updates, and only your updates.

Cookies can also be used to make your web browsing a little faster. Some web sites will show you a different version depending on your country. Small and medium-sized websites look up your IP address’ country from a public online database, and this is a relatively slow process. (Plus, they have to pay if there are too many look ups in a day.) If the web site sets a cookie after the first look up, it can keep track of your country and show you the right version without doing any more look ups.

Cookies are also frequently used when you fill out a form on the web or request information that’s in a database. One way to do this is with a query string — if you see a ? in the URL, a query string has been used. One problem with query strings is not much information can be sent. A bigger problem is that users can guess at other query strings because they see what’s being sent. If you fill out a form and see that the URL you go to is, there’s nothing to stop you from changing that URL to and accessing that user’s information. This isn’t much of a problem if the request is for a product description or a blog post, but it can cause havoc in lots of other situations. Cookies solve that problem — by using what’s called a session cookie which is stored in your browser’s memory and goes away when you close your browser, more information can be sent and it can be hidden a little better from snoopers.

So much for the good side of cookies. Next up — when cookies go bad.

* Actually, there are ways such as using a query string, matching up IP addresses, or a few other techniques. Although cookies have security issues, most of these methods are far less secure than cookies, or cause other problems for the user.

Web 101: Are Cookies Yummy?


CC Cookie image courtesy of azmichelle on flickr

Just like real life cookies, a few cookies in your browser make life fun, but too many are a bad thing. Before we can get into what’s good, what’s bad, and what you can do to manage cookies, we need to learn a bit about what cookies are.

Let’s say you have just navigated to this page. When the web server send the text that will display this page on the screen (see Web 101: Text and the Web for more on this), the web server also sends what’s called a header for the file. The header has things like a line telling your browser that the server found the file, a line saying what kind of file is being sent, a line saying how long the file is, and a few other things your browser needs to know. If the web site wants to put a cookie on your computer, it also includes the cookie in this header. For example, the header might include the line:

Set-cookie: 1127

Your browser keeps a file of cookies that it has saved, and when it sees this in the header, it adds a line of the cookie file that has the name of the website and the id number (1127 in this case) of the cookie. There will also be some other information, like when the cookie was first set, when it should expire, and the type of cookie.

The next time you visit that website (even if it’s just clicking on another page in the website), your browser will add a line to the request for the webpage. That line will be

Cookie: 1127

The web server will then look up cookie 1127 in its database, and know that you are the same person it saw before. If you have logged in to the site in the past, it will be able to match up that cookie to your account.

Because cookies are stored by your web browser, if you visit the site again on a different computer or with a different browser (say you use Firefox instead of Chrome), the site won’t recognize you and you will get a new cookie.

In the next couple of posts we’ll learn when cookies are good for you and when they are troublesome.

(Thanks to Computer Networking: A Top-Down Approach for much of the information in this post.)

Web 101: Images on the Web

Now we know that the web is based on text, but images are what make the web a killer app. Imagine if you went to Amazon to buy something and only had a paragraph describing each item. Facebook is built on the idea that you see pictures of your friends. And Pinterest without images wouldn’t even exist. (They all rely on databases too. But that’s another post.)

How does your browser know where to get those images? Continue reading

Web 101: Text and the Web

If you’re reading this blog, you’re probably someone who uses the Web just about every day. Most people I know who have a computer at home got that computer in order to send email and surf the Web. And most of what I’m studying at school has to do with the web one way or another, whether it’s building a database that powers a website, handling networks, or even just using the web to collaborate on a project.

There is quite a bit going on behind the scenes when you go to a site like Amazon. Understanding what’s going on under the hood can help you recognize good and bad website design, decide what browser to use, know what browser settings you would like to tweek, and figure out where to start looking when something goes wrong. All without writing a single line of code.

First, we need a little bit of background that will set us up for the fun and useful stuff. Let’s dig in!

Continue reading