Overview of this Lesson

In order to create web sites and web applications, it's important to understand how web applications work and how the Internet works. The internet is a huge entitiy - a network of computers all over the world, or a network of networks. The primary architecture of these networks is the client-server network. This network architecture allows us to connect our own individual machines to the millions and billions of machines that make up the networks of the Internet.

What is Client-Server?

When people refer to a client-server model, or client-server architecture, they're referring to a design of networked machines with the main files, programs, and services on a single computer called a server. Those resources can be shared among other machines on the network, which are called clients. The Internet is an example of client-server architecture: Many resources are stored on servers, and we can all use our own client machines to access those resources. The Internet is a huge, complex map of clients and servers, all connected together, allowing the world to share an infinite supply of electronic resources.

client server architecture
Client-Server Example: Internet

A server is also the software that manages the server's connections and access, and "serves" up requests for resources that are made by client machines. The machine that the server software resides on can be any computer that's powerful enough to run the server and perform the tasks the server needs to perform.

There are different types of servers: web servers host web sites, database servers host databases full of data, file servers host files that users can upload and download, game servers host games that users can connect to and play with each other, a print server hosts printing services that manage print jobs and a network of printers, and there are many others.

A client can refer to a machine, or it can refer to an application. Right now your computer is connected to a local network, and is therefore a client machine. If you're also using your smart phone or tablet, you're probably also connected to a local network or your carrier's network (your phone service provider's network), so they're also clients.

Additionally, any software program that allows your client machine to request information from a server, receives the returned results, and then presents those results in a human-readable format is also a client (client software, to be more precise). Your web browser is a client, for example: You use the web browser to request web pages, which are then rendered in the browser window for you to see and interact with. If you run an email program on your computer such as Thunderbird or Outlook, you're using an email client. An FTP client is a piece of software (such as WS-FTP, Core FTP, AnyClient, FileZilla, etc) that allows you to connect to a server so you can upload and download files. Think of any program on your computer, smart phone, tablet or other device that allows you to access shared resources on a server out there somewhere on the Internet: that's a client program!

Web Applications

How does client-server architecture apply to a course in web development? Some of the technologies used to develop these applications are server-side technologies, and others are client-side technologies. Server-side technologies, such as PHP, Perl, and MySQL perform their processing on the server. Client-side technologies, such as HTML, CSS, and JavaScript perform their processing on the client.

Examples of technologies used in web development:

The data flow in a client-server network can be complex for interactive web pages and for web applications. However, in these tutorials we start by learning the fundamentals, which means we will be creating static web pages (web pages with no interactivity, whose content doesn't change unless the programmer changes it and uploads a new version). Static pages have a very simple data flow on a web server:

client server data flow
Example of Client-Server Data Flow

In later tutorials, you'll learn how to use technologies such as CSS, JavaScript, and PHP to create interactive and even dynamic pages (pages whose content can change, or pages whose content is read from sources like a database or JSON (JavaScript Object Notation)).

The Request/Response Model

The Request/Response Model is how clients and servers communicate with each other. The client sends requests to the server, the server then processes and fulfills these requests, and then sends the response back to the client.

For example, say you visit the page Wendi's Cats:

  1. You click the link/bookmark or type the URL into your browser.
  2. An HTTP Request object is created:.
    • HTTP stands for Hypertext Transfer Protocol. This is a system of rules that allows "hypertext" to travel from one spot on the Internet to another.
    • An HTTP request contains information about the URL you're asking for, when the request is being made, where the request is coming from, etc.
    • In this case, it requests the file wendisCats.html in the virtual directory ~jollymor (this is an alias for a directory with a much longer path!).
  3. Your browser sends the Request object to the www-acad.sheridanc.on.ca server.
  4. The Request arrives at the server and is processed:
    • The server locates the wendisCats.html file in the ~jollymor/misc directory.
    • The server converts the file into a long string of data.
  5. A Response object is created:
    • The Response object contains the destination address (the machine that made the original request) and the string of data that contains the requested file.
    • The Response object also includes a response code of 200 and the message "OK". This means that the request was fulfilled successfully.
    • Other codes include 403 (forbidden - you're not allowed to view this resource), 404 (resource not found), 408 (request timeout - the client got tired of waiting for the response), 500 (internal server error).
  6. The server sends the response back to the machine that made the request (your browser).
  7. Your browser receives the Response object, opens it, and parses the string of data:
    • It builds the HTML document out of the received data.
    • If there are any references to files in the data, such as an image or a .CSS file, new requests are made and sent to the server for each one of those assets.
    • In this example, the file has a couple of images, and it also references a javascript file and a css file. New requests are made for each of those assets.
    • After the HTML document is created, the browser parses the responses from any other asset requests: CSS files are loaded and added to the HTML, images are added, etc.
    • The browser renders the complete HTML document so that it's visible to the user.
request/response model
Request/Response model: Shamelessly stolen from http://demosthenes.info/blog/137/The-Client-Server-Model

If the request involves server-side processing, then the process is a bit more complicated, as there are more steps to be completed. For example, what happens when you log into SLATE? You'll learn more about this process in the Web Programming course in term 2.