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Here I am, with another question about aggregation and association. I wanted to learn some basics of UML, so I started reading "UML distilled" by Martin Fowler. I read both chapters about classes, and there is one thing that I can't fully grasp I think, and that is aggregation vs association. In the book there is this quote:
In the pre-UML days, people were usually rather vague on what was aggregation and what was
association. Whether vague or not, they were always inconsistent with everyone else. As a result,
many modelers think that aggregation is important, although for different reasons. So the UML
included aggregation (Figure 5.3) but with hardly any semantics. As Jim Rumbaugh says, "Think of it
as a modeling placebo" [Rumbaugh, UML Reference].
As I understand from this quote and topics that I read on Stack Overflow it doesn't matter which one of those two relations I use, they mean basically the same, or is there any situation where the usage of aggregation instead of association would be justified and/or I could not change one to the other without changing the "meaning" of a class diagram?
I am asking this, because this book is from 2003 and some things could have changed during those few years.
Maybe this can help you, but i don't think you will find the perfect explanation:
The difference is one of implication. Aggregation denotes whole/part
relationships whereas associations do not. However, there is not
likely to be much difference in the way that the two relationships are
implemented. That is, it would be very difficult to look at the code
and determine whether a particular relationship ought to be
aggregation or association. For this reason, it is pretty safe to
ignore the aggregation relationship altogether. [Robert C. Martin | UML]
And an example for each situation:
a) Association is a relationship where all object have their own
lifecycle and there is no owner. Let’s take an example of Teacher and
Student. Multiple students can associate with a single teacher and
single student can associate with multiple teachers, but there is no
ownership between the objects and both have their own lifecycle. Both
can create and delete independently.
b) Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child
object can not belong to another parent object. Let’s take an example
of Department and teacher. A single teacher can not belong to
multiple departments, but if we delete the department, the teacher object
will not be destroyed. We can think about “has-a” relationship.[Maesh | GeeksWithBlogs]
Rumbaugh's statement is the most telling and Uncle Bob's good advice. As I've said elsewhere, Aggregation is semantically so weak as to offer nothing practically beneficial. It only has one valid corner case (acyclicity of recursive relationships) however few people know and understand that. So you end up having to point out in comments anyway.
I just don't use it. And have never felt any loss. Stick with simple binary associations and focus on what really matters - getting the cardinality and naming right. You'll get far more from that than trying to decide the undecidable association vs. aggregation.
hth.
I tend to use Aggregation to show a relation that is the same as a Composition with one big distinction: the containing class is NOT responsible for the life-cycle of the contained object. Typically, a (non-null) pointer or reference to the object-to-be-contained is passed to the containing class's constructor. The containing object, for the duration of its life-cycle, depends upon the contained object existing. The containing object cannot do its job (fully) without the contained object. This is my interpretation of the "Part/Whole" relationship implied by Aggregation.
This term often gets confused.
Aggregation and composition are some of the types of association. There is
hardly a difference between aggregations and associations during
implementation, and many will skip aggregation relations altogether in
their diagrams with association relation.
You can get the idea from this analogy.
Class:A(person) and Class:B(car) has association relation, if
Class:A has a Class:B declaration, and also Class:B(car) object is not essential to create a Class:A(person) object.
Class:A(car) and Class:B(tyre) has aggregation relation, if
Class:A has a Class:B declaration, and also Class:B(tyre) object is essential to create a Class:A(car) object.
Cheers!
In UML aggregation is under-defined and since they haven't got any clearly defined semantic.
A valid use-case of an aggregation is the encapsulation of a several classes, as stated in "Domain Driven Design" by Eric Evans.
E.g. a car has four wheels.
You might want to calculate the total amount of meters each wheel has driven, for each car.
This calculation is done by the car-entity, since it knows which wheels it has and you don't care which wheels belong to which car.
The car is the aggregation-root for all it's parts, like wheels, and you can't access the parts of a car from outside the aggregation, just the root.
So basically an aggregation encapsulates a set of classes which belong to each other.
They do not mean the same! I can put it in this way:
Association relationship: A class references another class. Actually it shows that a class is related to another class but they
don't necessarily have attributes to show this relationship... e.g
'Teacher' and 'Student' classes, although 'Teacher' class has no
attribute that refer to students, but we do know that in reality a
teacher do have students... And also 'School' class has 'teachers' and
'students' properties that now make those two classes related to each
other.
