1. CobWeb – A Constraint Based Extension of XML
Srivatsava Shanker
Sangeetha Raghavan
Tim Mckernan
Dr. Bharat Jayaraman

2. Motivation
XML defines structure of websites using document type definitions (DTDs).
Essentially, CobWeb = DTDs + Constraints
Constraints specify semantic integrity conditions on the website, whereas DTDs specify syntactic ‘well-formedness’ and ‘validity’ conditions.
CobWeb can be used for website specifications and also for B2B applications.

3. CobWeb Syntax Constraint_expr ::= [NOT] Constraint (Lop Constraint)*
Constraint_expr ::= [NOT] Constraint (Lop Constraint)*
Lop ::= AND|OR
Constraint ::= Complex_id (Rop Complex_id)*
Rop ::= ge|gt|le|lt|=
Complex_id ::= Simple_id(.Simple_id)* [:attribute| :href(Complex_id)]
Simple_id ::= identifier | identifier ‘[‘ index ‘]’
Index ::= first | last | positive_integer
Aggregate_constraint ::= Agg_term Rop Agg_term | constraint Agg_op (Complex_id)
Agg_term ::= Complex_id | Agg_func ( Complex_id)
Constraint ::= Complex_id (Rop Agg_func Complex_id)*
Agg_op ::= Agg_func | Agg_pred
Agg_func ::= SUM |AVE |COUNT
Agg_pred ::= ASCENDING|DESCENDING
Quantified constraint ::= [EVERY | EXIST] (x:Complex_id) Constraint_expr

4. Aggregate Constraints
Since many web pages contain collections of entities, we often wish to express a constraint in terms of the aggregate value of some collection. In the example below, we show a constraint that makes use of the average value of a set of student grades.
<?xml encoding="US-ASCII"?>
<!ELEMENT class (course, average, student+)>
<!ELEMENT course (#PCDATA)>
<!ELEMENT student (name, grade)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT grade (#REAL)>
<!ELEMENT average (#REAL)>
<!CONSTRAINT (average = AVERAGE(student.grade))>
The other Rop operators that are available are SUM, COUNT, etc.

5. Tree Structure of an XML document
class
class.student[30].grade= “A-”
course
student
“ CSE605”
student[1]
student[30]
name
grade
name
grade
“ZsaZsa”
“A-”
“Aaron”
“B+”

6. Quantified Constraints
<!CONSTRAINT (EXIST (x : Complex_id) constraint_expr)>
<!CONSTRAINT (EVERY (x : Complex_id) constraint_expr)>
<!ELEMENT member (name, rank)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT rank (#INTEGER)>
Every faculty member in the promotion committee must be a professor:
<!ELEMENT promotion_committee (member*)>
<!CONSTRAINT EVERY(x:hiring_committee.member.rank ) x = “professor”>
At least one member of the Graduate Affairs Committee must be a graduate student:
<!ELEMENT graduate_affairs (member*)>
<!CONSTRAINT EXIST(x:graduate_affairs.member.rank) x = “graduate student”>

7. Inheritance <?Xml encoding="US-ASCII"?>
<!ELEMENT student (student-id,name, transcript)>
<!ELEMENT transcript (student-id,course-list)>
<!ELEMENT course-list (course-id,semester,grade)+>
<!ELEMENT student-id (#PCDATA)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT grade (#PCDATA)>
<!ELEMENT semester (#PCDATA)>
<!ELEMENT course-id (#PCDATA)>
<!CONSTRAINT (student .transcript .course-list.course-id .grade!= ”F”)>
<!ELEMENT TA :: student (work, stipend)>
<!ELEMENT work (#PCDATA) default = ’grade students’>
<!ELEMENT stipend (#REAL)>
<!CONSTRAINT (TA.transcript.course-list.course-id.grade ge “B+”) >
<!ELEMENT RA :: student (work, stipend)>
<!ELEMENT work (#PCDATA) default = ’research project’>
<ELEMENT stipend (#REAL)>
<!CONSTRAINT (RA.transcript.course-list.course-id.grade ge “A-”) >

8. Online-Brochure
CobWeb can be used as a solution to a common problem in websites: verifying data content and links across multiple, interconnected web pages.

