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
• Apples 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
• lttag attr1val1 attr2val2gt

11
Structure of XML (2)

• Strict nesting is required
• Unstructured text is properly quoted
• A single unique root element
• Elements
• lttag gt Scope of tag lt/taggt
• lttag /gt
• lt? ?gt Meta-information
• lt!-- --!gt Comments

12
Structure of XML (3)

• Within an elements 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