by Waldemar Hummer, Philipp Leitner, Schahram Dustdar

1. A Step-By-Step Debugging Technique
To Facilitate Mashup Development and
Maintenance
Waldemar Hummer, Philipp Leitner, Schahram Dustdar
Distributed Systems Group, Vienna University of Technology
4th International Workshop on Web APIs and Services Mashups
8th IEEE European Conference on Web Services
December 1, 2010

2. Outline
 Motivation
 Mashups in WS-Aggregation
• Query Language
• Declarative Mashup Definition
 Integrated Mashup Debugging
• Goals
• Techniques
• Benefits
 Conclusion
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3. Motivation
 Mashups combine data from heterogeneous Web sources
 Clear vision of the desired output
 Trade-off between abstraction and flexibility [10]
 Proposed Simplifications
• Domain-Specific Languages (e.g., [7,15])
• Approaches for non-programmers (e.g., table-based [13,23])
 In any case, arbitrarily querying/combining data is complex
• Complexity of mashup programming can pose a barrier [24]
• „The biggest problem of mashup is the data“ [14]
• Further challenge: non-integrity of data, evolution of data sources
 Focus on development and debugging process
• SW-Engineering techniques not well-supported for Mashups [10]
• Important aspect: lack of support for interactive debugging [17]
 Demand for integrated mashup debugging facilities
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4. WS-Aggregation
 Aggregation of heterogeneous Web services data
 Loose coupling
• Data sources stored in registry [16]
 Scalable, distributed execution [11]
• Configurable distribution strategies
• Ad-hoc aggregator topologies
 Web services Aggregation Query Language (WAQL)
• Based on XQuery
• Integration of non-XML data sources (e.g., CSV, JSON, HTML, ...)
• Expressing data dependencies
• Possibility to generate sub-requests from templates
 Mashup Definition
1. Set of inputs for sub-requests {s1, s2, ..., sn}
2. WAQL Queries for transforming results
3. Data dependencies between sub-requests 4

5. Mashup Scenario
 Tourist information mashup
• Input: city, date
• Output: visa information, hotel rooms, wheather forecast
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6. Mashup Scenario (2)
Request Input (HTTP GET):
/getVisaInfo?c=${//country}
Data Source Response:
..<div id="visas">...</div>..
Preparation Query:
<visaInfo>
{//div[@id=’visas’]/node()}
</visaInfo>
Prepared Data Source Result:
<visaInfo>...</visaInfo>
 Query Types
Query Type Applied when? Applied to what?
Preparation Query immediately data source response
Intermediate Query before passing data to parent aggregator all prepared DS results
Finalization Query before returning data to client all prepared DS results
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7. Data Dependencies
Request Input (HTTP GET):
/getVisaInfo?c=${//country}
Data Dependency
resolved
at runtime
Data
Dependency
Graph Explicit Dependency:
/getVisaInfo?
c=$1{//country}
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8. Generated Inputs
Request Input (HTTP GET):
/getRooms?
h=$(${//hotelName/text()})
&d=${date}
Data Dependency: ${...}
Generated Inputs: $(...)
resolved
at runtime
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9. Mashup Debugging
 Debugging considered a core domain of SW-Engineering
 Different goals and purposes [25], e.g.:
• Asserting expectations
• Detecting anomalies
• Tracking origins
 Ultimate Goal: Fixing a Defect
• Defects in mashups:
− Data source(s) unavailable
− Data mistakenly dropped
− Data falsely added
− Data improperly transformed
 Different stages
• During development
• When underlying data sources change (→ maintenance)
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10. Asserting expectations
 Specify XQuery (or XPath) expressions that need to
evaluate to true.
Apply assertion Evaluate assertion
to response or before or after
total result? applying query?
When should
the assertion
be evaluated?
# Assertion Expression [E]nd / Data [R]esponse / [B]efore /
Source ID [T]otal Result [A]fter
1 count(//div[@id=’hotels’]/div)<=5 E T A
2 //div[@id=’visaInfo’]/b/text() E T A
3 //div[@id=’wheather’][a[1]/text()][a[2]/text()] E T A
4 every $h in //hotelName/text() satisfies E T B
//hotel[name=$h]/rooms
5 string-length(//country/name)>0 1 R B
6 every $r in //row satisfies count($r/col)>=3 4 R A
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11. Detecting Anomalies
 Different Types of Anomalies
 Type implies Required Action(s)
Anomaly Required Action
Fault Response from DS Correct Request / Change Endpoint
Preprocessing Error Fix Invalid WAQL Query
Unresolvable Dependency Add Data Source
Ambiguous Dependency Refactor WAQL Preparation Query
Circular Dependency Revise Explicit Dependencies
Failed Assertion (Before Q.) Correct Request / Change Endpoint
Failed Assertion (After Query) Correct WAQL Query
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12. Tracking Origins
Dependency Graph
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13. GUI Integration
 4 views: Design, Debug, Result (source), Preview (HTML)
 GUI indicates anomalies and possible origins
 Integrated support for top-down development 13

