In this overview chapter, we've focused on how two web applications (web browsers and web servers) send messages back and forth to implement basic transactions. There are many other web applications that you interact with on the Internet. In this section, we'll outline several other important applications, including:
HTTP intermediaries that sit between clients and servers
HTTP storehouses that keep copies of popular web pages close to clients
Special web servers that connect to other applications
Special proxies that blindly forward HTTP communications
Semi-intelligent web clients that make automated HTTP requests
As shown in Figure 1-11, a proxy sits between a client and a server, receiving all of the client's HTTP requests and relaying the requests to the server (perhaps after modifying the requests). These applications act as a proxy for the user, accessing the server on the user's behalf.
Proxies are often used for security, acting as trusted intermediaries through which all web traffic flows. Proxies can also filter requests and responses; for example, to detect application viruses in corporate downloads or to filter adult content away from elementary-school students. We'll talk about proxies in detail in Chapter 6.
A web cache or caching proxy is a special type of HTTP proxy server that keeps copies of popular documents that pass through the proxy. The next client requesting the same document can be served from the cache's personal copy (see Figure 1-12).
A client may be able to download a document much more quickly from a nearby cache than from a distant web server. HTTP defines many facilities to make caching more effective and to regulate the freshness and privacy of cached content. We cover caching technology in Chapter 7.
Gateways are special servers that act as intermediaries for other servers. They are often used to convert HTTP traffic to another protocol. A gateway always receives requests as if it was the origin server for the resource. The client may not be aware it is communicating with a gateway.
For example, an HTTP/FTP gateway receives requests for FTP URIs via HTTP requests but fetches the documents using the FTP protocol (see Figure 1-13). The resulting document is packed into an HTTP message and sent to the client. We discuss gateways in Chapter 8.
Tunnels are HTTP applications that, after setup, blindly relay raw data between two connections. HTTP tunnels are often used to transport non-HTTP data over one or more HTTP connections, without looking at the data.
One popular use of HTTP tunnels is to carry encrypted Secure Sockets Layer (SSL) traffic through an HTTP connection, allowing SSL traffic through corporate firewalls that permit only web traffic. As sketched in Figure 1-14, an HTTP/SSL tunnel receives an HTTP request to establish an outgoing connection to a destination address and port, then proceeds to tunnel the encrypted SSL traffic over the HTTP channel so that it can be blindly relayed to the destination server.
User agents (or just agents) are client programs that make HTTP requests on the user's behalf. Any application that issues web requests is an HTTP agent. So far, we've talked about only one kind of HTTP agent: web browsers. But there are many other kinds of user agents.
For example, there are machine-automated user agents that autonomously wander the Web, issuing HTTP transactions and fetching content, without human supervision. These automated agents often have colorful names, such as "spiders" or "web robots" (see Figure 1-15). Spiders wander the Web to build useful archives of web content, such as a search engine's database or a product catalog for a comparison-shopping robot. See Chapter 9 for more information.