FAQ on Web Services and Apache Axis2

Apache Axis2 Web Services, 2nd Edition

Create secure, reliable, and easy-to-use web services using Apache Axis2.

• Extensive and detailed coverage of the enterprise ready Apache Axis2 Web Services / SOAP / WSDL engine.
• Attain a more flexible and extensible framework with the world class Axis2 architecture.
• Learn all about AXIOM - the complete XML processing framework, which you also can use outside Axis2.
• Covers advanced topics like security, messaging, REST and asynchronous web services.
• Written by Deepal Jayasinghe, a key architect and developer of the Apache Axis2 Web Service project; and Afkham Azeez, an elected ASF and PMC member.     

Q: How did SOA change the world view?

A: The era of isolated computers is over. Now "connected we stand, isolated we fall" is becoming the motto of computing. Networking and communication facilities have connected the world in a way as never before. The world has hardware that could support the systems that connect thousands of computers, and these systems have the capacity to wield power that was once only dreamed of.

Yet, computer science lacked the technologies and abstraction to utilize the established communication networks. The goal of distributed computing is to provide such abstractions. RPC, RMI, IIOP, and CORBA are a few proposals that provide abstractions over the network for the developers to build upon.

These proposals fail to consider one critical nature of the problem. The systems are a composition of numerous heterogeneous subsystems, but these proposals require all the participants to share a programming language or a few languages. Service Oriented Architecture (SOA) provides the answer by defining a set of concepts and patterns to integrate homogenous and heterogeneous components together.

SOA provides a better way to achieve loosely coupled systems, and hence more extensibility and flexibility. In addition, similar to object-oriented programming (OOP), SOA enables a high degree of reusability. There are three main ways one can enable SOA capabilities in their systems and applications:

• Existing messaging systems: for example, JMS, IBM MQSeries, Tibco, and so on
• Plain Old XML (POX): for example, REST, XML/HTTP and so on
• Web services: for example, SOAP, WSDL, WS-*

Q: What are the shortcomings of Java Messaging Service (JMS)?

A: Among the commonly used messaging systems, Java Messaging Service (JMS) plays a major role in the industry and has become a common API for messaging systems. We can find a number of different message types of JMS, such as Text, Bytes, Name-Value pair, Stream, and Object. One of the main disadvantages of these types of messaging systems is that they do not have a single wire format (serialization format). As a result, interoperability is a big issue: if two applications are using JMS to communicate, then they must be on the same implementation. Sonic, Tibco, and IBM are the leaders in the commercial markets, and JBoss, Manta, and ActiveMQ are the commonly used open source implementations.

Q: What is POX and how does it serve the web?

A: Plain Old XML or POX is another way of exposing functionality and enabling SOA in the system. With the widespread use of the Web, the POX approach has become more popular. Most of the web applications expose the XML APIs, where we can develop components and communicate with them. Google Maps, Auto complete, and Amazon services are a few examples of applications that heavily use XML APIs to expose the functionality. In most cases, POX is used in combination with REST (Representational State Transfer). REST is a model of an underlying architecture of the Web, and it is based on the concept that every URL identifies resources. GET, PUT, POST, and DELETE are the verbs that are used in the REST architecture. REST is often associated with the theoretical standpoints, and for this reason, REST is generally not used for complex interactions.

Q: What are web services?

A: The fundamental concept behind web services is the SOA where an application is no longer a large monolithic program, but it is divided into smaller, loosely coupled programs. The provided services are loosely coupled together with standardized and well-defined interfaces. These loosely coupled programs make the architecture very extensible due to the possibility to add or remove services with limited costs. Therefore, new services can be created by combining existing services. To understand loose coupling clearly, it is better to understand the opposite, which is tight coupling, and its problems:

• Errors, delays, and downtime spread through the system
• The resilience of the whole system is based on the weakest part
• Cost of upgrading or migrating spreads
• It's hard to evaluate the useful parts from the dead weight

The benefits a web service provides are listed below:

• Increased interoperability, resulting in lower maintenance costs
• Increased reusability and composablity (for example, use publicly available services and reuse them or integrate them to provide new services)
• Increased competition among vendors, resulting in lower product costs
• Easy transition from one product to another, resulting in lower training costs
• Greater degree of adoption and longevity for a standard, a large degree of usage from vendors and users leading to a higher degree of acceptance

Q: What contributes to the popularity of web services?

A: Among the three commonly used methods to enable SOA, a web service can be considered as the most standard and flexible way. Web services extend the idea of POX and add additional standards to make the communication more organized and standardized. There are several reasons behind the web services being the most popular SOA-enabled mechanism, as stated here:

• Web services are described using WSDL, and WSDL can capture any complex application and the required quality of services.
• Web services use SOAP as the message transmission mechanism, as SOAP is a special type of XML. It gains all the extensibility features from XML.
• There are a number of standard bodies to create and enforce the standards for web services.
• There are multiple open source and commercial web service implementations. By using the standards and procedures, web services provide application and programming language-independent mechanism to integrate and communicate. Different programming languages may define different implementations for web services, yet they interoperate because they all agree on the format of the information they share.

Q: What are the standard bodies for web services?

A: In web services, there are three main standard bodies that helped to improve the interoperability, quality of service, and base standards:

• WS-I
• OASIS
• W3C

Q: How do organizations move into web services?