Aggregation relationship: A class contains another class. But if the container(ClassRoom) is destroyed, the contained(Chair) is not.
Actually the ClassRoom owns the Chair. Aggregation is a more stronger
relationship than the Association relationship.
Here is also a tutorial about it and the whole UML2.0 which explains everything easy and simple, you may find it useful: https://github.com/imalitavakoli/learn-uml2
TIP: Also let me mention that because the Association relationship exists between classes most of the times, we sometimes don't draw it to prevent unnecessary complexity.
Implementation wise there is not much of a difference but conceptually there is big difference: aggregations are used to express a hierarchy. When you work with a hierarchy of components there are certain type of operations you need to have in the root interface:
find subcomponents in the hierarchy
add/remove subcomponents to/from the hierarchy
change common attributes of all components
traverse the hierarchy recursively (Visitor pattern)
reconfigure the hierarchy and the links (associations) between the components
Most of these operations are not needed when dealing with associations.
To add, I would just suggest to download the UML specification from the OMG site: best reference and see p 110.
None indicates that the property has no aggregation semantics.
Shared indicates that the property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler.
Composite indicates that the property is aggregated compositely, i.e., the composite object has responsibility for
the existence and storage of the composed objects (see the definition of parts in 11.2.3).
Here I am, with another question about aggregation and association. I wanted to learn some basics of UML, so I started reading "UML distilled" by Martin Fowler. I read both chapters about classes, and there is one thing that I can't fully grasp I think, and that is aggregation vs association. In the book there is this quote:
In the pre-UML days, people were usually rather vague on what was aggregation and what was
association. Whether vague or not, they were always inconsistent with everyone else. As a result,
many modelers think that aggregation is important, although for different reasons. So the UML
included aggregation (Figure 5.3) but with hardly any semantics. As Jim Rumbaugh says, "Think of it
as a modeling placebo" [Rumbaugh, UML Reference].
As I understand from this quote and topics that I read on Stack Overflow it doesn't matter which one of those two relations I use, they mean basically the same, or is there any situation where the usage of aggregation instead of association would be justified and/or I could not change one to the other without changing the "meaning" of a class diagram?
I am asking this, because this book is from 2003 and some things could have changed during those few years.
Maybe this can help you, but i don't think you will find the perfect explanation:
The difference is one of implication. Aggregation denotes whole/part
relationships whereas associations do not. However, there is not
likely to be much difference in the way that the two relationships are
implemented. That is, it would be very difficult to look at the code
and determine whether a particular relationship ought to be
aggregation or association. For this reason, it is pretty safe to
ignore the aggregation relationship altogether. [Robert C. Martin | UML]
And an example for each situation:
a) Association is a relationship where all object have their own
lifecycle and there is no owner. Let’s take an example of Teacher and
Student. Multiple students can associate with a single teacher and
single student can associate with multiple teachers, but there is no
ownership between the objects and both have their own lifecycle. Both
can create and delete independently.
b) Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child
object can not belong to another parent object. Let’s take an example
of Department and teacher. A single teacher can not belong to
multiple departments, but if we delete the department, the teacher object
will not be destroyed. We can think about “has-a” relationship.[Maesh | GeeksWithBlogs]
Rumbaugh's statement is the most telling and Uncle Bob's good advice. As I've said elsewhere, Aggregation is semantically so weak as to offer nothing practically beneficial. It only has one valid corner case (acyclicity of recursive relationships) however few people know and understand that. So you end up having to point out in comments anyway.
I just don't use it. And have never felt any loss. Stick with simple binary associations and focus on what really matters - getting the cardinality and naming right. You'll get far more from that than trying to decide the undecidable association vs. aggregation.
hth.
I tend to use Aggregation to show a relation that is the same as a Composition with one big distinction: the containing class is NOT responsible for the life-cycle of the contained object. Typically, a (non-null) pointer or reference to the object-to-be-contained is passed to the containing class's constructor. The containing object, for the duration of its life-cycle, depends upon the contained object existing. The containing object cannot do its job (fully) without the contained object. This is my interpretation of the "Part/Whole" relationship implied by Aggregation.
