Web service
computer networks like the Internet. This interoperability is due to the use of open standards. OASIS and the W3C are the steering committees responsible for the architecture and standardization of web services. To improve interoperability between web service implementations, the WS-I organisation has been developing a series of profiles to further define the standards involved.
Standards used
- XML: All data to be exchanged is formatted with XML tags. This encoding can be performed by SOAP or XML-RPC.
- Common protocols: XML data can be transported between applications using common protocols such as HTTP, FTP, and SMTP.
- WSDL: The public interface to the web service is described by Web Services Desciption Language, or WSDL. This is an XML-based service description on how to communicate using the web service.
- UDDI: The web service information is published using this protocol. It enables applications to look up web services information in order to determine whether to use them.
Advantages of web services
- Web services provide interoperability between various software applications running on various platforms.
- Web services leverage open standards and protocols. Protocols and data formats are text based where possible, making it easy for developers to see and understand what is going on.
- By piggybacking on HTTP, web services can work through many common firewall security measures without requiring changes to their filtering rules.
Disadvantages of web services
- Web services standards for features such as security and transactions are currently nonexistent or still in their infancy compared to more mature distributed computing open standards such as CORBA.
- Web services suffer from poor performance compared to other distributed computing approaches such as RMI, CORBA, or DCOM. This is a common trade-off when choosing text based formats. XML explicitly does not count among its design goals either conciseness of encoding or efficiency of parsing.
- By piggybacking on HTTP, web services can evade existing firewall security measures whose rules are intended to block or audit communication between programs on either side of the firewall.
Reasons for creating web services
The main reason web services are used seems to be that they rely on HTTP over TCP port 80. To understand this, keep in mind that many enterprises have protected themselves (see computer security) by using firewalls that filter and block much Internet traffic for security reasons. In this milieu, typically many (almost all) ports are closed to incoming and outgoing traffic, and the administrators of these firewalls are not eager to open them up. Port 80, however, is always open because it is used for web browsers. Web services tunnel everything through port 80, making the technology very appealing.
Another reason may be that prior to SOAP, there were no really good interfaces to accessing functionality on different computers on a network. Most of them were ad hoc, and few people knew or could handle EDI, RPC, and similar APIs. Web services may have more solid frameworks and would thus be easier to use.
A third reason that web services are useful: They can provide very loose coupling between an application that uses the web service and the web service itself. This should allow either piece to change without negatively affecting the other. This flexibility may become increasingly important as software is built by assembling individual components into a complete application.
Platforms
Web services can be deployed by using application server software. A sample of application servers:
See also
