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Laying it out: Onion Architecture
Fanie Reynders
Team Lead
@FanieReynders
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The centre of your application is
not the database, it's the use cases
of your application.
Fanie Reynders
@FanieReynders
“
”
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Traditional Layered Architecture
Infrastructure /
DAL
Core / BLL
UI
DB
Utilities / Common / Shared
nHibernate /
LINQ to SQL
ASP.NET
Log4Net
AutoMapper
StructureMap
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Problems with Traditional Architecture
Very easy for developers to put increasing business logic
in the UI layer
Application is built on top of specific technology that is
sure to change over time – counter productive
Logic is scattered all over – difficult to locate & track
Where should the code go? BLL? DAL? Utilities?
Business Logic with extending tentacles
Library Explosion – Easy to take a dependency
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Different View: Traditional Architecture
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Onion Architecture: Concept Overview
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Onion Architecture: Concept Details
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Inverted Dependencies
Infrastructure /
DAL
Core / BLL
UI
DB
Utilities / Common / Shared
nHibernate /
LINQ to SQL
ASP.NET
Log4Net
AutoMapper
StructureMap
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Onion Architecture
Infrastructure (DAL)
Core
(BLL)
UI
DB
Dependency Resolution (Utilities)
nHibernate /
LINQ to SQL
ASP.NET
Log4Net
StructureMap
AutoMapper
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Benefits of Onion Architecture
True loosely coupled
Limit re-write over time – Business Logic self contained
Very portable
Business Logic has no dependencies
Architecture more sustainable over time
Infrastructure free to use any data access or external
service to accomplish task
UI & Data Access smaller – deal directly with
technology-related code
Compiler enforces dependency boundaries
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Example
ProductsController
Browser hits:
/Products/List
IProductRepository
IUserSession
HttpUserSession
<> ProductRepository
<>
DB
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Traditional Layered Architecture
• Unique to business: Domain model, validation, business workflows
• Defines contracts
• CANNOT reference external libraries
• NO technology specific code
Core
• Provide implementations for Core interfaces
• Call web services, access database
• CAN reference external libraries
• ONLY contains technology specific code (non-business)
Infrastructure
• Wires up Core interfaces with Infrastructure implementations
• Runs start-up / configuration / binding logic
Dependency
Resolution
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Thank you