App server, Web server: What's the difference?

Q:[b]What is the difference between an application server and a Web server?[/b]
A:Taking a big step back, a Web server serves pages for viewing in a Web browser, while an application server provides methods
that client applications can call. A little more precisely, you can say that:

A Web server exclusively handles HTTP requests, whereas an application server serves business logic to application programs
through any number of protocols.

Let's examine each in more detail.

The Web server

A
Web server handles the HTTP protocol. When the Web server receives an
HTTP request, it responds with an HTTP response, such as sending back
an HTML page. To process a request, a Web server may respond with a
static HTML page or image, send a redirect, or delegate the dynamic
response generation to some other program such as CGI scripts, JSPs
(JavaServer Pages), servlets, ASPs (Active Server Pages), server-side
JavaScripts, or some other server-side technology. Whatever their
purpose, such server-side programs generate a response, most often in
HTML, for viewing in a Web browser.

Understand that a
Web server's delegation model is fairly simple. When a request comes
into the Web server, the Web server simply passes the request to the
program best able to handle it. The Web server doesn't provide any
functionality beyond simply providing an environment in which the
server-side program can execute and pass back the generated responses.
The server-side program usually provides for itself such functions as
transaction processing, database connectivity, and messaging.

While a Web server
may not itself support transactions or database connection pooling, it
may employ various strategies for fault tolerance and scalability such
as load balancing, caching, and clustering—features oftentimes
erroneously assigned as features reserved only for application servers.

The application server

As
for the application server, according to our definition, an application
server exposes business logic to client applications through various
protocols, possibly including HTTP. While a Web server mainly deals
with sending HTML for display in a Web browser, an application server
provides access to business logic for use by client application
programs. The application program can use this logic just as it would
call a method on an object (or a function in the procedural world).

Such application
server clients can include GUIs (graphical user interface) running on a
PC, a Web server, or even other application servers. The information
traveling back and forth between an application server and its client
is not restricted to simple display markup. Instead, the information is
program logic. Since the logic takes the form of data and method calls
and not static HTML, the client can employ the exposed business logic
however it wants.

In most cases, the
server exposes this business logic through a component API, such as the
EJB (Enterprise JavaBean) component model found on J2EE (Java 2
Platform, Enterprise Edition) application servers. Moreover, the
application server manages its own resources. Such gate-keeping duties
include security, transaction processing, resource pooling, and
messaging. Like a Web server, an application server may also employ
various scalability and fault-tolerance techniques.

An example

As an example, consider an
online store that provides real-time pricing and availability
information. Most likely, the site will provide a form with which you
can choose a product. When you submit your query, the site performs a
lookup and returns the results embedded within an HTML page. The site
may implement this functionality in numerous ways. I'll show you one
scenario that doesn't use an application server and another that does.
Seeing how these scenarios differ will help you to see the application
server's function.

Scenario 1: Web server without an application server

In
the first scenario, a Web server alone provides the online store's
functionality. The Web server takes your request, then passes it to a
server-side program able to handle the request. The server-side program
looks up the pricing information from a database or a flat file. Once
retrieved, the server-side program uses the information to formulate
the HTML response, then the Web server sends it back to your Web
browser.

To summarize, a Web server simply processes HTTP requests by responding with HTML pages.

Scenario 2: Web server with an application server

Scenario
2 resembles Scenario 1 in that the Web server still delegates the
response generation to a script. However, you can now put the business
logic for the pricing lookup onto an application server. With that
change, instead of the script knowing how to look up the data and
formulate a response, the script can simply call the application
server's lookup service. The script can then use the service's result
when the script generates its HTML response.

In this scenario,
the application server serves the business logic for looking up a
product's pricing information. That functionality doesn't say anything
about display or how the client must use the information. Instead, the
client and application server send data back and forth. When a client
calls the application server's lookup service, the service simply looks
up the information and returns it to the client.

By separating the
pricing logic from the HTML response-generating code, the pricing logic
becomes far more reusable between applications. A second client, such
as a cash register, could also call the same service as a clerk checks
out a customer. In contrast, in Scenario 1 the pricing lookup service
is not reusable because the information is embedded within the HTML
page. To summarize, in Scenario 2's model, the Web server handles HTTP
requests by replying with an HTML page while the application server
serves application logic by processing pricing and availability
requests.

Caveats

Recently,
XML Web services have blurred the line between application servers and
Web servers. By passing an XML payload to a Web server, the Web server
can now process the data and respond much as application servers have
in the past.

Additionally, most
application servers also contain a Web server, meaning you can consider
a Web server a subset of an application server. While application
servers contain Web server functionality, developers rarely deploy
application servers in that capacity. Instead, when needed, they often
deploy standalone Web servers in tandem with application servers. Such
a separation of functionality aids performance (simple Web requests
won't impact application server performance), deployment configuration
(dedicated Web servers, clustering, and so on), and allows for
best-of-breed product selection.

应用服务器有哪些：
BEA  WebLogic  Server，IBM  WebSphere  Application  Server，Oracle9i  Application  Server，jBoss，Tomcat