14. Conclusion
 Mashup development remains complex
• Demand for enhanced testing and debugging techniques
 WS-Aggregation
• Framework for mashup definition and distributed execution
• Query language WAQL
• Integration of non-XML data sources
• Data dependencies, generated inputs
 Debugging aspects
• Asserting expectations
• Detecting anomalies
• Tracking origins
 Benefits of explicit debugging support
• Detects and locates anomalies
• Enables top-down development
• Facilitates maintenance
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15. Discussion
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16. Top-Down Development
 Define assertions first
 Successively add data source requests
 Similarity to Test-Driven-Development (TDD)
# Defined DS Assertions Unresolved
Requests Defined Violated Dependencies
1 - 1,2,3,4 1,2,3 -
2 3 1,2,3,4 2,3,4 -
3 2,3 1,2,3,4 2,3,4 ${//country}
4 1,2,3 1,2,3,4,5 3,4 -
5 1,2,3,4 1,2,3,4,5,6 3 -
6 1,2,3,4,6 1,2,3,4,5,6 3 ${//coords}
7 1,2,3,4,5,6 1,2,3,4,5,6 - -
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17. Scenario Details
# WAQL DS Request DS Response (Ex) WAQL Prep. Query Prepared Result (Ex)
1 <getCountry><city>${city} <country <country>{country/ <country>Austria
</city></getCountry> name=“Austria“/> @name}</country> </country>
2 /getVisaInfo?c=${//country} ..<div id="visa"> <visaInfo>{//div[@id=’visa’ <visaInfo>..</visaInfo>
...</div>.. ]/node()}</visaInfo>
3 <getHotels><city>${city} ..<hotel><name> <hotelNames>{for $h in <hotelNames>
</city></getHotels> Sacher</name> //hotel[position()<6] return <hotelName>Sacher
<stars>5</stars>... <hotelName> </hotelName>
</hotel>.. {$h/name/text()} ...
</hotelName> </hotelNames>
}</hotelNames>
4 /getRooms? {"hotel":{"name":{" jsonToXML(/) <hotel><name>Sacher
h=$(${//hotelName/text()}) $":"Sacher"}, "rooms": </name><rooms>
&d=${date} {"room":[{"beds":{" <room><beds>2</beds>
$":"2"}, "price":{" <price>350</price>
$":"350"}},{..},..]}}} </room>..</rooms>
</hotel>
5 /cityCoords? Innsbruck,47N,11E let $c=csvToXML(/)// <coords><lat>48N</lat>
c=${//country} Salzburg,47N,13E row[col[1]=’${city}’]/col <long>16E</long>
Wien,48N,16E return <coords><lat>{$c[2] </coords>
... /text()}</lat><long>{$c[3]
/text()}</long></coords>
6 <getWheather>${//coords} <wheather date=".." - <wheather date=".."
</getWheather> temperature="23" temperature="23"
humidity="81%"/>.. humidity="81%"/>.. 17

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