XML

Part 1 of this series talks about SOAP vs. REST. In this installment I'll discuss the reasons for defining the web service contract between client and server, the existing methods for doing it, and the important concepts of each.

Defining the Contract

An important part of any web service is the contract (or interface) which it defines between the service and any clients that might use it. This is important for a number of reasons: visualization with tools, interaction with other specifications (e.g., web service choreography), code generation, and enforcing a high-level agreement between the client and service provided (that still gives the service freedom to change the underlying implementation). Taken together, they give pretty compelling use cases for having web services contracts, although advocates of minimalism may disagree.

When IBM, Microsoft, and Ariba submitted WSDL 1.1 to the W3C in 2001 as a language for describing web services in conjunction with SOAP 1.1, HTTP POST and GET, and MIME, it quickly became a standard used by every SOAP toolkit. This happened in spite of the fact that it never progressed beyond being a W3C Note (which, according to W3C, is a document available for "discussion" and not officially endorsed by the W3C). In fact, though there is both a WSDL 1.1 and 1.2, WSDL 2.0 is the only version of the specification officially endorsed by the W3C.

With the rise in popularity of RESTful web services, there also became a need to describe contracts for these types of web services as well. Although WSDL 2.0 attempts to fill the gap by providing support for HTTP binding, another specification fills this need in an arguably better way: WADL , a specification developed at Sun by Marc Hadley. Though it has not been submitted to any official standards body (OASIS, W3C, etc.), WADL is promising because of its more comprehensive support for REST-style services.

Developers new to web services are often intimidated by parade of technologies and concepts required to understand it: REST, SOAP, WSDL, XML Schema, Relax NG, UDDI, MTOM, XOP, WS-I, WS-Security, WS-Addressing, WS-Policy, and a host of other WS-* specifications that seem to multiply like rabbits. Add to that the Java specifications, such as JAX-WS, JAX-RPC, SAAJ, etc. and the conceptual weight begins to become heavy indeed. In this series of articles I hope to shed some light on the dark corners of web services and help navigate the sea of alphabet soup (1). Along the way I'll also cover some tools for developing web services, and create a simple Web Service as an example. In this article I will give a high-level overview of both SOAP and REST.

I've read an article titled Does XML have a future on the web?, and it does not surprise me, because the author himself is the creator of JSON. Naturally, he would love to promote his stuff. I would like to ignore him but would like to address his questions.

Lets see what he got.

"For data transfer applications, XML is losing ground to JSON because JSON is simply a better data transfer format".
I want to know how? Everyone in the world knows XML is more recognized and widely used and it is well supported by every one of the vendors in the web world (Editors, Application Servers, Parsers, Web Servers, Loaders etc.). JSON's role is what? Simply converting raw text in to a JavaScript object. And how is that a better data transfer format? Can I take the same raw text and use it else where? I don't think so.

I heard there are some frameworks available for JSLT (yeah you heard it right, JavaScript - SLT) available as open source, still I would challenge the performance about those frameworks. I haven't tried them myself, but you cannot beat the performance provided by the native browser XSLTs. I could reuse same XML data and apply different XSLTs to get different sets of transformed structures. You can't even think about this using JSON. And that to you can transform using XSLT's in milliseconds. With JavaScript JSON, you'll be lucky if you can get it in seconds.

Also, what about reusability? My server still can send the same XML and I could put a SOAP wrapper on top of that and I could expose it to web services. Now anybody can invoke that web service. Not necessarily only on the web side, it could be invoked by any server side programs as well. XML is the de-facto standard for Data integration projects, Data warehousing projects. More and more corporations are moving towards a SOA (Service oriented architecture) model and more and more applications have to use XML as the data transport mechanism.

JSON may be used by applications which are not enterprise level, only for applications which deal with less data, and have no need for extensibility. But for more scalability, more robustness, and more extensibility, XML is your best bet. XML has more support and that's what your boss would like to hear.