HTTP: The Definitive Guide
8.1 Gateways
The history behind HTTP extensions and interfaces was driven by people's needs. When the desire to put more complicated resources on the Web emerged, it rapidly became clear that no single application could handle all imaginable resources.
To get around this problem, developers came up with the notion of a gateway that could serve as a sort of interpreter, abstracting a way to get at the resource. A gateway is the glue between resources and applications. An application can ask (through HTTP or some other defined interface) a gateway to handle the request, and the gateway can provide a response. The gateway can speak the query language to the database or generate the dynamic content, acting like a portal: a request goes in, and a response comes out.
Figure 8-1 depicts a kind of resource gateway. Here, the Joe's Hardware server is acting as a gateway to database contentnote that the client is simply asking for a resource through HTTP, and the Joe's Hardware server is interfacing with a gateway to get at the resource.
Figure 8-1. Gateway magic
Some gateways automatically translate HTTP traffic to other protocols, so HTTP clients can interface with other applications without the clients needing to know other protocols (Figure 8-2).
Figure 8-2. Three web gateway examples
Figure 8-2 shows three examples of gateways:
In Figure 8-2a, the gateway receives HTTP requests for FTP URLs. The gateway then opens FTP connections and issues the appropriate commands to the FTP server. The document is sent back through HTTP, along with the correct HTTP headers.
In Figure 8-2b, the gateway receives an encrypted web request through SSL, decrypts the request,[1]and forwards a normal HTTP request to the destination server. These security accelerators can be placed directly in front of web servers (usually in the same premises) to provide high-performance encryption for origin servers.
[1] The gateway would need to have the proper server certificates installed.
In Figure 8-2c, the gateway connects HTTP clients to server-side application programs, through an application server gateway API. When you purchase from e-commerce stores on the Web, check the weather forecast, or get stock quotes, you are visiting application server gateways.
8.1.1 Client-Side and Server-Side Gateways
Web gateways speak HTTP on one side and a different protocol on the other side.[2]
[2] Web proxies that convert between different versions of HTTP are like gateways, because they perform sophisticated logic to negotiate between the parties. But because they speak HTTP on both sides, they are technically proxies.
Gateways are described by their client- and server-side protocols, separated by a slash:
<client-protocol>/<server-protocol>
So a gateway joining HTTP clients to NNTP news servers is an HTTP/NNTP gateway. We use the terms "server-side gateway" and "client-side gateway" to describe what side of the gateway the conversion is done for:
Server-side gateways speak HTTP with clients and a foreign protocol with servers (HTTP/*).
Client-side gateways speak foreign protocols with clients and HTTP with servers (*/HTTP).