9. <?xml encoding="US-ASCII"?>
<!ELEMENT toc (toc_section+, toc_loc)>
<!ELEMENT toc_section (toc_description)>
<!ATTLIST toc_section
xmlns:xlink CDATA #FIXED "
xlink:type (locator) #FIXED "locator"
xlink:href CDATA #REQUIRED
xlink:role CDATA #IMPLIED>
<!ELEMENT toc_description (toc_number, toc_title)>
<!ELEMENT toc_number (#INTEGER)>
<!ELEMENT toc_title (#PCDATA)>
<!ELEMENT toc_loc EMPTY>
<!ATTLIST toc_loc
xmlns:xlink CDATA #FIXED "
xlink:type (locator) #FIXED "locator"
xlink:href CDATA #REQUIRED
xlink:role CDATA #FIXED "toc_loc"
xlink:title CDATA #FIXED "Table of Contents">
<!CONSTRAINT (toc_section:href(section.toc_loc:href)=/toc_loc:href) >
<!CONSTRAINT (toc_section.toc_description.toc_number =
toc_section:href:(section.toc_description.toc_number))>
<!CONSTRAINT (ASCENDING(toc_section.toc_description.toc_number)) >

10. <!ELEMENT section_data (#PCDATA)>
<!ELEMENT section (toc_description, section_data, toc_loc, prev_loc?, next_loc?)>
<!ELEMENT section_data (#PCDATA)>
<!ELEMENT prev_loc EMPTY>
<!ATTLIST prev_loc
xmlns:xlink CDAT #FIXED "
xlink:type (locator) #FIXED "locator"
xlink:href CDATA #REQUIRED
xlink:role CDATA #FIXED "prev_loc"
xlink:title CDATA #REQUIRED>
<!ELEMENT next_loc EMPTY>
<!ATTLIST next_loc
xmlns:xlink CDATA
#FIXED "
xlink:role CDATA #FIXED "next_loc"
<!CONSTRAINT ((section.prev_loc:href(section.toc_description.toc_number) + 1)
= section.toc_description.toc_number)>
 <!CONSTRAINT ((section.next_loc:href(section.toc_description.toc_number) - 1)

11. Simple Circuit Definition
<?xml encoding="US-ASCII"?>
<!ELEMENT component >
<!ATTLIST component
voltage #REAL (#REQUIRED)
current #REAL (#REQUIRED)
resistance #REAL (#REQUIRED)
>
<!CONSTRAINT (voltage = current*resistance)>
<!ELEMENT series::component (ser_comps)>
<!ELEMENT ser_comps (component+)>
<!CONSTRAINT voltage = SUM(ser_comps.component:voltage)>
<!CONSTRAINT current = EVERY(ser_comps.component:current)>
<!CONSTRAINT resistance = SUM(ser_comps.component:resistance)>
<!ELEMENT parallel::component (par_comps)>
<!ELEMENT par_comps (component+)>
<!CONSTRAINT voltage = EVERY(ser_comps.component:voltage)>
<!CONSTRAINT current = SUM(ser_comps.component:current)>
<!CONSTRAINT 1/resistance = SUM(1/(ser_comps.component:resistance))>
<!ELEMENT Battery >
<!ATTLIST Battery
voltage #REAL (#REQUIRED)>
 <!ELEMENT connect (battery, component)>
<!CONSTRAINT battery:voltage = component:voltage)>
Here only some of the information is given by the XML document , the others are found out by solving constraints
The circuit example illustrates how to construct series and parallel circuits, using attributes current, voltage and resistance.
The constraints used in
Parallel circuit- 1/R= 1/R1 +1/R2+…
Series circuit- R=R1+R2+…

12. Conclusions and Status
CobWeb views the web as a database, and constraints help specify conditions for database integrity.
New features include inheritance, aggregation, quantified constraints, etc.
A prototype implementation of most of the CobWeb features has been developed using an extension of the IBM XML4J parser.
Full implementation and applications under consideration.