This document summarizes a presentation about aggregates, entities, and value objects. It begins with introductions and definitions of these terms. It then discusses the nature of methods, models, and aggregates. Specifically, it defines aggregates as clusters of associated objects treated as a unit for data changes. It suggests modeling aggregates as actors with an incoming message queue to handle changes asynchronously. This improves concurrency and models aggregates as consistency and concurrency boundaries.

1. Devnology Community Day
Aggregates, Entities
and Value objects
Rick van der Arend – rvdarend@sogyo.nl

2. SOFTWARE INNOVATORS 2
Agenda
 Introduction
 Definitions
 … on the nature of Methods
 … on the nature of Models
 … on the nature of Aggregates
 End

3. SOFTWARE INNOVATORS 3
Introduction

4. SOFTWARE INNOVATORS 4
Introducing me
 Rick van der Arend
 Sogyo Consultancy
▫ Architect / Developer
▫ Consultant
▫ Coach / Trainer
 Interests:
▫ CQRS / Event Driven
▫ Actor Model
▫ Functional

5. SOFTWARE INNOVATORS 5
Definitions

6. Value Objects
 Many objects are a description of characteristics
 When (one of) the characteristics changes, it
becomes another (value) object
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7. Entities
 Some objects are not defined by their attributes
 They represent a thread of identity
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8. Aggregates
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9. Aggregates: definitions
An Aggregate is a collection
of items that are gathered
together to form a total
quantity
http://en.wikipedia.org/wiki/Aggregate
SOFTWARE INNOVATORS 9

10. Aggregates: definitions
An Aggregate is a cluster of
associated objects that we
treat as a unit for the
purpose of data changes
Domain Driven Design, Eric Evans (DDD-EE)
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11. Breakdown of this definition
1. A cluster of associated objects
2. That we treat as a unit
3. For the purpose of data changes
SOFTWARE INNOVATORS 11
C

12. Things to notice
 We‟re talking objects here, not classes
 It‟s for a certain purpose: data changes
▫ This means that for other purposes (like viewing),
we might not treat them as a unit
 This is all in one context & model
▫ A car engineer will have a very different model of
your car than you will have as a driver
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13. Implementation
 From the cluster, choose a root entity with global
identity – this will be the aggregate root
 Nothing from the outside can hold a reference to
anything inside, only to the root
 Inner objects may be handed over transiently
 All objects in the cluster may hold references
 Data changes on the aggregate should be ACID
 The aggregate (root) maintains its invariants
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14. But that sounds a lot like…
 An object
 A component
 A service
 An actor
 An agent
“High internal cohesion, Low external coupling”
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15. What are the advantages?
 These intra-aggregate links can be modelled as
messages
 A message is to be interpreted by the receiver,
in its own time
This means the receiver:
 Could be switched for another type
 Could be dormant and awakened on receipt
 Could be halfway around the world
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16. And the alternative?
 You could try to make a fully acid model, living in
/ supported by a relational database
 And use replication
 The problem is that this model cannot deal with
inconsistencies
 So it can only compromise availability when
there‟s a separation
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17. And there’s more
 A model that can deal with inconsistencies is
more robust
 Such a model has lower coupling with external
entities and can be understood more easily
 You are likelier to have a better model that can
deal with changes in the future
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18. SOFTWARE INNOVATORS 18
On the nature of Methods

19. What is a method?
A method is a combination of the following:
1. Checking of input parameters
2. Checking of caller rights
3. Checking of appropriateness of current state
4. Reading parts of the state
5. Performing calculations
6. Setting certain fields to a new value
7. Calling methods on certain fields
8. Returning a value
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20. What is a method?
A method is a combination of the following:
1. Checking of input parameters
2. Checking of caller rights
3. Checking of appropriateness of current state
4. Reading parts of the state
5. Performing calculations
6. Setting certain fields to a new value
7. Calling methods on certain fields
8. Returning a value
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This should be pushed out of the method: by modelling or code contracts
This is a cross cutting concern: use annotations
Tell, don‟t Ask: no can‟t do / deal with it && business rules as annotations
Do you really need this? Remember Tell, don‟t Ask
Ok, but when too complex, move this out to a service
Ok, setting a new state
Ok, relaying the command
Ok, querying

21. Option1: Calculation
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MyCalculatingMethod(param1, param2)
{
return param1 + param2;
}
This could be a static method!

22. Option 2: Relaying a call
MyRelayingMethod(param1, param2)
{
field1.HisMethod(param1, param2, field2);
}
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23. Option 3: Setting a field
MySettingValueMethod(param1)
{
field1 = field1.Add(param1);
}
MySettingEntityMethod(param1)
{
field1 = param1;
}
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24. Option 4: Querying
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MyQueryingMethod()
{
return field1;
}
This could be a static method! (That‟s why I said
this could/should be moved out to a service)

25. Single Responsibility Principle
 There is something that is called the Single
Responsibility Principle
 Could this mean that a method should only do
one of the previous things?
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26. Single Responsibility Principle
 There is something that is called the Single
Responsibility Principle
 Could this mean that a method should only do
one of the previous things?
 Well, I think a method can perform multiple
relays, if they combine to form one responsibility
▫ But only if they do this exhaustively
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27. SOFTWARE INNOVATORS 27
On the nature of Models

28. A domain model as a graph
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V
V
V
V
S
A Value (object)
An Entity
A Static object
(entry point)

29. A repository
SOFTWARE INNOVATORS 29
S
V
V
V
V
C
An object that has a collection in it, pointing to several entities
(a collection is special, it can add and remove references)

30. An Aggregate
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V
V
V
V
S
A couple of objects with high internal cohesion, low external coupling

31. Changes are reference re-routes
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V
V
V
S
V
A field is changed to point to another entity

32. Changes are reference re-routes
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V
V
V
S
V
A field is changed to point to another value (object)

33. Changes are reference re-routes
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V
V
V
S
V
A field is changed to point to another entity that is supplied as parameter

34. Changes are reference re-routes
SOFTWARE INNOVATORS 34
V
V
V
S
V
A field is changed to point to another value that is supplied as parameter

35. Just wondering...
 Wouldn‟t it be possible to make every method
call change only one such references?
 And wouldn‟t it be possible to make such a
change an event?
 Then there would be a uniform way of making
model changes into events.
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36. Identifying the event
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V
VV
S
V
A field is changed to point to another entity that is supplied as parameter
Reason
(Command?)
Path
new Value
Source
1
2
Time
Version

37. SOFTWARE INNOVATORS 37
On the nature of
Aggregates
…or should I say Actors?

38.  Incoming message queue
 Asynchronous message loop
 All mutable state internally
 All shared state is immutable
 An „independent‟ object
Let’s take an Actor
38SOFTWARE INNOVATORS
Actor

39.  Incoming message queue
 Asynchronous message loop
 Aggregate
What do we have then?
 An actor with a incoming, async message queue
 And a coherent, self-supporting model in it
And put an aggregate in it
39SOFTWARE INNOVATORS

40.  This is a combination of Actors & OO modelling
 The aggregate not only becomes a consistency
boundary, but a concurrency boundary too
 This Aggregate-as-an-Actor model is very close
to the one that is normally described in CQRS
 That one is just missing the queue and the loop
The Aggregate-as-an-Actor model
40SOFTWARE INNOVATORS

41. SOFTWARE INNOVATORS 41
~ The end ~

42. SOFTWARE INNOVATORS 42
42
Questions?

43. SOFTWARE INNOVATORS 43
Contact info
Rick van der Arend
rvdarend@sogyo.nl
030 - 220 22 16
Web: www.sogyo.nl
Blog: www.software–innovators.nl