Added best practice for domain service docs

docs/en/Best-Practices/Domain-Services.md

@@ -1,7 +1,68 @@
 ## Domain Services Best Practices & Conventions
 
-> **This document is not ready yet. Please see the [Domain Services](../Domain-Services.md) document.**
+
+
+### Domain Service
+
+- **Do** define domain services in the **domain layer**.
+- **Do not** create interfaces for the domain services **unless** you have a good reason to (like mock and test different implementations).
+- **Do** name your domain service with `Manager` suffix.
+
+For the example of a domain service:
+```cs
+public class IssueManager : DomainService
+{
+	//...
+}
+```
+
+### Domain Service Methods
+
+- **Do not** define `GET` methods. `GET` methods do not change the state of an entity. Hence, use the repository directly in the Application Service instead of Domain Service method.
+
+- **Do** define methods that only mutates data; changes the state of an entity or an aggregate root.
+
+- **Do not** define methods with generic names (like `UpdateIssueAsync`). 
+
+- **Do** define methods with self explanatory names (like `AssignToAsync`) that implements the specific domain logic.
+
+
+- **Do** accept valid domain objects as parameters.
+
+```cs
+public async Task AssignToAsync(Issue issue, IdentityUser user)
+{
+    //...
+}
+```
+
+- **Do** throw `BusinessException` or custom business exception if a validation fails.
+
+  - **Do** use domain error codes with unique code-namespace for exception localization.
+
+```cs
+public async Task AssignToAsync(Issue issue, IdentityUser user)
+{
+    var openIssueCount = await _issueRepository.GetCountAsync(
+            i => i.AssignedUserId == user.Id && !i.IsClosed
+        );
+
+        if (openIssueCount >= 3)
+        {
+            throw new BusinessException("IssueTracking:ConcurrentOpenIssueLimit");
+        }
+
+        issue.AssignedUserId = user.Id;
+}
+```
+
+- **Do not** return `DTO`. Return only domain objects when you need.
+- **Do not** involve authenticated user logic. Instead, define extra parameter and send the related data of ` CurrentUser` from the Application Service layer.
+
+
 
 ## See Also
 
-* [Video tutorial](https://abp.io/video-courses/essentials/domain-services)
+* [Video tutorial](https://abp.io/video-courses/essentials/domain-services)
+* [Domain Services](../Domain-Services.md)
+* [Exception Handling](../Exception-Handling.md)