Object and component “wiring” standards

1. Object and component “wiring” standards This presentation reviews the features of software component wiring and the emerging world of XML-based standards. By Michiel Pouw

2. Interproces communication • Wide variety of mechanisms for IPC • Files • Pipes • Sockect • Shared Memory

3. Disadvantage / Advantage • None of these mechanisms is portable across platforms • They can easily be extended to work across networks • But implementing complex interactions is painfull and error-prone

4. Remote procedure calls

5. Disadvantage / Advantage • Mapping all levels of communication on to a single abstraction • In-process • Interprocess • Intermachine • But also hides the significant cost difference between calls

6. From procedures to objects • A remote machine can be seen as an object and RPC entry points are its methods • To be generally useful, an object-oriented library must thus be distributed in source form • Not popular for component “wiring”

7. Back to basic: Compound documents • The components and compositition are intuitively meaningfull for the user • The Xerox Star System • Apple’s Hypercard • Microsoft Visual Basic • Later, web pages with embedded objects, such as Java applets and ActiveX controls added a new dimension

8. The rise of XML • Useful for representing any structured and semi-structured data • Prefers tree-shaped data • Can handle relational schemas • Even binary data using a standard encoding • New applications of XML arise by the day

9. The rise of XML (2) • Browsers directly support displaying and exploring XML documents • Wide range of tools for generic support • XML can be used as common language among applications of independent origin and operation • XML takes the notion of “wiring” standards from the level of protocols and wire formats to the level of durable data representation

10. Structure of XML • Three important catagories • XML elements • XML attributes • Unstructured text • Elements and attributes like HTML <tag attr1=“val1” attr2=“val2”>

11. Structure of XML (2) • Strict nesting is required • Unstructured text is properly quoted • A single unique root element • Elements: • <tag …> Scope of tag </tag> • <tag …/> • <? … ?> Meta-information • <!-- … --!> Comments

12. Structure of XML (3) • Within an element’s scope nested elements or unstructured text can be placed • Multiple nested elements can have the same tag • Ordering of nested elements matters • Attributes of an element form a set • Different names • Ordering is irrelevant

13. Document type descriptors • DTDs were originally used before XML • Cryptic, limited but compact

14. XML versus DTD • XML Schema itself is extensible • Much richer and expressive then DTD • Part of XML Schema is a flexible and expressive data type description system • Use XML to define a standard metaschema for other XML documents

15. XML support standards • Xpath • Tag separation by slashes (“/”) • Xpointer • Extends the set of idenifible locations in XML • Xlink • Focuses on the links among XML documents • XML stylesheet language • Transform an XML document into another form

16. XML document object and streaming model • XML document object model presents XML documents as • Abstract data structures • or as objects • For large documents this requires • Random acces • or streaming reading and writing

17. Simple object acces protocol • XML documents can be used as self-describing messages • Can be used for remote object invocation • The SOAP standard provides standard ways to • Describe the addressee of an invocation • Encode a wide range of typical programming data types into invocation messages • Define what parts of a message must be understood or can be ignored

18. XML web services • With SOAP web services can be established • These services offer computational services to other systems • Similar to deployed components from a client point of view • Standard interfaces • Implementation is not revealed

19. XML web services (2) • Not addressed at the level of SOAP • Authentication of communication parties • Encryption of exchange information • Compensation agreements • Subscription • Billing • Another standard is needed for these issues

20. Web services description language • WSDL documents are based on XML • Defines an extensible framework to describe web services. • Input and output messages are defined by WSDL ports • WSDL ports are grouped into WSDL services

21. Web services and programming models • Ports on services are similar to methods on interfaces • Tempting to model web service invocations as synchronous remote method calls • Also tempting to think of web service implementations as classes that implement the ports of a service as methods

22. Web services and programming models (2) • Special nature of web-distributed services • Unpredictable latencies • Flexible host selection due load balancing • Failures can occur at any time • A better model should use asynchronous call models against web services, similar to those found in message queuing systems

23. Conclusion Many other issues need to be addressed in the world of web services that have previously been addressed in remote procedure, distributed object, and messaging approaches