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239 lines
18 KiB
Markdown
239 lines
18 KiB
Markdown
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# 🍔 Domain Driven Design
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For many architects, the process of data modeling is driven by intuition. However, there are well-formulated methodologies
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for approaching it more formally. I recommend the [Domain-Driven Design](https://en.wikipedia.org/wiki/Domain-driven_design)
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and choose it as AppFlowy architecture.
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## 💥 Layered architecture
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The most common architecture pattern is the layered architecture pattern, known as the n-tier architecture pattern.
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Partition the software into `layers` to reduce the complexity. Each layer of the layered architecture pattern has a
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specific role and responsibility.`DDD` consists of four layers.
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```
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┌──────────────────────────────────────────────────┐ ─────────▶
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│ Presentation Layer │──┐ Dependency
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└──────────────────────────────────────────────────┘ │
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│ │
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▼ │
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┌──────────────────────────────────────────────────┐ │
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│ Application Layer │ │
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└──────────────────────────────────────────────────┘ │
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│ │
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▼ │
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┌──────────────────────────────────────────────────┐ │
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│ Domain Layer │◀─┘
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└──────────────────────────────────────────────────┘
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▲
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│
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┌──────────────────────────────────────────────────┐
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│ Infrastructure Layer │
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└──────────────────────────────────────────────────┘
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```
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**Presentation Layer**:
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* Responsible for presenting information to the user and interpreting user commands.
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* Consists of Widgets and also the state of the Widgets.
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**Application Layer**:
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* Defines the jobs the software is supposed to do. (Shouldn't find any UI code or network code)
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* Coordinates the application activity and delegates work to the next layer down.
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* It doesn't contain any complex business logic but the basic validation on the user input before
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passing to the other layer.
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**Domain Layer**:
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* Responsible for representing concepts of the business.
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* Manages the business state or delegated to the infrastructure layer.
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* Self contained and it doesn't depend on any other layers. Domain should be well isolated from the
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other layers.
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**Infrastructure Layer**:
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* Provides generic technical capabilities that support the higher layers. It deals with APIs, persistence and network, etc.
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* Implements the repository interface and hides the complexity of the Domain layer.
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As you see, the `Complexity` and `Abstraction` of these layers are depicted in the following diagram. Software system are composed in layers,
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where higher layers use the facilities provided by lower layers. Each layer provides a different abstraction from the layer above
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and below it. As developers, we should pull the complexity downwards. Simple interfaces and powerful implementations (Think about the
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[open](https://man7.org/linux/man-pages/man2/open.2.html) function). Another way of expressing this idea is that it is more important
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for a module to have a simple interface than a simple implementation.
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```
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▲
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│
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Level of ├───────────────────┐
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Abstraction │ Presentation │
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├───────────────────┴───────┐
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│ Application │
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├───────────────────────────┴─────────┐
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│ Domain │
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├─────────────────────────────────────┴────────┐
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│ Infrastructure │
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└──────────────────────────────────────────────┴─────▶
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Complexity
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```
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### Data Model
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DDD classifies data as referenceable objects, or entities, and non-referenceable objects, or value objects. Let's introduce some DDD terminology.
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**Entity**
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`Entities` are plain objects that carry an identity which allows us to reference them. e.g. user, order, book, etc.
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You use `entities` to express your business model and encapsulate them into Factory that provides a simple API to create Entities.
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**Value Object**
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`Value Object` can't be referenced. They can be only included into entities and serve as attributes. Value objects could be simple and treat as immutable. e.g. email, phone number, name, etc.
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**Aggregate**
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`Entities` and `Value objects` can be grouped into aggregates. Aggregates can simplify the model by accessing the entire aggregate.
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For instance, Table has lots of rows. Each row using the table_id to reference to the
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table. TableAggregate includes two entities: Table and the Row.
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```
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TableAggregate
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┌────────────────────────────────────────────────────────────────┐
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│ │
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│ ┌────────────────────┐ ┌─────────────────────────┐ │
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│ │struct Table { │ │struct Row { │ │
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│ │ id: String, │ │ table_id: String, │ │
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│ │ desc: String, │◀▶───────│} │ │
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│ │} │ │ │ │
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│ └────────────────────┘ └─────────────────────────┘ │
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│ │
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└────────────────────────────────────────────────────────────────┘
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```
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**Service**
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When a significant process of transformation in the domain is not a natural responsibility of an `Entity` or `Value object`, add an operation to the model as standalone interface declared as a Service. For instance: The `Value object` EmailAddress uses the function `validateEmailAddress` to verify if the email address is valid or not. `Service` exists in Application, Domain and Infrastructure.
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```
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class EmailAddress {
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final Either<Failure<String>, String> value;
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factory EmailAddress(String? input) {
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return EmailAddress._(
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validateEmailAddress(input),
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);
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}
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const EmailAddress._(this.value);
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}
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Either<Failure<String>, String> validateEmailAddress(String? input) {
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...