A: There are three ways in which an organization could possibly use to move into the web services, listed next:

• Create a new web service from scratch. The developer creates the functionalities of the services as well as the description (i.e., WSDL).
• Expose the existing functionality through a web service. Here the functionalities of the service already exist. Only the service description needs to be implemented.
• Integrate web services from other vendors or business partners. There are occasions when using a service implemented by another is more cost effective than building from the scratch. On these occasions, the organization will need to integrate others' or even business partners' web services.

The real usage of web service concepts is for the second and third methods, which enables other web services and applications to use the existing applications.

Web services describe a new model for using the web; the model allows publication of business functions to the Web and provides universal access to those business functions. Both developers and end users benefit from web services. The web service model simplifies business application development and interoperation.

Q: How does a Web services model look like?

A: Web service model consists of a set of basic functionalities such as describe, publish, discover, bind, invoke, update, and unpublish. In the meantime, the model also consists of three actors—service provider, service broker, and service requester. Both the functionalities as well as actors are shown in the next figure.

• Service provider is the individual (organization) that provides the service. The service provider's job is to create, publish, maintain, and unpublish their services. From a business point of view, a service provider is the owner of the service. From an architectural view, a service provider is the platform that holds the implementation of the service. Google API, Yahoo! Financial services, Amazon Services, and Weather services are some examples of service providers.
• Service broker provides a repository of service descriptions (WSDL). These descriptions are published by the service provider. Service requesters will search the repository to identify the required service and obtain the binding information for these services. Service broker can be either public, where the services are universally accessible, or private, where only a specified set of service requesters are able to access the service.
• Service requester is the party that is looking for a service to fulfill its requirements. A requester could be a human accessing the service or an application program (a program could also be a service). From a business view, this is the business that wants to fulfill a particular service. From an architectural view, this is the application that is looking for and invoking a service.

Q: What are web services standards?

A: So far we have discussed SOA, standard bodies of web services, and the web service model. Here, we are going to discuss more about standards, which make web services more usable and flexible. In the past few years, there has been a significant growth in the usage of web services as application integration mechanism. As mentioned earlier, a web service is different from other SOA exposing mechanisms because it consists of various standards to address issues encountered in the other two mechanisms. The growing collection of WS-* (for example, Web Service security, Web Service reliable messaging, Web Service addressing, and others) standards, supervised by the web services governing bodies, define the web service protocol stack shown in the following figure. Here we will be looking at the standards that have been specified in the most basic layers: messaging and description, and discovery.

The messaging standards are intended to give the framework for exchanging information in a distributed environment. These standards have to be reliable so that the message will be sent only once and only the intended receiver will receive it. This is one of the primary areas where research is being conducted, as everything depends on the messaging ability.

Q: Describe the web services standards, XML-RPC and SOAP?

A: The web services standards; XML-RPC and SOAP are described below.

XML-RPC: The XML-RPC standard was created by Dave Winer in 1998 with Microsoft. That time the existing RPC systems were very bulky. Therefore, to create a light-weight system, the developer simplified it by specifying only the essentials and defined only a handful of data types and commands. This protocol uses XML to encode its calls to HTTP as a transport mechanism. The message is sent as a POST request in which the body of the request is in XML. A procedure is executed on the server and the value it returns is also formatted into XML. The parameters can be scalars, numbers, strings, dates, as well as complex record and list structures. As new functionalities were introduced, XML-RPC evolved into what is now known as SOAP, which is discussed next. Still, some people prefer using XML-RPC because of its simplicity, minimalism, and the ease of use.

SOAP: The concept of SOAP is a stateless, one-way message exchange. However, applications can create more complex interaction patterns—such as request-response, request-multiple responses, and so on—by combining such one-way exchanges with features provided by an underlying protocol and application-specific information. SOAP is silent on the semantics of any application-specific data it conveys as it is on issues such as routing of SOAP messages, reliable data transfer, firewall traversal, and so on. However, SOAP provides the framework by which application-specific information may be conveyed in an extensible manner. The developers had chosen XML as the standard message format because of its widespread use by major organizations and open source initiatives. Also, there is a wide variety of freely available tools that ease the transition to a SOAP-based implementation.

Q: Define the scope of Web Services Addressing (WS-Addressing)?

A: The standard provides transport independent mechanisms to address messages and identifies web services, corresponding to the concepts of address and message correlation described in the web services architecture. The standard defines XML elements to identify web services endpoints and to secure end-to-end endpoint identification in messages. This enables messaging systems to support message transmission through networks that include processing nodes such as endpoint managers, firewalls, and gateways in a transport-neutral manner. Thus, WS-Addressing enables organizations to build reliable and interoperable web service applications by defining a standard mechanism for identifying and exchanging Web Services messages between multiple end points.

Q: What is Web Services Description Language (WSDL)?

A: WSDL developed by IBM, Ariba, and Microsoft is an XML-based language that provides a model for describing web services. The standard defines services as network endpoints or ports. WSDL is normally used in combination with SOAP and XML schema to provide web services over networks. A service requester who connects to a web service can read the WSDL to determine what functions are available in the web service. Special data types are embedded in the WSDL file in the form of XML Schema. The client can then use SOAP to call functions listed in the WSDL.

The standard enables one to separate the description of the abstract functionality offered by a service from the concrete details of a service description such as how and where that functionality is offered. This specification defines a language for describing the abstract functionality of a service as well as a framework for describing the concrete details of a service description. The abstract definition of ports and messages is separated from their concrete use, allowing the reuse of these definitions.