UOW introduction document.

docs/en/Unit-Of-Work.md

@@ -1,3 +1,29 @@
-## Unit of Work
+# Unit of Work
+
+ABP Framework's Unit Of Work (UOW) implementation provides an abstraction and control on a **database transaction** scope in an application.
+
+Once a new UOW started, it creates an **ambient scope** that is participated by **all the database operations** performed in the current scope and considered as a **single transaction**, **committed** (on success) or **rolled back** (on exception) all together.
+
+ABP's UOW system is;
+
+* **Works conventional**, so most of the times you don't deal with UOW at all.
+* **Database provider independent**.
+* **Web independent**, that means you can create unit of work scopes in any type of applications beside web applications/services.
+
+## Conventions
+
+The following method types are considered as a unit of work:
+
+* ASP.NET Core MVC **Controller Actions**.
+* ASP.NET Core Razor **Page Handlers**.
+* **Application service** methods.
+* **Repository methods**.
+
+A UOW automatically begins for these methods except if there is already a **surrounding (ambient)** UOW in action. Examples;
+
+* If you call a [repository](Repositories.md) method and there is no UOW started yet, it automatically **begins a new transactional UOW** that involves all the operations done in the repository method and **commits the transaction** if the repository method **doesn't throw any exception.** The repository method doesn't know about UOW or transaction at all. It just works on a regular database objects (DbContext for [EF Core](Entity-Framework-Core.md), for example) and the UOW is handled by the ABP Framework.
+* If you call an [application service](Application-Services.md) method, the same UOW system works just as explained above. If the application service method uses some repositories, the repositories **don't begin a new UOW**, but **participates to the current unit of work** started by the ABP Framework for the application service method.
+* The same is true for an ASP.NET Core controller action. If the operation has started with a controller action, then the **UOW scope is the controller action method body**.
+
+All of these are automatically handled by the ABP Framework. The rest of this document explains the UOW system in details and options provided to a fine control the UOW system.
 
-TODO