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Onion Architecture + Symfony Cheat Sheet by

How to apply the Onion architecture with Symfony framework

Onion Archit­ecture

Layer: Applic­ation Core (appli­cation + domain)
Layer: Presen­tation
Layer: Infras­tru­cture
Layer: Tests

1. Applic­ation Core

has no intera­ction direct with outerl­ayer. It represents the domain business and domain logic. It defines always the domain specific entities, value objects, events, except­ions, services, factories, interf­aces.
Applic­ation layer manages the internal domain logic. It provides different applic­ation services, which enable the commun­ication with presen­tation, tests and infras­tru­cture.

1.1 Domain Layer

consist of entities, value objects, aggregates
repository interface
interfaces to access the business models, which are used by applic­ation and implem­ented by outer layer. For example: infras­tru­cture
the business rules to adjust changes on business behavior and business models
domain services define the complex internal commun­ication among the domain models. For example: apply some changes cross different domain models.
which can be used to track the state changes of domain

1.2 Applic­ation Layer

events / Event Subcriber
defines the events, which represent the state changes in business domain. for example:
theser services enable the intera­ction with internal domains by using the predefined interfaces in the domain layer.
query interface
These interfaces are defined for fetching the domain data. They are commonly used by presen­tation layer and implem­ented by infras­tru­cture layer.
they are simple objects, which are used to change the state of business domain. For example: confir­mPa­yment

2. Presen­tation Layer

contro­llers are the typical gateways for intera­ction comming from end user. It can be a contro­ller, that represents REST endpoint; or a contro­ller, that renders the web page.
It enables the user to access and update the applic­ation core via console in terminal.
provide the template to define how to represent the business data. for example: template of email, template of exports, template of preview
provide the UX interface to end users
Data Transfer Object, defines the view model of request and response
Presen­tation layer provides the interfaces how end user can drive the business logic

3. Infras­tru­cture layer

query implem­ent­ations
repository implem­ent­ations
The infr­ast­ructure layer holds the most low level code. Anything in here should be easy to replace. Code here should never effect anything related to logic, or how your applic­ation behaves.

4. Tests Layer

unit tests
test if internal applic­ation core works well
test if the commun­ication between applic­ation core and external services in infras­tru­cture layer is possible
test if the intera­ction between end user and the presen­tation layer work well
Tests layer test the functi­onality of applic­ation core and integation between applic­ation core and outer layer.

Remark 01:

Appl­ication core is the indepe­ndent core, which defines the most of core logic and a couple of interf­aces, that must be implem­ented and used by outer layer. The inner applic­ation core should be indenp­endent from outlayer, and should be always runable, if you change any part of the outer layer.

Key tenets of Onion

The big advantag of Onon Archit­ecture is that business logic ends up coupled to ONLY applicaton layer concerns, not to infras­tru­cture layer anymore. The applic­ation is built around an indepe­ndent object model. Inner layers define interf­aces. Outer layers implement interfaces
Direction of coupling is toward the center. All applic­ation core code can be compiled and run separate from infras­tru­cture

Symfony Project structure: Core

├── Application
│   ├── Command
│   ├── Event
│   ├── Query
│   └── Service
└── Domain
    ├── Event
    ├── Model
    ├── Repository
    ├── Service
    └── Validation

Symfony Project structure: Presen­tation

├── Api
│   └── Rest
│   ├── Controller
│   └── DTO
├── Console
│   └── DTO
└── Web
    ├── Backoffice
    │   ├── Asset
    │   ├── Controller
    │   ├── DTO
    │   ├── Form
    │   └── Twig
    └── Pub
        ├── Asset
        ├── Controller
        └── Twig

Symfony Project structure: Infras­tru­cture

├── Mail
├── Persistence
│   └── Doctrine
│   ├── Migrations
│   └── Repository
├── Queue
└── SSO

Symfony Project structure: Tests

├── functional
├── integration
└── unit

Remark 02

Appl­ication layer should never use the concret implem­ent­ation from infras­tru­cture layer or presen­tation layer. It defines the applic­ation interfaces and manages the domainer interf­aces, so that the applic­ation core can work a wohle without outerl­ayer. By providing the different applic­ation services, the commun­ication with tests, presen­tation and infras­tru­cture is possible.

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