What value could custom client SDK can provide over SOAP toolkit(s)?

1. Single library to deal.
Developers need not have to spend time in knowing or generating stubs & which framework to choose for for the same (XML marshaling/un-marshaling).This single library client can hide this implementation methodology by exposing easy to use APIs & hiding these techniques behind the scene using some kind of dependency injection APIs. (Jax-ws, Axis, XFire, Xstream so on...). The single library can be collection of core + optional service libraries in the same fashion provided by the Spring Like spring-jdbc.jar, spring-jndi.jar so on... with spring-core.jar as kernal providing infrastructure for all other services.

2. Handling backward compatibility issues
Carefully designed client SDK can solve backward/upward compatibility issues without compromising on the design elegance, If we don't want to use new SOAP version, it should be OK to use existing version most of the time. Client SDK can be designed in such a way user is shielded from SOAP schema changes that does not require recompilation or rebuild what so ever.For using new APIs user just has to drop in new library(jar) that's it. Well designed Mock objects can even save the need to goto sandbox during development.

3. Easy to use fluent APIs.
Domain specific language (DSL) design can bring lot of changes & provide better user experience with fail safe APIs. Fluent APIs can bring lot of +ve changes. For me this is the single most important feature.
For example:

MailBuilder.mail().
.from(praveen.manvi@yahoo.com)
.to(pmanvi@aol.com)
.withSubject("Fluent Mail API")
.withBody("Fluent API & DSL can make developer life simpler")
.attach(new File("c:/test.txt")).
.send();

is definitely much easier to use. This is ok for 90% of users to start, if we have hooks to get into Transport & other Java Mail APIs,it can simplify the way we code & importantly understand without compromising the power for advanced users.

4. Meaningful Exception hierarchy stack for better recoverable actions
Users don't have to deal with SoapFault Error code. Just like Spring DAOException hierarchy translating the SQLException & SQL error codes giving meaningful RuntimeException (even exteding it to hibernate, ibatis,jpa) API providers can make developer's life easier. Like SQLException & SQL error code APIFaultException & fault code is difficult use & introduces lot of boiler plate code (because of checked exception) without providing the meaningful recovery options. Hibernate, Spring & C# have proved that checked exception is like communism, works best in theory only.

http://static.springframework.org/spring/docs/2.5.x/reference/dao.html


5. Automatic logging soap XML handshake (Making this switch on & off on need basis) between client & server. This comes as very handy & we can even make them asynchronous & log this information to to different data sources like file & database by providing proper hooks. This automation improves the performance as well as avoids the need to develop auditing APIs.

6. Most of error handling/validation can done by the SDK framework, there is very less chance for error requests coming to server improving server performance. 99% of the time server will deal with valid requests

7. Meaningful defaults for all the APIs & actions, can reduce initial learning curve & improve the perception. It can help user to get into the the details step by step instead of throwing him everything upfront.

8. Can enforce the best practices. & develop new interfaces that are very difficult to achieve with simple plane SOAP.

For example a utility can be developed to export the data in chunks from large files like Excel sheet, a csv file or a PDF., as the sending large files & batch processing with SOAP is not mature (mtom...)

9. Encourage more developer community participation. Easy to use APIs improves the possibility of
developer to develop new tools & application using the SDK especially desktop tools.

10. Get rid technical limitation of programming technique. SOAP inherintly not object orieneted & it's procedural style of programming is not what a modren developer would like to work.

I just evaluated different SDK provided by various well known API providers & most of them don't really provide above facilities & there is very good opportunity of 3'rd party developers to fill the gap over here.

Same concepts can be carried out for developing C# APIs. As for as dynamic langauges(Groovy,Ruby, PHP) concerned I guess they don't need any SDK, they are smart enough to deal with XML directly without any code generation.

Many a times developers need to design wrappers over SOAP apis -- that involves substantial effort. Client SDK can save the time & money here. Like Spring removed the necessity of having in-house framework for standard java enterprise project by not tying the implementation any specific technology but to the interface & also DSL developed with business in mind can make writing code like writing an essay.

References:
http://developer.ebay.com/DevZone/XML/docs/HowTo/BestPractices/JavaToJsdkToSoap.html - eBay SOAP SDK sample
https://www.paypal.com/IntegrationCenter/ic_sdk-resource.html - Paypal API
http://code.google.com/p/fluentmailapi/ - Fluent Mail APIs
http://www.javaworld.com/javaworld/jw-08-2008/jw-08-dsls-in-java-3.html - DSL in java
http://www.toolsforteams.com/code/facebook-api/ - Facebook APis
http://code.google.com/p/java-twitter/ - Twitter APIs
http://www.oracle.com/technology/tech/soa/mastering-soa-series/part4.html - Rich UI for web services
http://www.javabeat.net/articles/29-introduction-to-google-guice-5.html - Guice Samples