This term often gets confused.
Aggregation and composition are some of the types of association. There is
hardly a difference between aggregations and associations during
implementation, and many will skip aggregation relations altogether in
their diagrams with association relation.
You can get the idea from this analogy.
Class:A(person) and Class:B(car) has association relation, if
Class:A has a Class:B declaration, and also Class:B(car) object is not essential to create a Class:A(person) object.
Class:A(car) and Class:B(tyre) has aggregation relation, if
Class:A has a Class:B declaration, and also Class:B(tyre) object is essential to create a Class:A(car) object.
Cheers!
In UML aggregation is under-defined and since they haven't got any clearly defined semantic.
A valid use-case of an aggregation is the encapsulation of a several classes, as stated in "Domain Driven Design" by Eric Evans.
E.g. a car has four wheels.
You might want to calculate the total amount of meters each wheel has driven, for each car.
This calculation is done by the car-entity, since it knows which wheels it has and you don't care which wheels belong to which car.
The car is the aggregation-root for all it's parts, like wheels, and you can't access the parts of a car from outside the aggregation, just the root.
So basically an aggregation encapsulates a set of classes which belong to each other.
They do not mean the same! I can put it in this way:
Association relationship: A class references another class. Actually it shows that a class is related to another class but they
don't necessarily have attributes to show this relationship... e.g
'Teacher' and 'Student' classes, although 'Teacher' class has no
attribute that refer to students, but we do know that in reality a
teacher do have students... And also 'School' class has 'teachers' and
'students' properties that now make those two classes related to each
other.
Aggregation relationship: A class contains another class. But if the container(ClassRoom) is destroyed, the contained(Chair) is not.
Actually the ClassRoom owns the Chair. Aggregation is a more stronger
relationship than the Association relationship.
Here is also a tutorial about it and the whole UML2.0 which explains everything easy and simple, you may find it useful: https://github.com/imalitavakoli/learn-uml2
TIP: Also let me mention that because the Association relationship exists between classes most of the times, we sometimes don't draw it to prevent unnecessary complexity.
Implementation wise there is not much of a difference but conceptually there is big difference: aggregations are used to express a hierarchy. When you work with a hierarchy of components there are certain type of operations you need to have in the root interface:
find subcomponents in the hierarchy
add/remove subcomponents to/from the hierarchy
change common attributes of all components
traverse the hierarchy recursively (Visitor pattern)
reconfigure the hierarchy and the links (associations) between the components
Most of these operations are not needed when dealing with associations.
To add, I would just suggest to download the UML specification from the OMG site: best reference and see p 110.
None indicates that the property has no aggregation semantics.
Shared indicates that the property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler.
Composite indicates that the property is aggregated compositely, i.e., the composite object has responsibility for
the existence and storage of the composed objects (see the definition of parts in 11.2.3).
I mean, is there any PersonAggregate class? I understand it doesn't exist. I only have an entity acting as aggregate root. Is it correct?
I only have an entity acting as aggregate root. Is it correct?
That's correct.
The aggregate is implicit - it's the boundary that separates two disjoint sets of state that can be modified independently of each other. Equivalently, the aggregate is a graph of business state within a model that can be modified without consulting state outside the graph, and vice versa.
The aggregate root is explicit. That's the single entity in the graph that is exposed - which is to say that it serves as the entry point through which all modifications to the graph must pass.
Hypothetically, you could implement an aggregate that has two different exposed entities that can each execute commands to modify the state; Evans introduced the notion of a single aggregate root because multiple entry points is difficult to get correct.
I have seen both solutions used in projects, but most often people do not use this suffix.
One interesting solution for this is make aggregate classes public and non-aggregate classes package(default). You'd see directly from your IDE which classes have which visibility and you can determine easily where is an aggregate. Additionally non-public class cannot be used outside package which is an original intent.
My understanding is an Aggregate Root is an Entity but an Entity might not be an Aggregate Root. Therefore, I view 'Aggregate Root' as more of a stereotype.
