SOA Governance in Action: REST and WS-* Architectures

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Machine Learning in Action. MongoDB in Action. Mahout in Action. Hadoop in Action.

RESTful Web Services vs. “Big” Web Services: Making the Right Architectural Decision

Event Processing in Action. Lucene in Action 2e. Collective Intelligence in Action. Java Persistence with Hibernate 2e. Code Generation in Action. Net for Programmers. The power of SOA is in loose coupling, composition, and how it enables approaches like cloud computing. But when I needed to implement the services I had already determined were necessary, I faced a choice: use web services or REST-based styles as the means to interact with the services.

Web services in theory and in practice. T he main concepts behind Web Services were established in and during the height of the dot-com boom. RPC style we think not or Document style? How do you identify end points? And what about naming operations and methods? Clearly SOAP on its own leaves too much to interpretation. But writing and reading and understanding WSDL is a cumbersome affair. Data type matching can be a pain. Versioning is a bear. Minor server-side changes often result in different WSDL and a resulting different service interface, and on the client-side, XSD descriptions of the service are often similarly tied to a particular version of the SOAP endpoint and can break all too easily.

And you still have all the problems associated with SOAP. In my attempts to simply get a service up and running, I found myself fighting more with SOAP and WSDL than doing actual work to get services built and systems communicating. Writing and reading and understanding WSDL is a cumbersome affair. Sure, I could artificially insert UDDI into my use case, but in the scenario where I needed loose coupling, I could get that by simply abstracting my end points and data schema.

Bye, bye UDDI. These specifications are not web services dependent, and indeed, many of the largest Web-based applications use OAuth and the other specs to make their REST-based environments more secure. Why REST is ruling. I ended up using REST for a number of reasons, but the primary one is simplicity.

RESTful Web Services vs. “Big” Web Services: Making the Right Architectural Decision

The basic operation and scalability of the Web has proven the underlying premise of the fundamental REST approach. HTTP operations are standardized, widely accepted, well understood, and operate consistently. GET retrieves the current state of a resource in some representation. POST transfers a new state onto a resource. Self-descriptive messages. Resources are decoupled from their representation so that their content can be accessed in a variety of formats e. Meta-data about the resource is available and used, for example, to control caching, detect transmission errors, negotiate the appropriate representation format, and perform authentication or access control.

Stateful interactions through hyperlinks. Every interaction with a resource is stateless, i. Stateful interactions are based on the concept of explicit state transfer. Several techniques exist to exchange state, e. State can be embedded in response messages to point to valid future states of the interaction. Such lightweight infrastructure, where services can be built with minimal tooling, is inexpensive to acquire and thus has a very low barrier for adoption.

The effort required to build a client to a RESTful service is very small as developers can begin testing such services from an ordinary Web browser, without having to develop custom client-side software. Furthermore, thanks to URIs and hyperlinks, REST has shown that it is possible to discover Web resources without an approach based on compulsory registration to a centralized repository.

On the operational side, it is known how to scale a stateless RESTful Web service to serve a very large number of clients, thanks to the support for caching, clustering and load balancing built into REST. Due to the possibility of choosing lightweight message formats, e. There is some confusion regarding the commonly accepted best practices for building RESTful Web services. Lo-REST, on the other hand, focuses on the minimum common denominator.

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This is due to the fact that proxies and firewalls may not always allow HTTP connections that use any other verb. As with most non-standard workarounds, these may not be understood by all Web servers, and require additional development and testing effort. Another limitation makes it impossible to strictly follow the GET vs. POST rule. The size of the request notwithstanding, it may also be challenging to encode complex data structures into a URI as there is no commonly accepted marshalling mechanism.

Inherently, the POST method does not suffer from such limitations.

What SOA Governance Really Means

Architectural decisions [ 39 ] are the key metaphor in our comparison method. Architectural decisions capture the main design issues and the rationale behind a chosen technical solution; they can informally be defined as conscious design decisions concerning a software system as a whole or affecting one or more of its core components and determining the non-functional characteristics and quality factors of the system [ 52 ].

For each architectural decision, one or more architecture alternatives AAs enumerate the design options.


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In our earlier work, we proposed a structured, proactive approach to architectural decision modeling [ 52 ], which extends today's practices of rather informal, retrospective decision capturing for documentation purposes. When reviewing the SOAP vs. REST debate, it becomes clear that many of the issues discussed and arguments brought forward qualify as recurring architectural decisions that can be modeled. Architectural decision models have several use cases, for example decision support and governance during architecture design; they can also be used to facilitate technical quality assurance reviews [ 53 ].

In the context of this work, we used them as a metric for the REST vs. We took the following steps:.

A comparison of these principles comprises the first step in our comparison Table 1. This is the level of abstraction and detail on which most existing comparisons of the two styles reside. Principles such as dealing with heterogeneity and loose coupling are typically defined informally; hence, an assessment solely based on these criteria only is bound to be subjective and incomplete. Therefore, we added three more steps. The first architectural principle clarifies whether the Web is used as a publishing medium for delivering application services to clients or as a messaging medium for application integration - in other words, whether HTTP is considered as an application or as a transport protocol.

acknowledgments

In the context of REST, the Web is seen as the universal medium for publishing globally accessible information. In other words, the HTTP protocol is used as a tunneling protocol to enable remote communication through firewalls, but it is not used to convey the semantics of the service interaction. Heterogeneity is more related to the competition between different browser vendors that would lead, for instance, to different renderings of HTML pages or incompatible JavaScript libraries.

Still, all browsers support the same HTTP protocol and can process a large variety of standard document types.


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  • This domain can be characterized as a collection of heterogeneous, autonomous, distributed software systems [ 7 ]. In organizations having a long history, which may even predate the advent of the Web, these software systems are implemented in many different kinds of technologies. Thus, in such kind of loosely coupled system, clients are not affected when services suffer from temporary downtime.

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    In other words, when an HTTP server is down, its clients will be affected as their HTTP requests fail; they have to handle such connection timeout failures themselves. Synchronous interactions can nevertheless show a form of loose coupling, when clients discover the actual location of service providers at runtime.

    In REST, a form of location transparency can be achieved by DNS address translation, requiring to additional efforts to properly configure the networking infrastructure. When it comes to managing the evolution of a Web service , loose coupling implies the ability to make modifications to a Web service without affecting its clients. Freezing the basic protocol allows for complete decoupling of clients from servers, as no change on the server will ever break a client, as such change simply does not happen.

    However, independent of whether the set of operations is fixed or not, another important part of a service interface definition is the message parameter data model, which specifies the format i. As opposed to a binary protocol, the usage of SOAP or POX messages can give the ability to slightly modify a service interface in non-breaking ways [ 42 ].

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