1. From UML to ROLAP multidimensional databases using a pivot model
Nicolas PRAT, ESSEC Business School
Jacky AKOKA, CNAM Paris
BDA 2002, INT, Evry, October 2002

2. Overview 1. Introduction 2. Unified multidimensional metamodel
3. Design method
4. Conclusion

3. Introduction Data warehousing and OLAP market growing rapidly
=> need for systematic, tool-supported method for data warehouse/multidimensional database design.
Difficulty of data warehouse design often underestimated by OLAP tool vendors. However, crucial phase.
=>
Data warehouse design should follow the conceptual/logical/physical design phases (as in transactional database design).

4. State of the art
1. Introduction
Many papers proposing multidimensional data models (sometimes with associated algebra/query language).
Only a few data warehouse design methods (Akoka 97, Akoka 01, Golfarelli 98, Cabibbo 98, Moody 00).
Distinction between conceptual/logical/physical:
Often unclear and/or missing phases.
Our contribution:
Data warehouse design method based on UML, spanning the three design phases.
Metamodels for each design step (including unified multidimensional metamodel=>pivot model).
Transformations operating on the concepts of the metamodels.
Specification of the transformations in OCL (Object Constraint Language).

5. Multidimensional metamodel
2. Unified multidimensional metamodel
Problem with the multidimensional metamodel:
No agreement on the concepts of this model (e.g. facts).
No agreement on the level of this model: physical, logical, or conceptual.
We consider the multidimensional metamodel to be at the logical level:
It exists independently of implementation.
Its concepts (e.g. dimension) are not as close to reality as concepts like the object or the entity.
Strong parallel with the relational model.
We have defined a unified multidimensional model.

6. Multidimensional modeling
2. Unified multidimensional metamodel 4 6 9 3 1 12 8 11 5 P1 P2 P3 P4 P5 P6 P7
3 March 99
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Bordeaux
Brest
Lyon
Nantes
Paris
LEGEND
Measure
Quantity sold
MONTH
QUARTER
YEAR
Hierarchy
PRODUCT
DAY
DIMENSION
CITY
REGION
product name
unit
price
Attribute :
CATEGORY

7. Unified multidimensional metamodel
ModelElement
name :
Name
MultidimensionalModel
MultidimensionalModelElement 1
1..* +
ownedElement +
owner 1
attribute
0..* +
+source 1
dimensionLink
0..*
DimensionLink
target
1..*
2..*
dimensionHierarchy {
ordered }
level :
Integer
Dimension
DimensionAttribute
DimensionHierarchy
1..*
0..*
+dimension +
measure
Dimensioning
strong :
Boolean
Measure
dummyMeasure :
Boolean
1..*
0..*
AggregateFunction
name :
FunctionName
restrictionLevel
Integer

8. Overview 3. Design method Universe of discourse CONCEPTUAL DESIGN UML
schema
conceptual
modeling
model
enrichment
/transformation
Enriched /
transformed
UML
schema
model
LOGICAL
DESIGN
Logical mapping
Unified multidimensional schema
Unified multidimensional model
PHYSICAL
DESIGN
Physical mapping
ROLAP
snowflake
schema
ROLAP star
MOLAP
DATA
CONFRON -
TATION
Source confrontation
Data
Warehouse
Metadata

9. Conceptual design Multidimensional representation of data (OLAP).
3. Design method
Multidimensional representation of data (OLAP).
Conceptual phase necessary (vs. direct representation of data in ROLAP stars/snowflakes or MOLAP cubes).
Choice of UML for the conceptual phase:
Standard and well-known formalism
Simple and powerful constructs to represent data at a high level of abstraction
“Easy” mapping to relational and multidimensional systems.
2-step conceptual design:
Definition of a UML model (class diagram without operations)
Enrichment/transformation of this model to facilitate further automatic mapping to a unified multidimensional model.
=>need to enrich UML metamodel.

10. Enriched UML metamodel
3. Design method
UMLModel
UMLModel
ModelElement
ModelElement
name
name : :
Name
Name 1 1 1 1
1..*
1..*
1..*
1..* + +
ownedElement
ownedElement + +
constrainedElement
constrainedElement + +
constraint
constraint
UMLModelElement
UMLModelElement
0..*
0..*
0..*
0..* { {
ordered
ordered } }
0..*
0..*
0..*
0..*
Constraint
Constraint
Attribute
Attribute
AssociationEnd
AssociationEnd
Relationship
Relationship + +
owner
owner + +
attribute
attribute
Class
Class
measure
measure : :
Boolean
Boolean
aggregation
aggregation : :
AggregationKind
AggregationKind
multiplicity
multiplicity : :
Multiplicity
Multiplicity
GeneralizationConstraint
GeneralizationConstraint 1 1 1 1
0..*
0..*
0..*
0..* { {
ordered
ordered } } 1 1 1 1 + +
connection
connection
0..*
0..*
0..*
0..*
2..*
2..* { {
ordered
ordered } }
+participant
+participant
+association
+association
+association
+association
{disjoint,
{disjoint,
complete
complete } } 1 1 1 1
AttributeOfOrdinaryClass
AttributeOfOrdinaryClass
AttributeOfAssociationClass
AttributeOfAssociationClass
Association
Association
Generalization
Generalization
identifyingAttribute
identifyingAttribute : :
Boolean
Boolean
0..*
0..*
0..*
0..* 1 1 1 1
0..*
0..*
0..*
0..*
{disjoint,
{disjoint,
complete
complete } } + +
child
child
{disjoint,
{disjoint,
complete
complete } } + +
generalization
generalization
OrdinaryClass
OrdinaryClass
AssociationClass
AssociationClass
OrdinaryAssociation
OrdinaryAssociation + +
specialization
specialization 1 1 1 1
+parent
+parent

