Software Engineering II
di
Simon Lepore
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Terminology
• Model = A semantically closed abstraction of a system
• Meta-model = A semantic information model
• Meta-metamodel = The language in which to express
meta-models
 Metamodeling = Is an activity, and this activity produces
meta-models
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Scope
• An attempt at describing the world around us for
a particular purpose
• As a schema for data that needs to be exchanged or
that needs to be stored
• As a language that supports a particular methodology or
process(the original purpose of the UML meta-model)
• As a language to make information on the web
more semantic
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
What is Metamodeling
• To express kinds of things in order to model them
in a particular system
• To define our concepts in terms of other concepts
• It expresses various models that it wants to define
(the RELATIONSHIP are instances of E-R diagram)
• Then model dictate the way in which the next level
would be expressed
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What is Metamodeling
Some traditional types of users for meta-models:
1.
CASE tool vendor / modeling tool vendor:
In essence, what the chief architect does is extending the meta-model.The data structures in the tool are
the physical representation of the meta-model.
2.
Repository vendor:
For example, you may sell a repository (or database, or directory, ...) to customers who want to store
software/systems development information in it. They may call it "meta-data".
3.
Systems integrator:
One the first activities in such a project is to understand the meaning of the data in all the
systems that need to be integrated: which data has the same meaning, which data is
complementary, and how everything relates. Performing this analysis yields a semantic
model for the types of data to be integrated, in other words, a meta-model.
4.
End user:
Functionality is one aspect ("this one has a nice shiny button over here"), but the supported meta-model
is even more important. After all, the meta-model determines what you can express with the software
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Metamodeling
Obj.Type
meta-metamodel
level]
…
ProcessType
Attribute Type
EntityType
metamodel
level
…
Product
Employee
model
level
…
Bob
Betty
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data &
process
level
Metamodeling
• A metamodel is a model of models.
•
It contains meta-obj. type.
•
Object types whose istances are also object types.
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Maintaining modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Levels
•
Each level is supposed to define the basis for
instantiating the next
•
The number of the levels adapted depends on
the points of view.
•
OMG organized hierarchies in four levels:
– meta-meta-model level : description of the structure and
semantics of meta-meta-data
– meta-model level : the descriptions (i.e., meta-meta-data) that define the
structure and semantics of meta-data
– model level : meta-data that describes information
– user level : the information that we wish to describe
22/05/2002
Levels
Obj.Type
…
meta-model Process Type
Attribute Type
level
Entity Type
model
level
…
Person
Product
…
data &
process B&OTurnt
SonyCDp
level
22/05/2002
model
level
V45728
V45729
data &
process
level
Levels
• Metamodeling can define many levels of types whose
instances are also types
• Two various needs of approach: salesperson’s
object is the clerk’s object type
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Modeling levels
• For change the kinds of object types that can be defined
at the model level, a metamodel must be modified.
• These rigid structures are implemented in the most
CASEtool : this is a problem
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Modeling levels
• Most CASE tools maintain models and metamodels
as physically separate levels.
Metamodel
defines
Instances of
Model
defines
Instances of
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Data and
process
Modeling levels
• Problem solution :
Model level
Metamodel level
Object type
Object
Metamodel level
Model level
Object type is now an Object and Object is an Object type
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Modeling levels
• This cannot be supported by most CASE tools.
• Secondly, modifying separately maintained metamodels
is often impossible.
• No metamodel customization features from sw vendors
or may consider the contents of its metamodel to be
proprietary.
• Exist implicity as program code(difficult to modify).
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Modeling levels
• Level modeling falters when the bottom level also
instances(such as exsample)and so on
• The ability to instantiate at the model level were
allowed, any number of levels, could be defined within it
• This ability could be supported by defining Object
is an instance of one or more Object types.
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Modeling levels
• Each Object type classifies
any number of Objects.
• Each Object instance is
an instance of one or more
Object type.
Object
Type
classifies
Is an
instance of
Object
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Modeling levels
• In other worlds, by defining the is an instanced of
mapping at the metamodel level, all levels can have
types and instances.
• Only difference between the model and the metamodel
levels is that one has instances of the other.
22/05/2002
Modeling levels
Obj.Type
…
Process Type
Is an
Attribute Type
instances of
Entity Type
…
Is an
Person
instances of Product
…
Is an
B&OTurnt
instances of SonyCDp
V45728
Is an
instances of V45729
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Modeling in one framework
• Modeling in rigid, physically separate levels can create
problems.