Not in domain-driven design. That would be exposing technological jargon, essentially implementation detail, to the domain experts
From a post I read it seems that Entity is just a subset of Aggregate. I've read about the two patterns in both Domain-Driven Design and Implementing Domain-Driven Design, and I'm trying to understand the UML difference between them.
Let's consider a simple class. It's a Letter holding a message, a receiver, and possibly the sender.
I guess this Letter class would be considered an Entity?
Now let's say we want to expand our parcel business to be able to send also Packages, then it could look like the following.
Since all the Items in the Package will be lost if the whole Package is lost, we use a UML Composition relation (a filled diamond). We also want to preserve the Package's consistency by prohibiting Items from being changed or removed from outside the Package. The description of Aggregate reads
The aggregate root guarantees the consistency of changes being made
within the aggregate by forbidding external objects from holding
references to its members.
We therefore make sure the Composition relation is hidden, and with preserved invariants, within the Aggregate.
My question is:
Can we say that the UML difference between Entity and Aggregate is that Entity does not contain any Composition relation whereas Aggregate contains at least one Composition relation?
To answer your question, no you can't say that. An aggregate root is entity itself, and may or may not be composed of child entities. The child entities can also be composed of other entities as well (though not recommended usually).
The aggregate root is responsible for maintaining the state and enforcing the invariants of both itself and it's child entities.
So to recap, an aggregate and a child entity can each have 0 or more child entities. All child entities require an aggregate root however.
An Entity represents the M(odel) in MVC. It's usually denoted as a <<entity>> stereotyped class.
An Aggregate is a synonym for a class which aggregates different other classes. That means it needs the other classes for its life time. There's also a Composite which is similar except that the related class instances will die along with the composite class.
To answer your final question: an Entity is atomar. It does not aggregate anything.
Edit Since I just encountered it for my work: There are Entities which compose/aggregate other entities. 30 years ago at university we called them trapeze for they hang between two other entities and relate them. Nowadays I'd call them association class.
An Entity in Domain-Driven-Design (DDD) is just a class stereotype in UML terms. That stereotype indicates to you that the object is identified by an explicit unique identifier, rather than its attributes.
Objects in an object model collaborate together. Together they form object graphs. An Aggregate represents a group of objects that need to be considered together because not doing so would potentially leave one or more of the objects in an invalid state.
"Can we say that the UML difference between Entity and Aggregate is that Entity does not contain any Composition relation whereas Aggregate contains at least one Composition relation?"
No. A UML composition or aggregation association is unrelated to the concept of DDD Aggregate or Entity.
For example, one can represent a Transaction-LineItem relationship in UML without composition or (UML) aggregation.
Transaction --- 1 -------- 0..* --- LineItem
Both these objects as modeled above would need to be part of the same (DDD) Aggregate because they need to be treated as a pair. If mistreated individually, one could make their combined states invalid or incorrect.
Here I am, with another question about aggregation and association. I wanted to learn some basics of UML, so I started reading "UML distilled" by Martin Fowler. I read both chapters about classes, and there is one thing that I can't fully grasp I think, and that is aggregation vs association. In the book there is this quote:
In the pre-UML days, people were usually rather vague on what was aggregation and what was
association. Whether vague or not, they were always inconsistent with everyone else. As a result,
many modelers think that aggregation is important, although for different reasons. So the UML
included aggregation (Figure 5.3) but with hardly any semantics. As Jim Rumbaugh says, "Think of it
as a modeling placebo" [Rumbaugh, UML Reference].
As I understand from this quote and topics that I read on Stack Overflow it doesn't matter which one of those two relations I use, they mean basically the same, or is there any situation where the usage of aggregation instead of association would be justified and/or I could not change one to the other without changing the "meaning" of a class diagram?
I am asking this, because this book is from 2003 and some things could have changed during those few years.
