Web Services Protocol Stack

You may hear web services for many years and already get some ideas of it, but always get confused and lost in the set of protocol and technology in use and not sure how they are related to each other in the picture.

Here, i would like to give another explanation of them from underlying protocol stack in use for web service so that you can organized, relate them properly in mind.

The Web service protocol stack is a collection of computer networking protocols that are used to define, locate, implement, and make Web services interact with each other. The Web service protocol stack mainly comprises four areas (source:wikipedia):

* Transport Protocol: responsible for transporting messages between network applications and includes protocols such as HTTP(s), SMTP, FTP, as well as the more recent Blocks Extensible Exchange Protocol (BEEP). Now, most common protocol in use is HTTP(S).

* Messaging Protocol: responsible for encoding messages in a common XML format so that they can be understood at either end of a network connection. Currently, this area includes such protocols as XML-RPC, WS-Addressing, REST and SOAP, which is most common one in use today.

* Description Protocol: used for describing the public interface to a specific web service. The WSDL interface format is typically used for this purpose.

* Discovery Protocol: centralizes services into a common registry such that network web services can publish their location and description, and makes it easy to discover what services are available on the network. At present, the UDDI API is normally used for service discovery.

Web service protocol stack also includes a whole range of recently defined protocols: BPEL, SOAP-DSIG.

In general terms, a web service is a software component or service that has been published on the World Wide Web in one of several particular ways, but industry tends to use the term web services more specifically, to describe services that are exposed using SOAP, an XML-based message encoding that is typically transported using HTTP.

XML-RPC

XML-RPC is a remote procedure call protocol which uses XML to encode its calls and HTTP as a transport mechanism. XML-RPC is a very simple protocol, defining only a handful of data types and commands, and the entire description can be printed on two pages of paper. Some people still prefer XML-RPC to SOAP because of its simplicity, minimalism, and ease of use. (source:wikipedia)

SOAP

SOAP is a protocol for exchanging XML-based messages over computer networks, normally using HTTP/HTTPS. SOAP forms the foundation layer of the Web services stack, providing a basic messaging framework so that more abstract layers can build on.

There are several different types of messaging patterns in SOAP, but by far the most common is the Remote Procedure Call (RPC) pattern, in which one network node (the client) sends a request message to another node (the server) and the server immediately sends a response message to the client.

SOAP is the successor of XML-RPC, though it borrows its transport and interaction neutrality and the envelope/header/body from elsewhere, probably from WDDX (source:wikipedia).

A SOAP message is an ordinary XML document containing the following elements:

* A required Envelope element that identifies the XML document as a SOAP message
* An optional Header element that contains header information
* A required Body element that contains call and response information
* An optional Fault element that provides information about errors that occurred while processing the message

All the elements above are declared in the default namespace for the SOAP envelope (source: w3schools)

WSDL

The Web Services Description Language (WSDL) is an XML-based language that provides a model for describing Web services.The current version of the specification is the 2.0. The WSDL defines services as collections of network endpoints, or ports. WSDL specification provides an XML format for documents for this purpose.A port is defined by associating a network address with a reusable binding, and a collection of ports define a service.

WSDL is often used in combination with SOAP and XML Schema to provide web services over the Internet. A client program connecting to a web service can read the WSDL to determine what functions are available on the server. Any special datatypes used are embedded in the WSDL file in the form of XML Schema. The client can then use SOAP to actually call one of the functions listed in the WSDL.WSDL 1.2 was renamed WSDL 2.0 because of its substantial differences from WSDL 1.1. By accepting binding to all the HTTP request methods (not only GET and POST as in version 1.1) WSDL 2.0 specification offers a better support for RESTful web services, much simpler to implement by beginners. (source:wikipedia)

A WSDL 1.1 document uses the following elements in the definition of network services:

* Types– a container for data type definitions using some type system (such as XSD).
* Message– an abstract, typed definition of the data being communicated.
* Operation– an abstract description of an action supported by the service.
* Port Type–an abstract set of operations supported by one or more endpoints.
* Binding– a concrete protocol and data format specification for a (not more than one) particular port type.
* Port– a single endpoint defined as a combination of a binding (not more than one) and a network address.
* Service– a collection of related endpoints (port).

WSDL 2.0 is different from WSDL 1.1 check out the example of a structured WSDL 2.0 document from here.

UDDI

Universal Description, Discovery and Integration (UDDI) is a platform-independent, XML-based registry for businesses worldwide to list themselves on the Internet. UDDI is a cross-industry effort driven by all major platform and software providers like Dell, Fujitsu, HP, Hitachi, IBM, Intel, Microsoft, Oracle, SAP, and Sun, as well as a large community of marketplace operators, and e-business leaders.

JAX-RPC
Check out my previous two posts (1|2) in this blog.

JAXR

Java API for XML Registries (JAXR) defines a standard API for Java platform applications to access and programmatically interact with various kinds of metadata registries. JAXR provides a uniform and standard Java API for accessing different kinds of XML-based metadata registry.

Current implementations of JAXR support ebXML Registry version 2.0, and UDDI version 2.0. More such registries could be defined in the future.

The JAXR API insulates application code from the underlying registry mechanism. When writing a JAXR based client to browse or populate a registry, the code does not have to change if the registry changes, for instance from UDDI to ebXML.

WS-I
The WS-I Basic Profile (official abbreviation is BP), a specification from the Web Services Interoperability industry consortium (WS-I), provides interoperability guidance for core Web Services specifications such as SOAP, WSDL, and UDDI. The profile uses Web Services Description Language (WSDL) to enable the description of services as sets of endpoints operating on messages. BP complied web service means smooth internal operability (messaging, description and registry) between web services regardless of underlying implementation.