Entities | Domain Model | Domain Layer | Domain Driven Design
Such Concepts has Life cycle Attached to them and these Concepts take many States During their Life Cycle. Concept of Order, Concept of Bug, Work Item.
Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.
Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.
Some objects are not defined primarily by their attributes.
They represent a thread of identity that runs through time and often across distinct representations.
Sometimes such an object must be matched with another object even though attributes differ.
An object must be distinguished from other objects even though they might have the same attributes.
Mistaken identity can lead to data corruption.
Therefore,
When an object is distinguished by its identity, rather than its attributes, make this primary to its definition in the model.
Keep the class definition simple and focused on life cycle continuity and identity.
Define a means of distinguishing each object regardless of its form or history.
We can have Group Query on such Objects on the basis fo some attributes.
Be alert to requirements that call for matching objects by attributes. Query for matching objects based on some attributes.
Define an operation that is guaranteed to produce a unique result for each object, possibly by attaching a symbol that is guaranteed unique. ??????
This means of identification may come from the outside, or it may be an arbitrary identifier created by and for the system,
The model must define what it means to be the same thing.
Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.
Entity: An object that is not defined by its attributes, but rather by a thread of continuity and its identity.
JBogard Says
If two instances of the same object have different attribute values, but same identity value, are they the same entity?
If the answer is “yes”, and I care about an identity, then the class is indeed an entity.
I model entities with reference objects (classes), and I give them a surrogate identity (i.e., probably a GUID).
Additionally, my model must include what it means to have the same identity. That means overriding Equals, looking solely at the identity and not attributes.
Example: Most airlines distinguish each seat uniquely on every flight. Each seat is an entity in this context. However, Southwest Airlines (or EasyJet/RyanAir for Europeans) does not distinguish between every seat; all seats are the same. In this context, a seat is actually a value object.
Entity: An object that is not defined by its attributes, but rather by a thread of continuity and its identity.
JBogard Says
If two instances of the same object have different attribute values, but same identity value, are they the same entity?
If the answer is “yes”, and I care about an identity, then the class is indeed an entity.
I model entities with reference objects (classes), and I give them a surrogate identity (i.e., probably a GUID).
Additionally, my model must include what it means to have the same identity. That means overriding Equals, looking solely at the identity and not attributes.
Example: Most airlines distinguish each seat uniquely on every flight. Each seat is an entity in this context. However, Southwest Airlines (or EasyJet/RyanAir for Europeans) does not distinguish between every seat; all seats are the same. In this context, a seat is actually a value object.
Entities
- Entities are domain concepts that have a unique identity in the problem domain.
- Natural keys from the problem domain, application generated, and datastore generated are all techniques for creating entity IDs.
- Having a life cycle is also a distinguishing characteristic of entities.
- Entities should always be valid for the given context.
- Invariants are fundamental truths about an entity, so they should always be enforced.
- Be careful of modeling physical concepts as single entities. The typical Customer entity can often be logically split across multiple bounded contexts into numerous entities.
Entity
If any term in the domain is unique and differs from other objects in the system, we need to use entity to model it. These value objects may differ in form within a given lifecycle, and yet you should always identify and be able to find them using queries. For that end, you can rely on unique identifiers, which need to be created from the very beginning of the development process (when you develop entities).
There are several strategies you can use to create unique identifiers:
#1 Input of unique value by a user
Use this approach if you need your app’s identifiers to be easily readable. Here’s a thing, though: You need to make sure that all identifiers are checked for uniqueness and accuracy within the app itself. Moreover, implementing any changes to identifiers is costly, and users are not usually allowed to manipulate or change them. Therefore, you should rely on methods that guarantee the quality and consistency of every identifier.
#2 Identifier generation by an app
There are some fast and reliable generators that can be used to automatically generate unique identifiers. For instance, in Java we have java.util.UUID class that allows generating universally unique identifiers using four different methods: time-based, DCE security, name-based, randomly generated UUIDs.
Here’s a UUID example:
Hide Copy Code
046b6c7f-0b8a-43b9-b35d-6489e6daee91
(basically, a 36-byte string)
Long identifiers (like shown below) are hard to store due to memory overload. This is why, it makes sense to use one or two segments of this 36-byte UUID string. (Of course, you need to be sure that these segments are well-protected.) The contracted identifier can be protected much better if it is used as a local entity identifier within aggregate. To demonstrate this, let’s have a look at the following identifier:
Hide Copy Code
APM-P-08-14-2016-046B6C7F
- APM is a specific context to control the design process
- P is the project itself
- 08-14-2016 is the date when it was created
- 046B6C7F is the first segment of UUID.
