According to the book I am reading, in the three layer architecture (presentation, application ,storage), storage layer is responsible for looking after the persistence storage of data in the system. If so, I think entity classes are in the application layer.Is that correct? What classes will be in the storage layer? What is the difference between entity classes and the classes of storage layer? Can you please give me some examples?
There is not one truth here. People have different opinions and have their own definitions.
You have tagged your question with the UML tag, but UML does not define such things like "storage layer" or "entity class".
You have tagged your question with the "ecb-pattern" tag, but this pattern defines logical layers and does not prescribe a certain physical layering and vice versa, a physical layering does not prescribe a logical layering. The three-tier architecture you mention is probably a physical layering. Each physical layer may or may not use the ecb-pattern. This is a decision made by the software architect.
Most authors define the storage layer as being implemented by a database management system (DBMS) usually relational (RDBMS), where object-oriented notions like classes are not used. In that case, if the ecb-pattern is used, entity classes are part of the application layer and/or the presentation layer.
Example 1: The entity class Person could be a class in the application layer, which persists itself by sending a SQL message to the RDBMS (= storage layer). Class Person typically also implements business rules like "date of birth should be in the past".
Example 2: The entity class Person could be a class in the presentation layer, which persists itself by sending a message to the application layer, where the ecb-pattern is not used (suppose).
Entities are the low end database "parts". They represent the bits and pieces you put in the data storage for persistence. UML uses a stereotype which resembles a tape/head (if still someone remembers what that is):
The stereotypes for a boundary resemble a screen (or you can think of that being some projection). A boundary presents some information to an actor.
The control stereotype resembles a turning wheel (where you think of work being done) and it hides the business logic which react no boundary events and changes/reads entities.
You can also use rectangle notation:
Edit As #www.admiraalit.nl notes, this notation is not part of Superstructures itself (so not basic UML) but a common used notation in various UML profiles.
Related
I have a logical data model and multiple physical data models for different systems, all documented in UML. I want to show how the data in the physical models trace back to the logical model so that when changes are made to the logical model we can readily determine all of the physical models that are impacted.
Are these relationships best represented as a trace or a realization? I suspect the answer is "trace", but wanted to see what others thought prior to investing too much time in this and being wrong. Does a trace or realization relationship offer extra semantics over the other that would be useful in this context?
My interpretation so far (intentionally quoted): "I usually make them <<trace>> dependencies. A Realization is between an interface or an abstract class and a 'regular' class. The physical model is derived from the logical model, not 1:1 realized. In most cases you make modifications on a physical level which are not foreseen in a logical model (de-normalization and all sorts of optimization you need on a DB level). Somehow this is sort of a realization."
However, and this is the pure truth, this is what the UML spec says on p. 54:
7.8.14 Realization [Class]
Realization is a specialized Abstraction relationship between two sets of model Elements, one representing a specification (the supplier) and the other represents an implementation of the latter (the client). Realization can be used to model stepwise refinement, optimizations, transformations, templates, model synthesis, framework composition, etc.
So you can actually use a realization here. The <<trace>> is more informal, but not necessarily wrong.
These relationships are best represented as a trace, i.e. a dependency relationship with stereotype <<trace>>. The trace is meant to relate elements in different models, which is exactly your situation.
The UML 2.5.1 specification defines the trace stereotype as part of the 'standard profile' (section 22.3), as follows:
Specifies a trace relationship between model elements or sets of model
elements that represent the same concept in different models. Traces are
mainly used for tracking requirements and changes across models. As
model changes can occur in both directions, the directionality of the
dependency can often be ignored. The mapping specifies the relationship
between the two, but it is rarely computable and is usually informal.
Although the UML specification defines the realization relationship broadly enough to fit your purpose as well, the examples in the UML specification (e.g. figure 7.21) and the way I have seen this relationship been used in real world projects lead me to the conclusion that this type of relationship is primarily intended to be used to relate elements within the same model, in particular interface elements with classes that implement these interfaces.
