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

Interproces communication

Wide variety of mechanisms for IPC Files Pipes Sockect Shared Memory

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

Remote procedure calls

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

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”

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

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

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

Structure of XML

Three important catagories XML elements XML attributes Unstructured text

Elements and attributes like HTML

<tag attr1=“val1” attr2=“val2”>

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

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

Document type descriptors

DTDs were originally used before XML

Cryptic, limited but compact

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

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

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

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

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

XML web services (2)

Not addressed at the level of SOAP Authentication of communication parties Encryption of exchange information Compensation agreements

SubscriptionBilling

Another standard is needed for these issues

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

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

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

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