11. Conceptual design (step 1)
3. Design method
Product
Product
_type
_type
Media_type
Media_type
may
may _ _ be be _ _
advertised
advertised
_in
_in
product
product
_type
_type
media_type
media_type
product
product
_unit
_unit
insertion
insertion
1..*
1..*
1..*
1..*
Region
Region * * * *
region
region
Conceptual design (step 1) 1 1 1 1 1 1 1 1
number
number
_of_
_of_
inhabitants
inhabitants
1..*
1..*
1..*
1..*
gets
gets * * * * * * * * * * * *
Media
Media 1 1 1 1
Product
Product
exposure
exposure
media_
media_
name
name
product
product
_code
_code
percentage
percentage
_of_
_of_
region
region
media_
media_
exposure
exposure
advertising
advertising _ _
price
price
product
product _ _
name
name * * * * * * * * * * * *
1..*
1..*
1..*
1..*
1..*
1..*
1..*
1..* 1 1 1 1
main_
main_
Target
Target
shareholder
shareholder * * * *
target
target
_code
_code
consumption
consumption
status
status
for
for * * * * * *
product
product _ _
consumption
consumption
minimum_age
minimum_age 1 1 1 1
maximum_age
maximum_age * * * * * *
sex
sex
Shareholder
Shareholder is is _ _
strongly
strongly _ _
influenced
influenced
_by
_by * *
shareholder
shareholder _ _
name
name * * * * * * * *
Advertising
Advertising _ _
campaign
campaign
Year
Year
campaign
campaign
_code
_code
{overlapping,
complete}
year
year * * * * * * * * 1 1 1 1
Private
Private _ _
shareholder
shareholder
Public_
Public_
shareholder
shareholder
during
during
1..*
1..*
1..*
1..*
public_
public_
shareholder
shareholder _ _
level
level
Quarter
Quarter 1 1 1 1
quarter
quarter
{disjoint,
complete} in in 1 1 1 1
1..*
1..*
1..*
1..*
Person
Person
Company
Company
Date
Date
manager_
manager_
name
name
dd_mm_
dd_mm_ yy yy

12. Conceptual design (step 2)
3. Design method
Enrichment/transformation of the UML model with 4 types of successive transformations:
Determination of identifying attributes
Determination of attributes representing measures
Migration of association attributes
Transformation of generalizations.
Determination of identifying attributes:
Identifier=not a standard concept in UML
Necessary in order to define dimensions in the logical phase
Necessary for ordinary classes only
Use of the tagged value {id}.

13. Conceptual design (step 2)
3. Design method
Determination of attributes representing measures:
Measures vs. qualitative values
Distinction cannot based performed automatically based on types
Not necessary for identifiers (defined previously)
Use of the tagged value {meas}.
Migration of 1-1 and 1-N association attributes:
Check validity of representation first.
Transformation Tcc3 : Each attribute belonging to a 1-1 association is transferred to one of the classes involved in the association.
Transformation Tcc4 : Each attribute belonging to a 1-N association is transferred to the N-class, i.e. the class involved several times in the association.
Transformation of generalizations:
No direct mapping of UML generalizations to multidimensional hierarchies.
UML generalizations transformed into aggregations and classes.

14. Conceptual design (step 2)
3. Design method
Transformation of generalizations (cont’d):
Transformation Tcc5 : For each level i of specialization of a class C, a class named Type-C-i is created. The occurrences of these classes define all the specializations of C. In case of overlapping between specializations, a special value is created for each overlapping between two or more sub-classes of C. In case of incomplete specialization, the special value “others” is created. A N-1 aggregation is created between the classes C and Type-C-i.
Shareholder
_type
shareholder
_type {id} _
name
{id}
public_
level
manager_ 1 *
Private
private
Occurrences of shareholder_type: {private,public,both}
Occurrences of private_shareholder_type: {person,company,others}
Private _
shareholder
Public_
public_
level
Person
Company
manager_
name
{overlapping,
complete}
{disjoint,
Shareholder
_type
_type {id}
_name {id}
Transformation Tcc5

15. Logical design
3. Design method
From enriched/transformed UML model to unified multidimensional model. Mapping of:
Ordinary classes and their attributes (transformations Tcl1 to Tcl3)
Associations and their attributes (transformations Tcl4 to Tcl6).
Mapping ordinary classes and their attributes :
Transformation Tcl1: The identifying attribute of each ordinary class is mapped into a dimension in the multidimensional model.
Transformation Tcl2: The non-identifying attributes of each ordinary class are mapped into dimension attributes in the multidimensional model if these non-identifying attributes are not measures of interest.
Transformation Tcl3: The non-identifying attributes of each ordinary class are mapped into measures in the multidimensional model if these non-identifying attributes are measures of interest.