I am trying to understand how to classify the classes as boundary/control/entity classes. I can understand boundary and entity classes although my understanding may not beperfect. Boundary is the classes which interacts with the user. So the classes used for the userinterface will be boundary classes. Entity class handles data. So entities I use in the ER diagram will be entity classes. But I understand nothing why control object is used. It is said that control object is used to encapsulate domain functionalities. What if the control classes are not used. Can you please explain me with example.I found some explaination but I am still confused.Why boundary should not interact directly with entity? There are also classes which are not boundary/control/entity. What are they?
Background
The Entity/Boundary/Control approach was introduced by Ivar Jacobson in 1992 as part of his use case driven Objectory development method.
At that time Jacobson used the terminology Entity/Interface/Control. The strange circle notation that you can find in relaction with ECB was already used in his books in 1992 and in 1994. By the way, the use case of his methods were integrated into UML and his development process was merged into RUP, when Rational acquired Objectory.
The idea behind his method was to adopt a very logic and formal and deductive analysis and design approach. It starts with identifying the systems behavioral requirement with use cases. Each link of use case to the outside world would then be represented as an interface object responsible for encapsulating completely the user interface.
Each use case would be represented as one or several control objects:
Control object: An object that encapsulates functionality of one or
several use cases - I.Jacobson in The Object Advantage, ACM Press, 1994
Finally the business objects managed by the system can be partly inferred from the use cases, and during the analysis.
Additional information
The fundaments of the Iconix process were introduced in 1999 as part of the book "Use Case Driven Object Modeling with UML" by Rosenberg & Stephen. Some additional robustness constraints were introduced, certainly to improve separation of concerns. For example, the direct link between entity and boundary is prohibited. Everything has to be channelled through control objects:
Control objects (which we usually call controllers because they often
aren't real objects), serve as the "glue" between boundary objects and
entity objects - D.Rosenberg, in the linked DDJ article.
They add a recommendation to clarify the intent :
Both boundary objects and entity objects are nouns, and controllers
are verbs.
Conclusion
So the principle is that the control object represents the business logic offered by use cases, interacting on one side with the boundaries, and on the other side with the entities. Control objects can't be invoked/accessed directly by the outside world.
If you would want to avoid the control objects, you would have a boundary objects with methods corresponding to the verbs/functions/use-cases that your system is supposed to provide. This wouldn't be according to the modern ECB, but perfectly valid according to Jacobson's original approach. Nevertheless your boundary would no longer comply with the single responsibility principle of a SOLID design.
Boundary interact with actors (e.g. users).
Entity classes represent data.
Control mediates between the boundary and the entities (e.g. executes an operation on the entities)
Source: http://www.cs.sjsu.edu/~pearce/modules/patterns/enterprise/ecb/ecb.htm
The control classes contain the business logic. It's the most important part of a system. While the boundary just controls whether the text is green or blue (very basically) and the entities control whether data is stored in text files or databases (also very basically) the control classes do all the business logic. What to change in the entity when the boundary send mouse/keyboard events an vice verse what to show from the entities in the boundary.
If I create a conceptual class diagram such that each class captures 'name' and 'attributes' but not 'operations', have I not basically created what would be otherwise considered an ERD? I'm trying to gain an understanding of what the differences are between creating a conceptual class diagram as I have described versus calling it a ERD? If these are still two different animals, can somebody please explain what the differences are?
The class diagram contains just the classes in your object model with eventual links/relationships connecting diagram elements. However those links don't necessarily correspond to physical relationships like in an ERD diagram, but instead they represent logical connections.
The class diagram is just the object model of your application and does not contain any persistence-specific information. When you think about the class diagram forget about the database or any other storage you may use.
The ERD diagram on the other side, is a persistence-specific diagram which display the entities (tables) existing in a (most often) relational database. It also displays the physical relations (and cardinalities) between those tables and all other database-specific information. The ERD diagram can sometimes look similar to the class diagram, but that doesn't mean is the same as a class diagram.
There´s little difference in the expressiveness of both (if we just focus on the attributes, classes and associations part) if you use Extended Entity Relationship diagrams (the most common case nowadays)
True, they look very different at the graphical level since they use different symbols for the elements but the "semantics" are quite similar. They both allow inheritance (again, I´m talking about EER), n-ary associations, association classes, ...
