DTDDirected Transformations for Data Exchange in XML

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DTD-Directed Transformations for Data Exchange
in XML

• Wenfei Fan
• Internet Management Research Dept., Bell Labs
• Dept. of CIS, Temple University

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Overview

• Ongoing application-oriented research
• DTD-directed transformations
• Attribute Transformation Grammars (ATGs)
• Open questions
• joint work with M. Benedikt, C. Chan, R. Rastogi

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• 1. DTD-directed transformations

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Data exchange on the Web

• A community (industry) agrees on a certain DTD.
  Subsequently all members of the community
  exchange data w.r.t. the DTD
• e-commerce
• health-care
• ...

Web
DTD
XML
XML
Q view
RDB
OODB
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Example

• An XML view for patients SSN, name and treatment
  procedure
• a relational database at a hospital
• Patient (SSN, name, tr-name)
• Treatment (name, cost)
• Procedure (name1, name2) -- composition
  hierarchy
• . . .
• a DTD predefined by an insurance company
• r ? patient
• patient ? SSN, name, treatment
• treatment ? name, procedure
• procedure ? treatment -- recursive

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DTD and the structure of the XML data

• XML rooted, node-labeled tree
• r ? patient
• patient ? SSN, name, treatment
• treatment ? name, procedure
• procedure ? treatment
• lt-- unbounded

...
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DTD-directed transformation

• Given a relational database, schema R
• a predefined DTD D (with certain semantic
  information)
• Question Is there any systematic way to define
  an XML view of R (i.e., ? R ? D) that guarantees
  to conform to D?
• R Patient (SSN, name, tr-name)
• Treatment (name, cost)
• Procedure (name1, name2), ...
• D r ? patient
• patient ? SSN, name, treatment
• treatment ? name, procedure
• procedure ? treatment

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More on DTDs

• DTD D (E, P, r) -- an Extended Context-Free
  Grammar (ECFG)
• E a set of element types (nonterminals)
• P production rules (in a normal form), e ? ?
• ? s ? e1, , en e1
  en e
• where s string, ? empty word,
  , concatenation, disjunction,
  Kleene closure
• r start symbol
• e.g., r ? patient
• patient ? SSN, name, treatment
• treatment ? name, procedure
• procedure ? treatment

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Difficulties

• Given an input type and a fixed (complex) output
  type, is there any systematic way to write a
  program that type checks?
• XML view of DTD D a parse tree of D, possibly
  nested to an arbitrary level that cannot be
  decided statically

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Existing systems

• ignore the requirement of DTD-conformance
• SilkRoute (ATT), XPERANTO (IBM), Microsoft,
  Oracle, incapable of coping with a predefined
  DTD
• simply define an XML view and then check its
  conformance
• undecidable in general, co-NEXPTIME for extremely
  restricted view definitions
• no guidance on how to define XML views that
  typecheck
• one gets an XML view that typechecks only after
  repeated failures and with luck

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• 2. Attribute Transformation Grammars (ATGs)

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Attribute Transformation Grammars (ATGs)

• ATG ? ( D, Attributes, Rules), ? R ? D
• D DTD
• Attributes an (inherited) attribute tuple e is
  associated with each element type e
• e to pass value as well as control
• Rules for each production e ? ? and each element
  type e in ?, a semantic rule is associated e
  Q(e) or e ? Q(e)
• Q SQL query on instances of R,
• e the attribute of the parent of e (in the
  production), treated as a constant parameter in Q
• ? iteration over tuples single tuple
  assignment

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Example ATG

• r ? patient
• patient ? select SSN, name, tr-name
• from Patient
• patient ? SSN, name, treatment
• SSN patient.SSN, name
  patient.name
• treatment patient.tr-name
• recall Patient (SSN, name, tr-name)

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Example (contd)

• treatment ? name, procedure
• name treatment -- value
• procedure treatment -- parameter
• procedure ? treatment
• treatment ? select name2
• from Procedure
• where procedure name1
• recall Procedure (name1, name2)

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Evaluation of an ATG

• Top-down, lazy-evaluation (data-driven)
  treatment is further expanded as long as
  procedure is not empty.
• DTD-directed the XML tree is constructed
  strictly following the productions (the
  evaluation aborts if inconsistency is detected)

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ATGs vs. Attribute Grammars (AGs)

• ATGs are inspired by AGs, but are not a mild
  variation of AGs.
• AGs (CFG G, Attributes, Rules)
• given a string s, parse s with the CFG
• defined with a CFG
• support both synthesized and inherited attributes
• ATGs (ECFG, Attributes, Rules)
• given a database, extract relevant information
  from the database with SQL to build a parse tree
  of the ECFG
• defined with an ECFG
• support inherited attributes and SQL queries
• it does not make sense to parse a database
  w.r.t. a DTD

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• 3. Open questions

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Open questions (1)

• The exact expressive power of ATGs
• powerful enough to deal with all/most found in
  practice?
• Strictly more expressive than SilkRoute,
  XPERANTO, ...
• extra power by incorporating synthesized
  attributes?
• termination analysis Does an ATG ? R ? D
  terminate without aborting on all instances/a
  fixed instance of R?
• for a fixed database I decidable in PTIME in
  I
• for an arbitrary database and any ATG defined
  with conjunctive queries and arbitrary DTDs
  decidable in EXPTIME in ?
• for an arbitrary database and ATGs defined with
  general SQL queries undecidable

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Open questions (2)

• Information-preserving transformations
• Question Given a relational schema R and a DTD
  D, is there any ATG ? R ? D such that for any
  instance I of R, ?(I) preserves the information
  of I?
• challenging after 20 years, the notion of
  information preservation (semantic
  equivalence) is still not clear!
• important central to data transformations, data
  integration, storage optimization, and database
  design
• Existing definitions
• either too weak to capture what people want in
  practice
• or completely impractical

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Open questions (3)

• Incorporating integrity constraints
• XML data is typically specified with (D, ?),
  where
• D a DTD
• ? constraints, e.g., keys and foreign keys
• Question Given a relational schema R and an XML
  specification (D, ?), is there any ATG ? R ? D
  such that for any instance I of R, ?(I) both
  conforms to D and satisfies ??
• challenging even the consistency of (D, ?) is
  undecidable
• important practical restrictions on D or/and ?

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Open questions (4)

• Combining XML queries and view definitions
• It is common to query XML views -- XQuery, XSL,
  ...
• Question Given an ATG ? R ? D and a query Q on
  XML data that conforms to D, is there another ATG
  ? R ? D such that for any instance I of R,
  ?(I) Q(?(I))?
• One needs new query composition/propagation
  techniques in the context of XML

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Summary

• ATGs yield a systematic way to conduct
  DTD-directed transformations they provide
  guidance to define DTD-conformant XML views
• ATGs can be naturally generalized to deal with
• DTD-directed transformations from OODBs, etc
• DTD-directed transformations between XML
  documents
• DTD-directed integration of heterogeneous data
  sources
• Many important questions in connection with ATGs
  remain open ? application-oriented theoretical
  problems