hello
What is the difference between an application server and a Web server?
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.