When identifiers like these are found by developers, they can easily figure out where and when they originated.
#3 Identifier generation by permanent storage mechanism
To create this type of identifier, you need to request the database. Thanks to this method, you can always be sure that your identifier is protected and unique. And it will be rather short as well, and you will be able to use it as a part of concatenated identifier.
This strategy has a fundamental disadvantage, though — performance. If you request the database each time you need to receive a specific value, it takes a lot of time. If identifiers are generated by the app, processing happens much faster.
#4 Identifier attribution by other bounded contexts
Sometimes you need to integrate different bounded contexts to obtain an identifier. For example, you can do it using context maps as I demonstrated in the previous article.
To find a specific identifier in another bounded context, you can specify several attributes (email, account number, etc.) that allow identifying the unique identifier of external entity, which you can use as a local identifier. You can also copy a specific additional value from external to local entity.
However, when we usually deal with entities, we need to take not only a domain identifier but also a surrogate identifier into account. The first identifier is subject to specific rules within the domain, while the second one should be used specifically for ORM (as Hibernate). To create a surrogate key, we need to create such entity attributes as long and int. Meanwhile, the unique identifier is generated in the database, and its then can be used as a primary key. After that, this key’s visualization is implemented to the attribute with ORM instruments. A surrogate identifier like this is usually hidden as it isn’t included in the domain itself.
To note! To protect the identifier’s uniqueness within the object’s lifecycle, the identifier and the object itself have to be wired against any type of modification. As a rule, the protection is achieved by hiding identifier’s setters or by developing specific check-up mechanism to scan the setters and locate any modifications and changes. The PFM system from the previous article demonstrates this principle well enough.
To begin, we should separate specific entities in a given domain. In our case, we have BankingAccount entity, which can be identified with the help of accountNumber. Though this number is unique only for a given bank and can be duplicated many times in other banks. (Of course, you can always rely on IBAN if you live in the EU.) In other words, you need to use not only accountNumber but also a specific UUID segment. This way, our identifier will consist of
Hide Copy Code
PFM-A-424214343245-046b6c7f
- PFM — context name
- A — account
- 424214343245 — accountNumber
- 046b6c7f — segment of UUID
You can set any identifier as value object as well. So, let’s examine this crucial DDD pattern in detail.
Entities
We define entity as a single domain concept / object (it has an associated identifier). The identifiers are unique and immutable, for that reason it is advisable to store the identifier in a Value Object .
Commonly, the modeling tendency of entities reflects the data structure, with tools such as ORMsgenerating a domain model totally mapped to the data structure by means of CRUD . This method is very useful for simple applications or systems with limited or no evolution. On the other hand, it is not advisable for those applications or systems that require an evolution and progressive increase in their complexity. A modeling decoupled from the data structure is more advisable, being more faithful to the domain logic of the organization through obicuous language .
Identity Generation
The two most common strategies that allow us to create identifiers are:
- Through identity creation patterns such as globally unique identifier (GUID) or universally unique identifier (UUID) .
- The database or persistence system generates the unique identity by sequence or auto increasing the value. Although we previously need to create the registry to have the new identifier: something that can be an impediment in what situations, where we should have the identifier before the entity persists.
It is advisable to use information specific to the entity instance as part of the identifier, providing us with valuable information in any context (for example, the creation date).
Construction
The constructors must be the minimum mandatory and necessary to correctly instantiate the entity. The always necessary instance initialization parameter must be the Value Object identifier of the new instance. The assignments or setters of the entity attributes must be hidden, encapsulating the functionalities in methods that allow us to validate the incoming parameters, ensuring the correct hydration of the instance.
For more exhaustive elaborations of instance of entity we will use Factory methods encapsulating in them the logics of instance construction.
Surrogate Identity
Some ORM like Hibernate need to have an identifier through a native type of database, generating a conflict if we want to use a Value Object.