The ER diagrams I've seen (most frequently ERWin IE notation) have focused on the design for a database. They are concerned with primary keys, foreign keys, have unnamed relationships, and usually have no generalization / specialization.
A good UML conceptual class diagram, on the other hand, is not concerned with keys, reflects the problem domain, and has association-end properties that at least hint at the semantics of why things are related. This helps communicate the domain down to more junior developers so they don't have to guess.
It depends on the situation where you may not like to do the ER-D. But imagine if you have a seperate data layer where the data logic is handled. In this case many details of data shall not be shared with the application layer. And you class diagram shall not go beyond the application layer. I must stress that both the diagrams are not equal. And there are situations where you need to do both, mainly in multi-tier architecture, and there are situations where you may be able to just use class diagram; e.g. single-tier application.
I strongly advocate the view that class diagram doesn't abrogate the E-R diagram.
Design class diagrams are made from conceptual model and collaboration diagrams.
Design class diagrams include:
Classes, associations and attributes
Methods
Types of attributes
Navigability
Dependencies
IMO In Simple terms
Class diagram depicts the details of how will the system work.
ER diagram depicts how the system persists 'state' as a blue print.
Goal:
Detail out state and behavior of the components(classes) of the system.
Design 'efficient', flexile system(less coupling and more cohesion) using Solid principles.
Goal:
Design a blue print of how to 'efficiently' persist the state of the system.
Consider what kind of queries will be made (read vs write), are there any joins required
consequently figure out the columns for indexing
Use Normalization, ACID properties.
PS: notice the both the diagram tries to efficiently do thing in their on respect.
What is the difference between a domain model and a data model?
A datamodel is a design model that only describes data and it's relations. The model contains entities, but they are described in terms of what data they own not how they act on this data or what their responsibilities are.
An domain model on the other hand, is a conceptual model used in analysis of a problem domain. It describes the domain in terms of entities that have relations, data and behaviour. It describes the responsibilities of those entities as relevant for understanding the problem domain.
BTW an excelent and very short introduction to UML is:
UML Distilled: A Brief Guide to the Standard Object Modeling Language
A data model is focused on the DB schema definition, including tables, columns, and relationships.
A domain model is focused on the business domain, including concepts (classes of objects), behavior (methods/logic), and relationships.
In both cases, the cardinality is used for relationships (e.g. 1:1, 1:Many, 0:Many, ...).
That said, you would ideally like the data model and domain model to be closely related, i.e. a Person with name, ... and a MailingAddress, ... relates to a PERSON table with a NAME column and a FK to a MAILING_ADDR table entry. You have to decide where logic is hosted - in the objects in the software system vs. in the DB via procedures, triggers, and such.
I think it's important to provide some clarity here for posterity.
A data model is a design for how to structure and represent information. By structure, I mean concerns like "fifth normal form". By representation, I mean choosing a computer serialization, such as integer, floating point, or string.
The term domain model actually has two conflated meanings.
A model of essential characteristics of real or imaginary things in the world. In this kind of model, classes represent human conceptualizations and instances are things in the world. For example, a "Person" class would have instances including you and me, and an essential characteristic might be that every Person has a mother. This kind of model is often called an conceptual ontology or concept model and is intended to provide meaning.
A model of required information about things in the world, usually with some system in mind. In this kind of model, classes represent information that must be stored about things in the world. For example, a "Person" class would have instances representing required information about you and me, such as first name, last name, date of birth, current height, and current weight. This information often does not include all essential characteristics, such as our mothers, because, for the purposes of a particular system, that information is not required. This kind of model is often called an information model, conceptual data model, or operational ontology.
Both the UML and OWL languages can be used to represent either kind of domain model. Both can be considered analysis models, as they are used to analyze a domain. One is used to understand things in a domain, the other is used to gather requirements to build a particular software or database system for things in a domain. Both are necessary, and, unfortunately, they are usually conflated such that people building an analysis model are themselves confused about what they are modeling!