• These can remedied by modeling within a single framework instead.
 Model maintenance is performed on one coherent
and integrated model, not fragmented by levels.
 Object type and instances share the same
model.
 Any numbers of instantiates levels is possible.
since a rigid number of levels is not required.
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Modeling in one framework
• A single framework model can be used to decribe it self
• If a model is sufficently descriptive, it should be able to
specify its own object types.
• A small subset of the model can then describe a much
larger subset and so on.
• If a model can be a metamodel to describe itself, it can
be a metamodel to describe other models.
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Kernel Metamodel
• An approach of this kind can both describe different
models and provide a common framework for expressing
and comparing models.
Kerenel
metamodel
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Kernel Metamodel
• The kernel metamoddel with which the model describes
itdelf would contain such primitive obj types as Object
Type, Object, Relation and Function.
• Also contain Operation, Trigger Rule, ControlCondition… for support processing requirements.
• These primitives, can be used to form foundational
construct for other structures.
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
In last
• In a single framework model, the boundaries are nonexistent.
• The only reason for inhibiting changes to the kernel metalevel is to maintain standardization.
• The standards for the future are most important for the
interoperability
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
MOF(Meta Object Facility)
• It is a Standard of Object Management Group(OMG).
• In September of 1997, finalized a standard.
• MOF is the M3 levels (meta-meta-model)
• Is Object Oriented.
M3
M2
M1
M0
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Meta-meta-model
Meta-model
Model
Object
Four levels(OMG)
• M0 it represents the data busines level
(client n° 1234.)used from DBMS, SO.
• M1(model) describes the structure of M0 level
(identifier client=1234)1234 is an instances of M1 level.
• M2(metamodel) describes the structures of M1 level
(atribute=identifier client) identifier client is an
instances of M2.
• M3(meta-metamodel)its a model to decribe another
model and the structures of m2 level(i.e.meta-atr = atribute).
22/05/2002
OMG levels
Specify meta-meta classes
for metamodel.
Specify meta classes
for metamodel.
i.e. Class
UML
Package
notation
Specify classes for UML
model of designer.
M3
M2
M1
<<instanceOf>>
Object
Secify a pure Object
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M0
OMG levels
MOF Meta-metamodel
Meta-Attributes
Meta-Classes
<<instanceOf>>
MetaOperation
<<instanceOf>>
<<use>>
UML metamodel
Atributes
Classes
<<instanceOf>>
Analisys model
UML
Object
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Operation
<<instanceOf>>
<<use>>
<<instanceOf>>
<<use>>
Girl
Name : String
Age : Integer
GetName(): String
GetAge(): Integer
Maria : Girl
Goal of MOF
•
M3 describe itself (MOF Model is formally defined using its own metamodeling constructs).
•
UML is an instancies of MOF.
•
Defines a set of CORBA IDL interfaces that can be used to define and
manipulate metamodels and their models
– provides the infrastructure for implementing CORBA-based design
and reuse repositories
– specifies precise mapping rules to automatically generate CORBA
interfaces for metamodels,
thus encouraging consistency in manipulating metadata in all phases
of the distributed application development cycle
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Goal of MOF
•
•
•
•
•
•
Interoperability, interchange of meta data in
distribuited systems, distribuited object systems.
Beyond to the repository MOF-based we can use(for interchange meta-data)the format stream-based(standard XMI).
XMI defines the rules for mapping from MOF to XML to generate automatic XML
code.
Essentially a subset of the UML core:
Class
– attributes
– operations (only name and functionality, no associated method)
Generalization
– sub-Class inherits all features of its super-Classes
– any explicit Constraints that apply to a super-Class and
– any implicit behavior for the super-Class apply equally to the sub-Class
– At the M1 level, an instance of an M2-level Class is type substitutable for instances
of its M2-level super-Classes
–
–
Abstract classes
Root and leaf classes
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Goal of MOF
•
•
•
•
•
Association
– Association links
– Multiplicity specification
– Navigability setting
Aggregation
asymmetrical association
Component is part of at most one composite
– A component cannot be part of itself (also indirectly)
– When a composite is deleted, all its components are deleted
References
(attributes corresponding to navigable associations ends to offer a simple way to update links)
Datatypes
Meta-model definitions often need to use attribute and operation parameter values that have
“ordinary” types
Packages
–
–
•
Groups elements into a meta-model
Mechanisms to compose packages
Constraints
–
–
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Used to attach consistency rules to other meta-model components
A Constraint comprises: constraint name, a language used to express the consistency
rules
Description of Mof
•
•
Can be described by using the MOF model itself
–
circularity problem
–
Direct “description” by natural language and UML diagrams
Abstract and Concrete Mappings
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Description of Mof
• The Abstract mapping (defined in“The MOF Abstract Mapping” chapter)
fleshes out a MOF meta-model into an abstract information model (i.e., by spelling
out the logical structure of the meta-data described by the meta-model).
