What are cookies?

Key Takeaways

[Music] By design, the web has no memory. Every time you open a web page, the website you're visiting has no way of knowing if you're the same person who visited previously. Well, this helps keep communication efficient and simple since there's no need for a mechanism to track requests and responses. But it also turns out to be a problem. For example, how can an online store have a shopping cart if it can't remember what you added? Well, cookies were invented to solve that problem. So, imagine that you visit the homepage of the website cats.ample and the site wants to show you a random image of a cat and keep a record of which cat you saw. Well, here's how to do that with cookies. So, when you visit the site, your web browser makes a request to the cats.ample web server for the files that are used on the page, including the cat.jpeg image file, and the web server at cats.ample responds with the file. A request to a web server and the response to that request can include extra information called headers along with the file itself. A set cookie header in a response from a server tells your browser to store some text, a name, and a value in a file. And this stored file is known as a cookie. The cats.ample Example, web server includes the set cookie cat equals tabby header in response to the request for the cat.jpeg image file. Your browser receives the file, displays the image, and processes the set cookie header that went with it. As a result, your browser stores a cookie with the name cat and the value tabby. And from now on, your browser sends the header cookie cat equals tabby with requests to the cats.ample web server. And when the cats.example web server receives a request, it can process the cookie and do whatever it wants with that value, such as making sure it doesn't send you an image of the same tabby cat again. So once again, here's the whole process from start to finish. Your browser requests an image file from the cats.ample example web server. The cats.example server responds with the file and the response includes the header set cookie cat equals tabby. Your browser stores the cookie and from now on your browser includes the header cookie cat equals tabby whenever it makes a request to cats.ample. In other words, the web server includes a set cookie header with a response and that tells your browser to store the cookie. And because of that, your browser includes a cookie header with a request and that sends the cookie back to the web server. Note that a cookie is only sent to the site that sets it. For example, a cookie set by cats.ample example won't be sent in requests to a different site such as dogs.ample. And the cookies I've described in this example are set by code running on a web server. Now, websites can also create cookies using JavaScript code in web pages running locally in your web browser. So, thanks for watching. Be sure to check out all the videos in the cookie basics series. We have a guide and demos to go along with this video and more resources on our website to help you understand how cookies work. [Music]