I think that domain model and data model are now pretty much the same with new top down modelling technologies. I mean that you can model in a class diagram and only add database stereotypes in your diagram. If you use the tool that I use then your ejb3 annotation would be immediately synchronized with your code. The next step is only to use a mapper to create your database. This technology only works with Java
I'm now using NetBeans as my IDE-of-choice, and it has a plugin for UML modeling. In the class diagram, there are model elements known as Boundary Class, Control Class, and Entity Class. However, I can't find a good definition of them, but I did find this site on UML Class Diagrams.
Robustness diagrams are written after use cases and before class diagrams. They help to identify the roles of use case steps. You can use them to ensure your use cases are sufficiently robust to represent usage requirements for the system you're building.
They involve:
Actors
Use Cases
Entities
Boundaries
Controls
Whereas the Model-View-Controller pattern is used for user interfaces, the Entity-Control-Boundary Pattern (ECB) is used for systems. The following aspects of ECB can be likened to an abstract version of MVC, if that's helpful:
Entities (model)
Objects representing system data, often from the domain model.
Boundaries (view/service collaborator)
Objects that interface with system actors (e.g. a user or external service). Windows, screens and menus are examples of boundaries that interface with users.
Controls (controller)
Objects that mediate between boundaries and entities. These serve as the glue between boundary elements and entity elements, implementing the logic required to manage the various elements and their interactions. It is important to understand that you may decide to implement controllers within your design as something other than objects – many controllers are simple enough to be implemented as a method of an entity or boundary class for example.
Four rules apply to their communication:
Actors can only talk to boundary objects.
Boundary objects can only talk to controllers and actors.
Entity objects can only talk to controllers.
Controllers can talk to boundary objects and entity objects, and to other controllers, but not to actors
Communication allowed:
Entity Boundary Control
Entity X X
Boundary X
Control X X X
Often used with/as a part of OOAD and business modeling. The definition by Neil is correct, but it is basically identical to MVC, but just abstracted for the business. The "Good summary" is well done so I will not copy it here as it is not my work, more detailed but inline with Neil's bullet points.
Good summary - Conceito: Entity-Control-Boundary Pattern
OOAD
These are class stereotypes used in analysis.
boundary classes are ones at the boundary of the system - the classes that you or other systems interact with
entity classes classes are your typical business entities like "person" and "bank account"
control classes implement some business logic or other
Actually, the Robustness Diagrams (or Analysis Diagrams, as they are sometimes called) are just specialized Class Diagrams. They are a part of UML, and have been from the beginning (see Jacobson's book, The Unified Software Development Process - part of the "Three Amigos" series of books). The aforementioned book has a good definition of these three classes on pp 183-185.
Boundary Control Entity pattern have two versions:
- old structural, described at 127 (entity as an data model elements, control as an functions, boundary as an application interface)
- new object pattern
As an object pattern:
- Boundary is an interface for "other world"
- Control in an any internal logic (like a service in DDD pattern)
- Entity is an an persistence serwis for objects (like a repository in DDD pattern).
All classes have operations (see Fowler anemic domain model anti-pattern)
All of them is an Model component in MVC pattern. The rules:
- Only Boundary provide services for the "other world"
- Boundary can call only to Controll
- Control can call anybody
- Entity can't call anybody (!), only be called.
jz
For the records, the Entity-Boundary-Control pattern comes from use-case driven software development and is much older than Scott Ambler's robustness diagrams who only reused the concept.
The pattern is not part of UML, but is closely related to it because it was promoted by Ivar Jacobson (who initiated it in 1992), Grady Booch and Jim Rumbaugh, the founding fathers of UML... and of the Unified process (UP, RUP). In UML it's just stereotypes like any others.
The Wikipedia explains it best, but Control classes correspond to Use Cases, Boundary classes correspond to the association between a Use-Case and an Actor, and Entities correspond to domain objects identified as being involved in the use-case.
The circulars symbols that most UML tools use with predefined BCE stereotypes are also from Jacobson. The robustness rules are just the transposition of the use-case mapping explained above.