• The IDL Mapping (Section 2.4.2, “CORBA Meta-data Services - The MOF
IDL Mapping,” on page 2-22) produces the standard OMG IDL and associated
behavioral semantics for meta-objects that can represent meta-data conforming to
the meta-model.
• The XML Mapping (see Section 2.4.3, “Meta-data Interchange - The MOF
XML Mapping,” on page 2-22) produces the standard XML DTD for interchanging
metadata conforming to the meta-model.
22/05/2002
Index
•
•
•
•
Index
Terminology
Scope
What is Metamodeling
- Metamodeling
- Level architectures
- Modeling levels
- Modeling in one framework
- Kernel metamodel
- In last
• Mof
• Case Study
22/05/2002
Case study
•
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
Conclusions and future work
22/05/2002
Introduction
 Web-based hypermedia systems are becoming more and
more sophisticated, new modeling requirements constantly
arise, and design models must constantly evolve.(an example of this
evolutions is HDM Hyper text design model)
 Since design tools should complement models to support an
efficient design process, model evolution raises technological
issue: Design tools must be modified when their underlying
model changes. This is why the paper proposes a general
approach to efficiently update design tools in response
to model evolutions.
22/05/2002
Introduction
The key ideas are:
 the description of a hypermedia model
in terms of a general meta-model, powerful enough to
express the semantics of current and future design constructs.
 the transformation of a hypermedia design tool into a meta-CASE tool, able to
cope with model updates without requiring to be redefined and rebuil from
scratch.
22/05/2002
Introduction
 The approach is presented by discussing a case study, that is,
the feasibility study to transform the design toolkit, Jweb, into
a meta-CASE tool (Jweb3).
 This tool will support the latest version of our model (called
W2000), and will easily evolve with the model it supports.
 In this document it comes discussed the adoption of the OMG
metamodeling standards MOF and XMI as enabling technology
a sample of the representation of W2000 in terms of MOF, and
the sketch of the architecture of the under-implementation
Jweb3.
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Case study
•
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
Conclusions and future work
22/05/2002
Feasibility Study
• As to the first aspect, they decided to probe the possibilities of
using existing UML CASE tools as temporary front-ends for
the forthcoming schema editor.
The idea was not to select “the best” available tool, but simply
do some experiments and delay the implementation of a special
purpose editor to better refine W2000 constructs. As side effect,
this implied that they did not want to work with proprietary
formats to store models(W2000 schemas), but they needed a
cross -platform standard to be able to change the front-end
according to our needs.
• They decided to exploit the OMG novel
way to meta-modeling to find a cumulative solution.
22/05/2002
Feasibility Study
• Using the OMG jargon, they can say that each model (i.e.artifact)
must be compliant to a given meta-model (i.e., notation).
• This definition allows their to decompose a CASE tool into a
provider of modeling features, which depends on the supported
meta-model, and a consistency checker between defined models
and their meta-model. Meta-CASE tools do a similar job at a
meta-meta-level. Meta-CASE tools let the developer define his
own meta-model and express the semantics of the primitives of
this meta-model in terms of a meta-metamodel.
22/05/2002
Feasibility Study
• Describing a new modeling feature of a meta-model (e.g., a new
primitive for W2000) with a meta CASE tool would require the
designer to describe the semantics of the new construct in terms
of the meta-meta-model.
• The key point is that a meta-metamodel
(MOF) supplies all modeling features to render extensions and
mechanisms to (automatically) update the tools with respect to
consistency checking and manipulation operations.
22/05/2002
Case study
•
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
Conclusions and future work
22/05/2002
W2000 as a Profilable
MOF Meta-model
• W2000 can be seen as a UML extension (profile, according to
the UML terminology).UML, in fact, supplies ad-hoc features
(stereotypes, tagged values, and constraints) to extend the
Original UML to cope with particular needs and specific
application domains.
• Defining W2000 as a UML profile allows us to exploit
UML commercial tools, but also XMI-UML, that is, the UML
instantiation of XMI , which is rapidly becoming the ”lingua
franca” to exchange UML models in a vendor-independent way.
