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What is a Web Service?
By Stephen Bucaro
Try to find an article on the Web that explains, in plain
English, what a "Web Service" is and you'll be going around
in rhetorical circles with no simple explanation, and no
examples. Most articles start out with some ambiguous
explanation like: "Web services identified with WSDL and
UDDI protocols make functionality available over the
Internet using SOAP encapsulated in XML envelopes" and
then the articles start spewing out programming code.
Examples of programming code don't help if you don't have
an overview of what a Web Service is.
David Berlind in his article "What are Web services
anyway?" for ZDNet.com says, "At last fall's Gartner
Symposium I asked several attendees presumably C-level
technology executives if they could give me a
definition of Web services... No one knew. Before the
session's end, over half the attendees had left because
they were expecting a discussion about something else."
When you do find examples of Web Services, they are
applications like; retrieving a stock quote, finding the
best price for a product, saving an appointment to a
calendar, or validating a credit card number. These are
all things that we have been doing on the Web for years.
So, what's the big deal about Web Services?
Web services can be thought of as an evolution of the
software components concept. For example, say you have
several different word processors on your computer, or on
your network. In the early days of software, each
application needed to contain it's own separate spell
checking code. With components, the spell checking
function is programmed into a separate module that can be
shared by several different word processor programs. Every
programmer doesn't have to write their own spell checking
code, they can license the use of a spell checking module
from a components vender.
The same thing is possible over the internet using DCOM,
CORBA, JavaBeans, etc. But these technologies were all
created by different organizations. The components find it
difficult to communicate with each other. It requires a
lot of information sharing and pre-planning to make these
components work together. "Web Services" is a set of
vendor-neutral specifications and protocols developed by
standards organizations such as OASIS and the W3C.
Using these standard protocols, Web sites can share Web
applications in a manner similar to how a spell checking
component can be shared between word processor programs.
Every Web site does not have to write it's own program
code to retrieve a stock quote, find a best price, saving
an appointment to a calendar, or validate a credit card
number. They can license the use of these functions from
Web Service providers.
From the human Web users point of view, the Web still
appears to work the same. There is nothing new. That is
why it's so difficult to find a simple explanation of
"Web Services". The new technology of Web Services is what
goes on under the hood. That's why most articles about the
subject jump right into the programming code. Many "Web
Service" articles are just articles about XML.
The html code used by the Web today is inadequate for the
Web services task. Html is great for formatting Web pages,
but it tells you nothing about the data contained in the
Web page. XML (eXtensible Markup Language) allows
programmers to define their own tags to describe the data
on a Web page, and communicate those definitions to the
receiving system. XML is not binary computer code, similar
to html, XML is just ASCII text.
When a Web site wants to use a Web Service, it makes a
request in a standard format called SOAP (Simple Object
Access Protocol). SOAP messages are written in XML. SOAP
messages are sent back and forth between the service
provider and service consumer. How does a programmer know
what services are provided and what messages to use in
accessing them? Web services use a standard message format
called WSDL (Web Services Description Language) to
describe themselves. WSDL is written in XML.
How does a developer find available Web Services? They can
be found by using a protocol called UDDI (Universal
Description, Discovery and Integration) to search a
directory of services available. Web Service providers
register their services in the directory. Developers can
locate and select a provider for the service that meet
their requirements. Similar to the licensing fees paid by
the developer of the word processor for the spell checking
component, web developers will have to pay for the use of
Web Services.
Developers don't have to use Web Services developed by
other people. Microsoft's.NET and Sun Microsystems's
J2EE are frameworks that contain resources and class
libraries for developing Web Services. There are also a
number of open source framework's available, for example
the Mono project hosted by the open Source software
company Ximian. www.ximian.com/devzone/projects/mono.html
When you use a Web Service or a Web site that uses a Web
Service, either licensed or directly, you will be required
to pay, and you WILL use Web Services, because desktop
applications will no longer be available. The former
practice of paying one-time to use a software application
as much as you want does not generate a continuous stream
of revenue for software companies. Web Services allow the
software companies to milk a continuous stream of money
from you. Maybe that is the simple explanation of Web
services that we can't seem to find.
Resource Box:
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