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}
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```
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**Repository**
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Repositories offer an interface to retrieve and persist aggregates and entities. They hide the database or network details from the domain.
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Repository interfaces are declared in the Domain Layer, but the repositories themselves are implemented in the Infrastructure Layer.
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You can replace the interface implementation without impacting the domain layer.
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For instance:
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```
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// Interface:
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abstract class AuthInterface {
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...
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}
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// Implementation
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class AuthRepository implements AuthInterface {
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...
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}
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```
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> More often than not, the repository interface can be divided into sub-repository in order to reduce the complexity.
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### Relation
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The diagram below is a navigational map. It shows the patterns that form the building blocks of Domain Driven Design and how they relate to each other.
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](imgs/domain_model_relation.png)
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## Operation Flow
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```
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presentation │ Application domain Infrastructure
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│ │
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7 Data Model
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┌──────────────────────────────┐ │ ┌────────────────────────┐ │ ┌─────────────────────┐
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│ │ │ │ ┌─────────────┐ │ │ Network Service │
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▼ Bloc │ │ │ │ Aggregate │ │ │ └─────────────────────┘
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┌─────────────┐ │ ┌─────────────────┴─────┐ │ └─────────────┘ │ ▲
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────────▶ Widget │ │ ┌────────┐ ┌────────┐ │ 2 │ ┌────────┐ │ │ │ 6
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└─────────────┘ │ │ │ Event │ │ State │ │────┬───▶│ │ Entity │ │ ┌─────────────────────┐
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User │ │ └────────┘ └────────┘ │ │ │ │ └────────┘ │ │ │ Persistence Service │
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interaction │ │ └──────▲────────────────┘ │ │ ┌─────────────────┐ │ └─────────────────────┘
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│ │ │ │ │ │ Value Object │ │ │ ▲
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└──────────┼────────┘ │ │ └─────────────────┘ │ │ 5
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1 │ │ └────────────◈───────────┘ │ ┌─────────────────────┐
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│ │ │contain │ Unit of Work │
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│ │ ┌────────────────────┐ │ └─────────────────────┘
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│ │ │ Service │ ▲
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│ │ └────────────────────┘ │ │
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│ │ │ 4
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│ │ Repository │ │
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│ │ ┌─────────────────────────────────────────────┴───────────────┐
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│ │ │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ 3 ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
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│ └───┤ Interface ────▶ Implementation │
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│ │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
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│ └─────────────────────────────────────────────────────────────┘
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│
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```
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1. Widget accepts user interaction and translates the interactions into specific Events. The events will be sent to the Application layer, handled by the specific `bloc`. The `bloc` sends the states changed by the events back to the widget, and finally the `Widget` updates the UI according to the state. The pattern is depicted in this diagram. (More about the flutter [bloc](https://bloclibrary.dev/#/coreconcepts?id=bloc))
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```
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┌──────────── State ────────────┐
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│ │
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▼ Bloc │
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┌─────────────┐ ┌─────────────┼─────────┐
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────────▶│ Widget │ │ ┌────────┐ ┌┴───────┐ │
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└─────────────┘ │ │ Event │ │ State │ │
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User interaction │ │ └────────┘ └────────┘ │
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│ └───────────────────────┘
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│ ▲
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│ │
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└────────── Event ────────────┘
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```
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2. The `bloc` processes the events using the services provided by the `Domain` layer.
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1. Convert DTO (Data Transfer Object) to domain model and Domain Model to DTO.
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2. Domain model is the place where all your business logic, business validation and business behaviors will be implemented.
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The Aggregate Roots, Entities and Value Objects will help to achieve the business logic.
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3. Calling repositories to perform additional operations. The repositories interfaces are declared in the `Domain` layer and are implemented in the `Infrastructure` layer.
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You can reimplement the repository interface with different languages, such as `Rust`, `C++` or `Dart`. etc.
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```
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Domain Infrastructure
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Repository A │
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┌────────────────────────────┼────────────────────────────────┐
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│ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
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│ Interface ──┼─▶ Implementation │
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│ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
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└────────────────────────────┼────────────────────────────────┘
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Repository B │
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┌────────────────────────────┼────────────────────────────────┐
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│ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
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│ Interface ──┼─▶ Implementation │
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│ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
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└────────────────────────────┴────────────────────────────────┘
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```
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4. The responsibility of [Unit of Work](https://martinfowler.com/eaaCatalog/unitOfWork.html) is to maintain a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems((No intermediate state)).
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If any one persistence service fails, the whole transaction will be failed so, roll back operation will be called to put the object
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back in initial state.
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5. Handling operations (INSERT, UPDATE and DELETE) with SQLite to persis the data.
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6. Saving or querying the data in the cloud to finish the operation.
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