22/05/2002
W2000 as a Profilable
MOF Meta-model
• The adoption of UML profiles is enough to use UML CASE
tools as front-ends for the schema editor, but does not allow
semantic checks. Currently, available CASE tools support
profiles partially(only Objecteering provides some support)
• Evolution and consistency checks would not be possible. This is
why they decided to adopt MOF to describe the W2000 metamodel, that is,to give a precise definition of W2000 concepts in
terms of a flexible meta-metamodel.
22/05/2002
W2000 as a Profilable
MOF Meta-model
• The W2000 meta-model reuses many UML concepts, and this
means that we can see W2000 as a UML profile and all possible
versions of W2000 constructs can always be mapped onto
instances of UML primitives, but with respect to conventional
UML profiles it satisfies also an additional condition
• All concepts (both new and borrowed ones) are defined using
MOF. Thus, they can say that W2000 is a profilable MOF meta
-model (hereafter, p-meta-model). The reason for introducing
the concept of p-meta-model is that if the UML metamodel can
be described in terms of MOF, the opposite does not hold true,
not all meta-models described in MOF can be mapped onto
the UML meta-model.
22/05/2002
Case study
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
– Remarks
• A p-meta-model for W2000
• An example W2000 model
• Conclusions and future work
22/05/2002
Technological solution
• The gray box in the upper left corner identifies meta-modeling technology;
all other tools belong to the usual modeling chain, which must be enabled by
a set of preliminary meta-modeling steps. They start by defining the W2000
p-meta-model:
• Usually they do this using Rational Rose integrated with the Unisys add-on
for XMI. This model, stored in XMI-MOF, is the main input to the two metamodeling tools.
22/05/2002
Technological solution
• The translator generator : produces a translator from XMI-UML to XMIW2000 as a set of XSLT rules.
• The checker generator uses the same meta-model to produce a set of
Schematron rules.
• The designer specifies his W2000 models using his preferred UML editor
and stores them in XMI-UML. The translator, through the XSLT rules,
transform the XML file compliant to XMI-UML to another XML file, but
compliant to XMI-W2000. This file is then the input to the checker that
validates it through the Schematron rules.
22/05/2002
Remarks
• Even if p-meta-models have been defined to solve a specific problem,
that is, to define the meta-model of W2000, they are absolutely general
and can be used in all those cases where either we need (want) a MOF
definition of an extended UML notation or simply we want to integrate
MOF-based and UML-based tools.
• They implemented all logical components of Figure through specialpurpose scripts and XSLT sheets, all coordinated by HTML interfaces.
• This implementation style should better fit evolution and simplify
maintenance.
22/05/2002
Case study
•
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
Conclusions and future work
22/05/2002
A p-meta-model for W2000
• This section presents an example application of the p-meta-model of
W2000 . The p-meta-model for W2000 (Figure 2) comprises the three
standard UML packages, and the new W2000 package.
Foundation package supplies all basic
modeling elements(classes,attributes,
associations, etc.)
BehavioralElements package supplies the
elements to specify the dynamic behavior
of a model. ModelManagement package supplies the means to cope with
complexity and organize a model into submodels according to the different
viewpoints. The new package, W2000, defines how we extended UML to
represent W2000 constructs.
22/05/2002
A p-meta-model for W2000
• The profilability of the p-meta-model implies that all new W2000 concepts
be specified as extensions to standard UML elements, that is, W2000
elements are subclasses of standard UML meta-model classes.
This organization is partially shown in Figure,which collects some excerpts
from the W2000 package;
• Information Model Design(fig. a): classes Entity and Component are both
subclasses of class Class (defined in the Core subpackage of the UML
Foundation package). Both classes are abstract classes because Entity are
always specialized in either EntityTypes or SpearEntities, while
Components become ComponentTypes or SpearComponents. Types are
similar to classes: They will be instantiated as many times as needed in the running
application; spear elements correspond to singletons: They specify special-purpose
elements singularly instantiated in the application.
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A p-meta-model for W2000
• Access Layer Design(fig. b): has a similar organization. A Collection,
abstract concept, comes from a Class and is specialized in SpearCollections
and CollectionTypes. Moreover, each collection has a
CollectionCenterType,which is a subclass of Center-Type.
• Navigation Design(fig. c):
Once more, Nodes must be either NodeTypes or SpearNodes. Links must be
either CollectionLinks or SemanticLinks, or StructuralLinks.
