1. Compressing XML Documents with Finite State Automata
S. Hariharan
Priti Shankar

2. Organization An introduction to XAUST
A brief introduction to XML & DTD
Previous work : XMill
Utility of DTD
Implementation
Experimental results
Conclusion

3. XAUST: Introduction XML Compression with AUtomata and STack
It is the only tool using DTD for compressing documents automatically
It generates code for an arbitrary DTD and compresses documents conforming to that DTD

4. XML The presence of tags makes the document verbose
XML describes a class of data objects called XML documents
The presence of tags makes the document verbose
Attributes are treated as tags
XML document is characterized as a tree
root
student id
name 1
Hariharan
opentag
<root>
<student id = “1”>
<name>
Hariharan
</name>
</student>
</root>
attribute
content
closetag

5. DTD DTD describes a document
DTD for the above XML fragment is given below
<!DOCTYPE StudentInfo [
<!ELEMENT root (student*)>
<!ELEMENT student (name | rollNo,
comment?)>
<!ATTLIST student id ID #REQUIRED>
<!ELEMENT name (#PCDATA)>
<!ELEMENT rollNo (#PCDATA)>
<!ELEMENT comment (#PCDATA)>
]>
* means zero or more occurrences
+ means one or more occurrences
? means zero or one occurrence
| means either-or
, means and
#PCDATA means textual content

6. Arithmetic coding
Arithmetic coding replaces a string of symbols with a single floating number
Symbol
Probability
Range a
0.2
[0, 0.2) b
0.3
[0.2, 0.5) c
0.1
[0.5, 0.6) d
[0.6, 0.8) e
[0.8, 0.9) !
[0.9, 1.0)
We wish to encode bacc!
After seeing b the encoder narrows it down to the interval [0.2, 0.5)
After seeing a the interval is further narrowed down to 1/5th of itself
Hence the new interval becomes [0.2, 0.6)
After seeing the first c the interval becomes [0.23, 0.236)
The final interval is [ , )

7. Previous work: XMill XMill separates structure from text
XMill has structure container and text containers
Consider the XML fragment
<book>
<title>
xyz
</title>
<isbn>
123
</isbn>
</book>
book = #1, title = #2, isbn = #3 Structure = #1 #2 C1 / #3 C2 / /
There will be many repeated sequences of structure like the one above and are compressed
The text is mapped to the container depending on the path from the root

8. Motivation for XAUST XMill does not use DTD
We propose a scheme using DTD to achieve better compression ratio
Automatic multiplexing is achieved

9. Utility of DTD Consider the Elements addressBook and card
<!ELEMENT addressBook ( card* )>
<!ELEMENT card ( (name | (givenName, familyName)), , note? )>
card
name
givenName
familyName
note
addressBook
A sample fragment of the XML document
Tags need not be encoded

10. Regular Expression regular expression
<!ELEMENT card ( (name | (givenName, familyName)), , note? )>
The DFA for the corresponding regular expression is given below 1 2 3 4 5
familyName
name
givenName
note

11. Regular Expression Contd.,1 2 3 4
familyName
name
givenName 5
note
We can see that we need not encode familyName and tags
They are the states with single output transition
But state 4 is a state with multiple transition. There is an implicit transition to the parent ELEMENT addressBook
( <!ELEMENT addressBook (card*)> )

12. Encoding text The tree for the DTD Consider a fragment of DTD
#PCDATA B A D
Consider a fragment of DTD
<!ELEMENT A (C, B, D) >
<!ELEMENT B (C) >
<!ELEMENT C (#PCDATA) >
<!ELEMENT D (#PCDATA) >
3 choices for encoding text
A single container for all text
A single container for each element
A single container for each leaf node of the DTD tree

13. Encoding text contd., Advantages and disadvantages of the 3 approaches
A single container for all the text
Advantage: Low memory consumption
Disadvantage: Low compression ratio
A single container for each Element
Advantage: Medium memory consumption
A single container for each leaf node in the DTD tree
Disadvantage: High memory consumption

14. Encoding text contd.,
Not much difference in the compression ratio between the options 2 and 3
Option 2 i.e., ‘A single container for each Element’ was chosen as it entails less memory consumption
As text is character data we used Arithmetic compression
Order-4 Adaptive Arithmetic compression is used

15. Implementation An automaton is generated for each Element
Stack is used for storing the current state of ancestor element
When encountering
Action taken
Open tag
Push the current state in stack. Start the automaton of the child element
Close tag
Pop the current state from stack. Make state transition
Text
Encode the text

16. Experiments
We conducted our experiments on XMark, DBLP, Uniprot, Michigan and X007 documents
XAUST is compared with XMill, XMLPPM and gzip
XMLPPM ran out of memory for Uniprot and Michigan documents
XMill, XMLPPM and gzip are better than XAUST for one document each
Compression ratio is defined as the ratio between the size of the compressed document and the size of the original document

17. Document size Name Size (in MB) Auction 113 Dblp 253 Uniprot 1070
Michigan
495
X007
128

18. Compression ratio of document

19. Compression ratio of tags

20. Conclusion and Future Work
The results amply justify the utility of DTD
Large documents are compressed without running out of memory
The only restriction placed by XAUST is the presence of DTD
Future work will concentrate on querying the compressed document

21. References
Hartmut Liefke, Dan Suciu.: XMill: An efficient compressor for XML data, Proceedings of ACM SIGMOD, 2000
Nelson, M: Arithmetic Coding. Dr. Dobbs Journal
UniProt :
Michigan:
XOO7:
XMark:
DBLP:

22. Thank you