16. Logical design Specifying transformation Tcl3 with OCL :
3. Design method
Specifying transformation Tcl3 with OCL :
Context UMLModel::Tcl3(nonIdentifier:Attribute,
multidimensionalModel:MultidimensionalModel)
:Measure
pre: nonIdentifier.owner.oclIsTypeOf(OrdinaryClass)
=true
and
nonIdentifier.identifyingAttribute=false
nonIdentifier.measure=true
post:result.name=nonIdentifier.name
post:nonIdentifier.owner.attribute->
forall(a1:Attribute|
if a1.identifyingAttribute=true
then result.dimension=Tcl1(a1) endif)
post:multimensionalModel->includes(result)

17. Logical design
3. Design method
Mapping ordinary classes and their attributes (example):
exposure
media_ {
meas }
Media_type
media_type {id}
insertion
Region
region
{id}
number
_of_
inhabitants
Media
name
advertising _
price 1 *
1..*
gets
Target
target
_code {id}
status
minimum_age
maximum_age
sex
percentage
Year
year
Quarter
quarter {id}
Enriched/transformed UML model
Transformation Tcl1
dimension target_code
dimension quarter
dimension year
dimension region
dimension media_name
dimension media_type
attribute status [target_code]
attribute minimum_age [target_code]
attribute maximum_age [target_code]
attribute sex [target_code]
attribute insertion [media_type]
attribute advertising_price [media_name]
measure percentage_of_region [target_code]
measure number_of_inhabitants [region]
Transformation Tcl2
Transformation Tcl3
Unified multidimensional model

18. Logical design Mapping associations and their attributes :
3. Design method
Mapping associations and their attributes :
Transformation Tcl4: The attributes of each association class are mapped into measures, associated with dimensions obtained by mapping the identifying attributes of the ordinary classes directly or indirectly participating in the association class (transformation Tcl1).
Transformation Tcl5: A path formed by N-1 associations is mapped into a hierarchy in the multidimensional model.
Transformation Tcl6: Every N-M or N-ary association without at least one attribute that is always defined is mapped into a dummy measure, associated with dimensions obtained by mapping the identifying attributes of the ordinary classes directly or indirectly participating in the association (transformation Tcl1).

19. Logical design Mapping associations and their attributes (example):
3. Design method
Mapping associations and their attributes (example):
dimension target_code
dimension quarter
dimension year
dimension region
dimension media_name
dimension media_type
attribute status [target_code]
attribute minimum_age [target_code]
attribute maximum_age [target_code]
attribute sex [target_code]
attribute insertion [media_type]
attribute advertising_price [media_name]
measure percentage_of_region [target_code]
measure number_of_inhabitants [region]
measure media_exposure
[media_name,target_code,quarter]
hierarchy time quarter->year
hierarchy media_type
media_name->media_type
dummy measure gets [region,media_name]
Media_type
Media_type
media_type {id}
media_type {id}
insertion
insertion
Region
Region
region
region
{id}
{id} 1 1 1 1
number
number
_of_
_of_
inhabitants
inhabitants { {
meas
meas } }
1..*
1..*
1..*
gets * * * * * *
Media
Media
exposure
exposure
media_
media_
name
name
{id}
{id}
media_
media_
exposure
exposure { {
meas
meas } }
advertising
advertising _ _
price
price * * * * * * * *
Target
Target
Year
Year
Transformation Tcl4
target
target
_code {id}
_code {id} * *
year
year
{id}
{id}
status
status
minimum_age
minimum_age
Transformation Tcl5 1
maximum_age
maximum_age
1..*
sex
sex
Transformation Tcl6
percentage
percentage
_of_
_of_
region
region { {
meas
meas } }
Quarter
Quarter
quarter {id}
quarter {id}
Unified multidimensional model
Enriched/transformed UML model

20. Physical design
3. Design method
For each type of target system: metamodel + associated transformations (elaborates on/completes OMG’s Common Warehouse Metamodel).
Example transformation (ROLAP star) :
Transformation Tls4: Every hierarchy D1->D2->…->Dn of the logical model is mapped by considering all the sub-hierarchies Dj->Dj+1…->Dn where 1<=j<n and Dj dimensions at least one measure. A sub-hierarchy Dj->Dj+1…->Dn is mapped in the physical model by defining in the dimension table identified by Dj a column corresponding to each of the Di (where j<i<=n).

21. Conclusion Data warehouse design method based on UML:
Spans the conceptual/logical/physical levels
Each step: metamodels + associated transformations
Unified multidimensional metamodel at the logical level (pivot metamodel).
Tool support (prototype developed).
Future works:
Complete/specialise set of transformations
Further experimentation
Reverse engineering.