1. Efficient Filtering of XML Documents with XPath Expressions
Chee-Yong Chan, Pascal Felber, Minos Garofalakis, Rajeev Rastogi
Information Sciences Research Center
Bell Laboratories, Lucent Technologies

2. Motivation
Efficient Filtering of XML Documents with XPath Expressions
Growing interest in content-based filtering & routing of data.
Data
Publishers
Subscription
Table
Filtering
Engine
Subset of
Relevant Data
Consumers
data
Content-based Router
XML => More expressive XPath-based subscriptions (e.g., Intel’s NetStructure XML Accelerator).
Challenge: How to efficiently filter XML data with XPath-based subscriptions?

3. Problem Abstraction document XPath Filter Subset of S that match D D
Efficient Filtering of XML Documents with XPath Expressions
Problem Abstraction
XML
document D
Subset of S that match D
XPath Filter
S, Set of XPath
expressions (XPEs)

4. Challenges Filtering with XPath expressions (XPEs) is non-trivial:
Efficient Filtering of XML Documents with XPath Expressions
Challenges
Filtering with XPath expressions (XPEs) is non-trivial:
Complexity of XPEs -- tree-structured patterns that include ``*’’ and ``//’’ operators.
Need for both unordered & ordered matchings.
//a /b /f
//e
/*/d /c
Example:
p = // a / b [ c / * / d ] / / e / f
XPE tree of p

5. Efficient Filtering of XML Documents with XPath Expressions
Our Solution: XTrie
Speed up XPE filtering with a novel index called XTrie.
Key idea:
Decompose
Complex, tree-
structured XPE
Set of simple,
linear patterns
(substrings)
XTrie
Index with
trie

6. XTrie Index Construction Algorithm
Architecture of XTrie
Efficient Filtering of XML Documents with XPath Expressions
XTrie Index Construction Algorithm
Complex,
tree -
structured
XPEs
XML
document D
XML Parser
(SAX based)
Start/End
Element
Events
XTrie Index
Set of XPEs
that match D
XTrie
Matching
Algorithm

7. Architecture of XTrie Complex, tree - structured XPEs Set of simple,
Efficient Filtering of XML Documents with XPath Expressions
Architecture of XTrie
Complex,
tree -
structured
XPEs
Set of simple,
linear patterns
(substrings)
Decompose
XPEs
Build XTrie
index
XML
document D
XML Parser
(SAX based)
XTrie Index
Start/End
Element
Events
Trie
Set of XPEs
that match D
XTrie
Matching
Algorithm
Substring
Table

8. Decomposition of XPEs
Efficient Filtering of XML Documents with XPath Expressions
Decompose each XPE p into a set of substrings that “cover” p.
Substring = Sequence of element names along some path in XPE tree, where each consecutive pair of nodes is related by a “/” operator (without any “*” or “//”).
Example: p = // a / b [ c / * / d ] // e / f
Substrings in p = {a, b, c, d, e, f, ab, bc, ef, abc }.
//a /b /f
//e
/*/d /c

9. Decomposition of XPEs
Efficient Filtering of XML Documents with XPath Expressions
Decompose each XPE p into a set of substrings that “cover” p.
Substring = Sequence of element names along some path in XPE tree, where each consecutive pair of nodes is related by a “/” operator (without any “*” or “//”).
Example: p = // a / b [ c / * / d ] // e / f
Substrings in p = {a, b, c, d, e, f, ab, bc, ef, abc }.
One possible decomposition of
p is { a, bc, d, ef }.
//a /b /f
//e
/*/d /c

10. Efficient Filtering of XML Documents with XPath Expressions
Decomposition of XPEs
In general, there are many possible decompositions.
Single-Element
Decomposition
Minimal
Decomposition
//a /b /f
//e
/*/d /c
//a /b /c
//e
/*/d /f

11. Decomposition of XPEs
Efficient Filtering of XML Documents with XPath Expressions
“Enhanced” min. decomp. = min. decomp. with a substring ending at each branching node.
//a /b /f
//e
/*/d /c
Single-Element
Decomposition
Minimal
. . .
“Enhanced”

12. XTrie XTrie index consists of 2 components: XTrie Index Trie Substring
Efficient Filtering of XML Documents with XPath Expressions
XTrie
XTrie index consists of 2 components:
Trie
Substring
Table
XTrie Index