Collection links relate the elements of a collection with their center, semantic links
correspond to semantic associations, and structural links render the component-based
decomposition of complex entities.
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A p-meta-model for W2000
22/05/2002
Case study
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
– Impact on Jweb
• Conclusions and future work
22/05/2002
An example W2000 model
• Figure presents the information model for the paper entity type. The entity
is structured in three main components: The abstract is always part of the
paper and it presents some basic information, like the paper title, authors,
and affiliation, along with other information required by the conference, that
is, paper id (number) and review status.
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An xample W2000 model
• Either we have a first submission, or after accepting the paper, we have
its camera-ready version.
These components reuse also some information already defined in the abstract.
Each time an attribute (slot) is reused in a component, we simply refer to its original
definition, instead of re-specifying it.
This way, we avoid inconsistent redefinitions and we maximize reuse.
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An xample W2000 model
• When we move to navigation design, Figure 6(a), we see that the paper has
been decomposed in three nodes. Once more, the main one contains all base
information, while the other two nodes correspond to the first submission
and camera-ready respectively. The actual contents of these nodes is
specified using the keyword body, which make readers refer to the
information design to understand the actual contents.
a)
b)
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An xample W2000 model
• For example, the contents of the AbstractNode node is defined by
the
body of the Abstract component. The correspondence between nodes
and components is not mandatory, but it helps modularize the design.
• Users are free to specify the contents of nodes as lists of attributes taken
from the entities and components that they embody.
Figure 6(b) shows how the user of the Web application can navigate among
papers and authors.
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Impact on Jweb
• A briefly introduce on toolset for clarify its main components.
Jweb assists designers during the whole design process: from information
design to prototype and enactment. Figure shows its main logical
components:
•
The editor lets designers design their applications using HDM/W2000 and
produces a HDM/W2000 schema.
• The mapper takes this schema and automatically generates the relational
schema for the editorial repository, suitable interfaces for populating the repository,
and a general description (XML mapping) of the relational schema.
22/05/2002
Impact on Jweb
• The configurator reads the HDM
schema, the description of the editorial repository, and the editorial contents and
allows for the creation of special-purpose filters to select data; the description of
these filters is rendered in XML.
• The generator uses the HDM/W2000 schema, the description of the editorial
repository and its contents, together with the filters defined to select data, and
generates the run-time database, populating it with the editorial contents suitably
filtered.
• The engine reads the contents from the run-time database, presents this contents
to user and is responsible formanaging the interaction between the user and the
application.
22/05/2002
Case study
•
•
•
•
•
•
•
Introduction
Feasibility Study
W2000 as a profilable MOF Meta-model
Technological solution
A p-meta-model for W2000
An example W2000 model
Conclusions and future work
22/05/2002
Conclusions and Future Work
• The group experiments in using the meta-modeling technology to support
design tools are so far limited to the editor, but starting from the p-metamodel they induced a non-trivial modification and they studied the impact of
these modifications.
• The main bottleneck was the definition and test of all (new) constraints in
Schematron, but all other changes were almost trivial and they could be
obtained automatically.
22/05/2002
Conclusions and Future Work
This exercise suggested two interesting considerations:
– The availability of a real MOF repository together with a friendlier notation
for specifying constraints would further shorten the maintenance process.
– If we had done the same exercise using our standard way, we would have
used the following process:
1. The new meta-model would have been coded directly using a DTD, with
suitable comments to explain the meaning of the new/changed features;
2. The implementer in charge of working on the new editor (component)
would have taken this definition as the reference for his work.
22/05/2002
Software Engineering II
The End
22/05/2002
References
•
www.metamodel.com
•
www.omg.org
•
www.jamesodell.com
•
saturne.info.uqam.ca/Labo_Recherche/Larc/MetamodelingWorkshop
/Odell/metamodeling/metamodeling.ps
[email protected]
– Site for to learn much about model metamodel end modeling
– There’s many definition about models
– Object Managment Group, leader of O-Otechniques
– Unified Modeling Language Specification version 1.4, Beta 1, November 2000.
– Meta Object Facility Specification version 1.3 New Edition: March 2000.
– Is a consultant, writer, and educator in the areas of object-oriented and agent-based
systems, business reengineering, and complex adaptive systems.
– Meta-modelling 1995
– Meta-modeling Techniques Meet Web Application Design Tools, L. Baresi, F.
Garzotto, L. Mainetti, and P. Paolini Dipartimento di Elettronica e Informazione
Politecnico di Milano www.elet.polimi.it
22/05/2002
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