Maybe this can help you, but i don't think you will find the perfect explanation:
The difference is one of implication. Aggregation denotes whole/part
relationships whereas associations do not. However, there is not
likely to be much difference in the way that the two relationships are
implemented. That is, it would be very difficult to look at the code
and determine whether a particular relationship ought to be
aggregation or association. For this reason, it is pretty safe to
ignore the aggregation relationship altogether. [Robert C. Martin | UML]
And an example for each situation:
a) Association is a relationship where all object have their own
lifecycle and there is no owner. Let’s take an example of Teacher and
Student. Multiple students can associate with a single teacher and
single student can associate with multiple teachers, but there is no
ownership between the objects and both have their own lifecycle. Both
can create and delete independently.
b) Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child
object can not belong to another parent object. Let’s take an example
of Department and teacher. A single teacher can not belong to
multiple departments, but if we delete the department, the teacher object
will not be destroyed. We can think about “has-a” relationship.[Maesh | GeeksWithBlogs]
Rumbaugh's statement is the most telling and Uncle Bob's good advice. As I've said elsewhere, Aggregation is semantically so weak as to offer nothing practically beneficial. It only has one valid corner case (acyclicity of recursive relationships) however few people know and understand that. So you end up having to point out in comments anyway.
I just don't use it. And have never felt any loss. Stick with simple binary associations and focus on what really matters - getting the cardinality and naming right. You'll get far more from that than trying to decide the undecidable association vs. aggregation.
hth.
I tend to use Aggregation to show a relation that is the same as a Composition with one big distinction: the containing class is NOT responsible for the life-cycle of the contained object. Typically, a (non-null) pointer or reference to the object-to-be-contained is passed to the containing class's constructor. The containing object, for the duration of its life-cycle, depends upon the contained object existing. The containing object cannot do its job (fully) without the contained object. This is my interpretation of the "Part/Whole" relationship implied by Aggregation.
This term often gets confused.
Aggregation and composition are some of the types of association. There is
hardly a difference between aggregations and associations during
implementation, and many will skip aggregation relations altogether in
their diagrams with association relation.
You can get the idea from this analogy.
Class:A(person) and Class:B(car) has association relation, if
Class:A has a Class:B declaration, and also Class:B(car) object is not essential to create a Class:A(person) object.
Class:A(car) and Class:B(tyre) has aggregation relation, if
Class:A has a Class:B declaration, and also Class:B(tyre) object is essential to create a Class:A(car) object.
Cheers!
In UML aggregation is under-defined and since they haven't got any clearly defined semantic.
A valid use-case of an aggregation is the encapsulation of a several classes, as stated in "Domain Driven Design" by Eric Evans.
E.g. a car has four wheels.
You might want to calculate the total amount of meters each wheel has driven, for each car.
This calculation is done by the car-entity, since it knows which wheels it has and you don't care which wheels belong to which car.
The car is the aggregation-root for all it's parts, like wheels, and you can't access the parts of a car from outside the aggregation, just the root.
So basically an aggregation encapsulates a set of classes which belong to each other.
They do not mean the same! I can put it in this way:
Association relationship: A class references another class. Actually it shows that a class is related to another class but they
don't necessarily have attributes to show this relationship... e.g
'Teacher' and 'Student' classes, although 'Teacher' class has no
attribute that refer to students, but we do know that in reality a
teacher do have students... And also 'School' class has 'teachers' and
'students' properties that now make those two classes related to each
other.
Aggregation relationship: A class contains another class. But if the container(ClassRoom) is destroyed, the contained(Chair) is not.
Actually the ClassRoom owns the Chair. Aggregation is a more stronger
relationship than the Association relationship.
Here is also a tutorial about it and the whole UML2.0 which explains everything easy and simple, you may find it useful: https://github.com/imalitavakoli/learn-uml2
TIP: Also let me mention that because the Association relationship exists between classes most of the times, we sometimes don't draw it to prevent unnecessary complexity.
Implementation wise there is not much of a difference but conceptually there is big difference: aggregations are used to express a hierarchy. When you work with a hierarchy of components there are certain type of operations you need to have in the root interface:
find subcomponents in the hierarchy
add/remove subcomponents to/from the hierarchy
change common attributes of all components
traverse the hierarchy recursively (Visitor pattern)
reconfigure the hierarchy and the links (associations) between the components
Most of these operations are not needed when dealing with associations.
To add, I would just suggest to download the UML specification from the OMG site: best reference and see p 110.
None indicates that the property has no aggregation semantics.
Shared indicates that the property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler.
Composite indicates that the property is aggregated compositely, i.e., the composite object has responsibility for
the existence and storage of the composed objects (see the definition of parts in 11.2.3).