Wrappers in Mediator-Based
Systems
Chapter 21.3 Information Integration
Presented By Annie Hii Toderici
Outline
Templates for Query Patterns
 Wrapper Generators
 Filters
 Other Operations at the Wrapper

Extractors in Data Warehouse

Extractors (wrapper) consist of:
◦ Predefined queries that are executed at the
source to produce data for the warehouse
◦ Suitable communication mechanisms
 Pass specific queries to the source
 Receive responses from the source
 Pass information to the warehouse
user query
result
Warehouse
Combiner
Extractor
Extractor
Source 1
Source 2
Wrappers in Mediator Systems
Mediator systems require more complex
wrappers than warehouse systems
 Must be able to accept a variety of
queries from the mediator and translate
any of them to the terms of the source

query
result
Mediator
query
result
Wrapper
query
result
Source 1
query
result
Wrapper
query
result
Source 2
Wrapper
Wrapper connects a mediator to a
source
 Wrapper (aka “adapter”) translates
incoming queries and outgoing answers
 Wrappers are defined by templates
 The wrapper generator creates tables of
all possible templates
 The driver and tables constitute the
wrapper

Templates for Query Patterns

Design a wrapper
◦ Classify the possible queries that the
mediator can ask into templates
Templates are queries with parameters
that represent constants (e.g., red)
 Uses the notation T=>S

◦ Template T is turned by the wrapper into the
source query S
Templates
Suppose we want to build a wrapper for
the source of Dealer 1
 Dealer 1

◦ Cars(serialNo, model, color, autoTrans, navi,
…)

Mediator
◦ AutosMed(serialNo, model, color, autoTrans,
dealer)
Template: cars of a given color ($c)
SELECT *
FROM AutosMed
WHERE color = ‘$c’;
=>
 SELECT serialNo, model, color, autoTrans,
‘dealer1’
FROM Cars
WHERE color = ‘$c’;

Templates

Other templates would be needed to deal
with other queries
◦ Type of models: parameter $m
◦ Type of transmission: parameter $t
◦ Seat type: parameter $s
N attributes, 2n possible templates
 Number of templates could grow
unreasonably large, it is possible to
simplify

Wrapper Generators
A collection of templates is given to the
wrapper generator (WG)
 WG creates a table that holds the various
query patterns contained in the
templates, and the source queries that are
associated with each

Templates
Wrapper
generator
Queries from
mediator
Results
Table
Driver
Queries
Result
Source
Task of the driver
Accept a query from the mediator
 Search the table for a template that
matches the query
 The source query is sent to the source
 The response is processed by the
wrapper, and then returned to the
mediator

Filters
Not always realistic to write a template
for every possible form of query
 To have the wrapper filter the results of
queries that it poses to the source
 The wrapper has a template that returns
a superset of what the query wants, then
filter the returned tuples at the wrapper
and pass only the desired tuples to the
mediator

Example Filter: blue Civic
SELECT *
FROM AutosMed
WHERE color = ‘$c’;
=>
SELECT serialNo,
model, color,
autoTrans, ‘dealer1’
FROM cars
WHERE color = ‘$c’;
Set $c = ‘blue’
TempAutos(serialNo,
model, color,
autoTrans, dealer)
SELECT *
FROM TempAutos
WHERE model =
‘Civic’;
Other Operations at the Wrapper

To transform data in other ways
◦ Columns may be projected out of the tuples
before transmission to the mediator
◦ To take aggregations or joins at the wrapper
and transmit the result to the mediator
Mediator: find blue Civic and retrieve
serial #, transmission type and dealers
SELECT *
FROM AutosMed
WHERE color = ‘$c’;
=>
SELECT serialNo, model,
color, autoTrans,
‘dealer1’
FROM cars
WHERE color = ‘$c’;
Set $c = ‘blue’
TempAutos(serialNo,
model, color,
autoTrans, dealer)
SELECT serialNo,
autoTrans, dealer
FROM TempAutos
WHERE model = ‘Civic’;
Examples


Mediator: find dealers s.t there are two red
cars of the same model, one manual and one
automatic
Query from mediator to wrapper:
SELECT A1.model A1.dealer
FROM AutosMed A1, AutosMed A2
WHERE A1.model = A2.model AND
A1.color = ‘red’ AND
A2.color = ‘red’ AND
A1.autoTrans = ‘no’ AND
A2.autoTrans = ‘yes’;
Example (Continued)

RedAutos(serialNo,
model, color,
autoTrans, dealer)

SELECT *
FROM AutosMed
WHERE color =
‘red’;

SELECT DISTINCT
A1.model, A1.dealer
FROM RedAutos A1,
RedAutos A2
WHERE A1.model =
A2.model AND
A1.autoTrans = ‘no’
AND A2.autoTrans =
‘yes’;
Thank You!

The End!