Coordination Middleware for document-centric applications

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Coordination Middleware for document-centric
applications

• April 2002

Paolo Ciancarini Robert Tolksdorf Franco Zambonelli
University of Bologna Technische Universität Berlin University of Modena and Reggio Emilia
ciancarini_at_cs.unibo.it tolk_at_cs.tu-berlin.de franco.zambonelli_at_unimo.it
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Motivations

• The Internet is "Document Centric"
• Document-centric computing models are needed
• A document is a communicable data structure
• Focus on two key issues
• coordination models and middleware for the
  engineering of complex content management
  applications
• XML as "The Standard" for passive and active
  documents

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Documents

• Passive document content representation
• "Formatter programs" like TeX assign rendering
  behavior to mostly procedural (sometimes
  declarative) markup
• Passive documents have at least two facets
• The abstract view on content (eg. WYSIWYG)
• The concrete machine-level representation

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Documents

• Documents were dependent from devices
• (a solution Postscript)
• Documents were dependent from applications
• (a solution Rich Text Format)
• Documents are dependent from the underlying os
• (think about the document fonts)
• Documents are dependent from the browser
• (no WYSIWTS-what you send is what they see)

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Taxonomy and History

time
2000
1995
1990
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Active documents

• Active document content structure behavior
• The document itself "knows" how to be handled
• Possible perspectives
• Reactive documents external applications are
  forced touse the internal behavior to handle
  documents
• Proactive documents the document/agent can
  handleitself in autonomy (Agents data
  behavior autonomy)
• Documents virtuels personnalisables (DVP2002)
• http//iasc.enst-bretagne.fr/DVP2002

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Middleware for XML Document Agents

• Middleware that offers services for agents that
• are specified using XML and
• "run'' in an XML environment
• Agents live in a completelyXML oriented world
• Middleware is implementedoutside the XML world
• Examples Displets, JXML, Adlets

Middleware
XML
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Displets Dept. Computer Science, Univ. of
Bologna

• General-purpose framework for active documents
  and for multi-document-agents applications
• XML document agents ("Displets") can
• include re-active behaviors related to rendering,
  printing, be searched..
• Include pro-active behaviors to produce
  animation, generate new documents, search, etc
• coordinate with each other

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The XML Compiler of Displets

• - XML document -gt DOM tree
• - DOM tree -gt DOM tree with XSL stylesheet
• - The DOM tree -gt tree of runnable Java objects
• The XML document has become a document agent!

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Coordinable Displets

• Displets can have associated
• their own private behavior
• when on a group the coordination behavior
  enacting coordination laws
• Static approach
• compiler-based
• the document agent and coordination laws
  generated atcompile time
• no dynamic enteringor leaving of groups

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XML Middleware for document Agents

• Coordination middleware thatexploits XML at the
  middlewarelevel in itself
• Coordination activities of application agents
  occur and are ruled via accesses to shared XML
  information spaces, in which the laws ruling
  coordination reside and are enacted
• Information space in itselfbecomes sort of an
  active document agent
• Examples XMLSpaces, MARS-X, XMIDDLE, XML
  Protocol/SOAP

Middleware
XML
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XMLSpaces TU Berlin XML documents as
field-data

• lt?xml version"1.0"?gtlt!DOCTYPE address SYSTEM
  "address.dtd"gtltlocationgt ltcity
  name"Trento"/gt lthostgtFACOLTÀ DI
  ECONOMIAlt/hostgt ltstreet name"Via Vigilio
  Inama" no"5"/gtlt/locationgt
• lt2001, __gt

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How to match?

• A formal is an object describing an XML document
• Can be
• Another document
  in(2001, ?__)
• Something elsein(2001, DOCTYPE"address.dtd")
• There is a variety of relations that identify XML
  documents

lt?xml version"1.0"?gtlt!DOCTYPE address SYSTEM
"address.dtd"gtltlocationgt ltcity
name"Trento"/gt lthostgtFACOLTÀ DI
ECONOMIAlt/hostgt ltstreet name"Via Vigilio
Inama" no"5"/gtlt/locationgt
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Multiple matching relations in XMLSpaces

• Relation Meaning Tool used
• Exact equality Exact textual equality DOM
  interfaces
• Restricted Textual equality ignoring
• equality comments, PIs, etc. DOM interfaces
• DTD Valid towards a DTD XML4J
• DOCTYPE Uses specific Doctype name DOM
• XPath Fulfills an XPath expression Xalan-Java
• XQL Fulfills an XQL expression GMD-IPSI XQL
• AND Fulfills two matching relations
• NOT Does not fulfill matching relation
• OR Fulfills one or two matching relations
• XOR Fulfills one matching relation