To solve and reconcile the conflict we must create both . The Value Object identity will be adjusted to the requirements of the domain and the other to the ORM known as Surrogate Identity . Since said Surrogate Identity is not part of the domain model, we will hide it using private or Layer Supertype (creating an abstract class that hides the identifier through protected ).
Validation
Following the principle of single responsibility, an entity should not be responsible for carrying out its validations internally, advising considering the extraction of functionalities or validation responsibilities.
Attributes / Properties Validation : Establishing and encapsulating in the methods of assigning properties the necessary preconditions for their correct initialization.
Whole Objects Validation : That the values of all the properties are correct does not mean that the composition of all of them generates a valid entity. For this we will use patterns such as Specification pattern . It is advisable to validate the complete entity as late as possible ( Deferred Validation ).
Object Compositions Validations : If the validation of an entity depends on other instances of other entities or aggregates, it is advisable to use Domain Services to encapsulate the validation logic and be able to recover those dependent entities through Repositories. Deferred Validation is also advised .
Entities
We define entity as a single domain concept / object (it has an associated identifier). The identifiers are unique and immutable, for that reason it is advisable to store the identifier in a Value Object .
Commonly, the modeling tendency of entities reflects the data structure, with tools such as ORMsgenerating a domain model totally mapped to the data structure by means of CRUD . This method is very useful for simple applications or systems with limited or no evolution. On the other hand, it is not advisable for those applications or systems that require an evolution and progressive increase in their complexity. A modeling decoupled from the data structure is more advisable, being more faithful to the domain logic of the organization through obicuous language .
Identity Generation
The two most common strategies that allow us to create identifiers are:
- Through identity creation patterns such as globally unique identifier (GUID) or universally unique identifier (UUID) .
- The database or persistence system generates the unique identity by sequence or auto increasing the value. Although we previously need to create the registry to have the new identifier: something that can be an impediment in what situations, where we should have the identifier before the entity persists.
It is advisable to use information specific to the entity instance as part of the identifier, providing us with valuable information in any context (for example, the creation date).
Construction
The constructors must be the minimum mandatory and necessary to correctly instantiate the entity. The always necessary instance initialization parameter must be the Value Object identifier of the new instance. The assignments or setters of the entity attributes must be hidden, encapsulating the functionalities in methods that allow us to validate the incoming parameters, ensuring the correct hydration of the instance.
For more exhaustive elaborations of instance of entity we will use Factory methods encapsulating in them the logics of instance construction.
Surrogate Identity
Some ORM like Hibernate need to have an identifier through a native type of database, generating a conflict if we want to use a Value Object.
To solve and reconcile the conflict we must create both . The Value Object identity will be adjusted to the requirements of the domain and the other to the ORM known as Surrogate Identity . Since said Surrogate Identity is not part of the domain model, we will hide it using private or Layer Supertype (creating an abstract class that hides the identifier through protected ).
Validation
Following the principle of single responsibility, an entity should not be responsible for carrying out its validations internally, advising considering the extraction of functionalities or validation responsibilities.
Attributes / Properties Validation : Establishing and encapsulating in the methods of assigning properties the necessary preconditions for their correct initialization.
Whole Objects Validation : That the values of all the properties are correct does not mean that the composition of all of them generates a valid entity. For this we will use patterns such as Specification pattern . It is advisable to validate the complete entity as late as possible ( Deferred Validation ).
Object Compositions Validations : If the validation of an entity depends on other instances of other entities or aggregates, it is advisable to use Domain Services to encapsulate the validation logic and be able to recover those dependent entities through Repositories. Deferred Validation is also advised .
Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.
Some objects are not defined primarily by their attributes. They represent a thread of identity that runs through time and often across distinct representations. Sometimes such an object must be matched with another object even though attributes differ. An object must be distinguished from other objects even though they might have the same attributes. Mistaken identity can lead to data corruption.
Therefore,
When an object is distinguished by its identity, rather than its attributes, make this primary to its definition in the model. Keep the class definition simple and focused on life cycle continuity and identity. Define a means of distinguishing each object regardless of its form or history. Be alert to requirements that call for matching objects by attributes. Define an operation that is guaranteed to produce a unique result for each object, possibly by attaching a symbol that is guaranteed unique. This means of identification may come from the outside, or it may be an arbitrary identifier created by and for the system, but it must correspond to the identity distinctions in the model. The model must define what it means to be the same thing.
No comments:
Post a Comment