13. XTrie XPEs p = // a / a / b / * / a / b q = / a / b [ c] // b / c
Efficient Filtering of XML Documents with XPath Expressions
XPEs
p = // a / a / b / * / a / b
q = / a / b [ c] // b / c

14. XTrie Decomposed Substrings XPEs /a /b /c //b //a /*/a p q
Efficient Filtering of XML Documents with XPath Expressions
Decomposed Substrings /a /b /c
//b
//a
/*/a p q
XPEs
p = // a / a / b / * / a / b
q = / a / b [ c] // b / c

15. XTrie Decomposed Substrings Substring-Table /a /b /c //b //a /*/a p q
Efficient Filtering of XML Documents with XPath Expressions
Decomposed Substrings /a /b /c
//b
//a
/*/a p q
Parent
Row
Rel.
Level
Num
Child
Rank
aab 1 2 3 4 5 1 3 1 2 1 2 ab ab
abc bc
Substring-Table

16. XTrie Trie Substring-Table aab ab abc bc a b a b c Next Row Parent
Efficient Filtering of XML Documents with XPath Expressions
Trie 1 a b 2 3 a b c
Substring-Table 4 5 6
Next
Row
Parent 1 3
Rank 2
Rel.
Level
Num
Child 4 5 b c
aab ab 7 8
abc
Child Node Ptr bc
Substring Table Ptr

17. XTrie Trie Substring-Table aab ab abc bc a b a b c Next Row Parent
Efficient Filtering of XML Documents with XPath Expressions
Trie 1 a b 2 3 a b c
Substring-Table 4 5 6
Next
Row
Parent 1 3
Rank 2
Rel.
Level
Num
Child 4 5 b c
aab ab 7 8
abc
Child Node Ptr
Substring Table Ptr
Max. Suffix Ptr bc

18. Optimizations for XTrie
Efficient Filtering of XML Documents with XPath Expressions
Optimizations for XTrie
“Lazy” variant of XTrie
Reduce number of accesses to substring-table by probing it only when the matched substring is a leaf substring of some XPE.
XTrie for single-path XPEs
Optimize data structures & algorithms by exploiting the simpler structures of single-path XPEs.

19. Related Work Commercial Products (e.g. BEA, Intel, etc).
Efficient Filtering of XML Documents with XPath Expressions
Related Work
Commercial Products (e.g. BEA, Intel, etc).
XFilter [ Altinel & Franklin, VLDB’00]
Model single-path XPEs as finite state machines (FSMs). /a /c
//b
p = / a // b / c
Build a hash index on FSMs’ transitions (ie element names). a b c
candidate-list
wait-list
Optimizations
XFilter-LB = XFilter with list balancing
Prefiltering = 2 parses over XML data to pre-filter some XPEs.

20. Experimental Evaluation
Efficient Filtering of XML Documents with XPath Expressions
Experimental Evaluation
DTD: NITF (News Industry Text Format)
123 elements, 513 attributes
XML data: Generated with IBM’s XML Generator (size = 20, 100, 1000 tag pairs)
XPath expressions: Generated using our own generator (P = #XPEs, L = max. depth, Pw = prob. of ‘*’, Pd = prob of ‘//’, z = skew of element names)
Algorithms: Eager & Lazy XTrie, XFilter & XFilter-LB [Altinel & Franklin, VLDB00]
System: Sun Ultra-250 (296MHz) with 512 MB memory running Solaris 2.7
NITF: News Industry Text Format

21. Efficient Filtering of XML Documents with XPath Expressions
Scalability (# XPEs)

22. Efficient Filtering of XML Documents with XPath Expressions
Scalability (# tags)

23. Efficient Filtering of XML Documents with XPath Expressions
Conclusions
XTrie -- A novel index structure that supports the efficient filtering of streaming XML data based on XPath expressions.
Features:
Index both simple single-path as well as complex tree-structured XPath expressions.
Handles ordered, unordered, and hybrid modes of matching.

24. Efficient Filtering of XML Documents with XPath Expressions
Speedup / # XPEs (m)

25. Efficient Filtering of XML Documents with XPath Expressions
Wildcards (m)

26. Efficient Filtering of XML Documents with XPath Expressions
Descendants (m)

27. Efficient Filtering of XML Documents with XPath Expressions
Number of levels (m)

28. Efficient Filtering of XML Documents with XPath Expressions
Skew (m)