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MARS-X University of Modena and Reggio Emilia

• A coordination infrastructure for mobile agents
• virtual mobility
• actual agent mobility
• physical (i.e., device) mobility
• Local XML dataspaces for mobile agent access
• shared by a node (or domain of nodes)
• Linda-like (JavaSpaces)interface to XML data
• Programmable tuplespace model
  coordinationlaws can be programmed in Java

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MARS-X Tuple-based Access to the XML Documents

• XML entries as JavaSpaces tuples
• on match, the effect of the access on a documents
  can be ruled via properly programmed reactions

class _infoN extends AbstractEntry static final
URL DTDfile new URL(http//mysite/myDTD.dtd)
public Integer f1 public String f2 public
String f3 public Integer f4 _infoN t new
_infoN() t.f1 null t.f2 foo t.f3
bl t.f4 17 myEntry result
space.read (t, null, NO_WAIT) ...
... ltinfoNgt ltf1gt3lt/f1gt
ltf2gtfoolt/f2gt ltf3gtblahlt/f3gt ltf4gt17lt/f4gt lt/info
Ngt ... ltinfoNgt ... lt/infoNgt ...
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XMIDDLE University College, London

• A Middleware (i.e., a coordination
  infrastructure) for mobile and ad-hoc networks
• The nodes/processes in the network share a global
  XML data structure
• The global data structure is an active document
• it includes protocols for handling data
  reconciliation and data merging
• also application specific protocols can be
  defined

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XMIDDLE The Architecture

• Coordinables are active processes
• mobile AND/OR executing on a mobile device
• Coordination media as XML tree
• possibly replicated (even partially) and shared
  among processes in the reach
• Coordination laws rules modifications to the XML
  tree

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XMIDDLE Coordination over the XML Tree

• Processes operate and coordinatewith each
  otherby manipulating a shared tree
• A disconnected process can continue operating
  on a replica of the tree
• When reconnecting, the globaldocument tree must
  be re-constructed in a consistent way

Disconnection of Process A
Reconnection of Process A with re-conciliation
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Self-contained XML Middleware

• Agents are represented as someXML documents as
  well as the data they operate onand the laws
  ruling theircoordination activities
• Examples WorkSpaces, XSL,XML Script, ADF

Middleware
XML
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Workspaces TU Berlin XML and XSL only

• Documents andworksteps asXML documents
• Workspacesengine is anextended XSLengine

Store Rating
exec("Excel")
ltRATING
ltRATING
Workspaces
PAPER"3"
PAPER"3"
engine
SCORE"4"/gt
SCORE"4"/gt
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Higher and guarded coordination in workflow graph

• AND-SPLIT, OR-SPLIT, AND-JOIN, OR-JOIN as
  generative (and destructive) coordination
  operation
• Transitions may also have conditions

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Workspaces

• An XML/XSL based workflow system
• Workflow graph is split into single steps that
  are represented as XSL documents

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Discussion

• Displets
• suitable for the definition and implementation of
  document agent apps
• Too static
• MARS-X
• suited for complex coordination patterns to be
  dynamically defined, in manipulation of shared
  XML documents by mobile agents
• restricts the application to use Java agents
• XMIDDLE
• suitable for document agents, and seems very
  suitable for mobility
• possibility of defining coordination laws very
  limited
• Workspaces
• provides more uniformity
• lacks an explicit support for mobility,
• general-purpose applicability unclear

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Goal Design ideal XML middleware

• directly handling, at the application level, the
  activities in XML document agents (Displets)
• making coordination activities occur in terms of
  manipulation of shared XML documents, (MARS-X,
  XMIDDLE, XMLSpaces)
• flexible to support user-defined XML grammars
  (XMLSpaces)
• effectively handling mobility and associated
  issues (XMIDDLE)
• enabling the ruling the coordination activities
  between application-level document agents in a
  dynamic way (MARS-X)
• expressing not only document agents behavior but
  also the laws ruling their coordination
  activities in term of XML documents and XML rules
  (WorkSpaces)

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Conclusions

• Further consider
• Mobility
• Coordination models
• Embedded systems
• Organizational structures
• Conclusion
• Taking the "coordination viewpoint" on XML based
  middleware is again fruitful