Readme for AddTransient - Deep Dive

Author: oneananda

034_AddTransient_DeepDive/README.md

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+# AddTransient - Deep Dive
+
+---
+
+## Request flow
+
+## 1. Client Sends a Request
+
+1. A user or client (such as a browser or an API consumer) sends an HTTP request to a specific URL/endpoint (e.g., `GET /api/customers/123`).
+
+2. The request arrives at your ASP.NET Core application, which is listening for incoming HTTP traffic.
+
+---
+
+## 2. Routing & Controller Activation
+
+3. **Routing**: ASP.NET Core’s routing matches the request path and HTTP verb (GET, POST, etc.) to a corresponding **controller action**.  
+   - For example, `GET /api/customers/123` might route to `CustomersController.GetCustomer(int id)`.
+
+4. **Controller Instantiation**: The ASP.NET Core framework **creates** an instance of the targeted controller.  
+   - At this point, **Constructor Injection** occurs:
+     - If your `CustomersController` requires `ICustomerService`, the framework checks its built-in DI container to see how `ICustomerService` is registered (e.g., `services.AddTransient<ICustomerService, CustomerService>();`).
+     - A **new** `CustomerService` object is created (because it’s registered as transient), and that object is passed into the `CustomersController` constructor.
+
+5. Once the controller is constructed, ASP.NET Core calls the relevant action method—e.g., `GetCustomer(int id)`.
+
+---
+
+## 3. Controller Action → Calls the Service
+
+6. Inside the action method, the controller delegates its main work to the **service layer**.  
+   - For example:  
+     ```
+     [HttpGet("{id}")]
+     public IActionResult GetCustomer(int id)
+     {
+         var customer = _customerService.GetCustomerById(id);
+         if (customer == null) return NotFound();
+         return Ok(customer);
+     }
+     ```
+     Here, `_customerService` is the transient service injected in the controller’s constructor. 
+     ```
+
+---
+
+## 4. Service → Calls Repository (or Other Dependencies)
+
+7. **Service Logic**: The `CustomerService` processes the request (e.g., performing business logic, validation, transformations). Often, it must retrieve or modify data.  
+   - If the service needs database access, it calls a **repository** interface (e.g., `ICustomerRepository`).  
+   - The service’s constructor injection also occurs (when the service is resolved). Because it is **transient**, each time an instance is requested, the DI container creates a fresh `CustomerService`. During its own construction, the service might request an `ICustomerRepository`. If `ICustomerRepository` is also registered with the container (e.g., `AddTransient<ICustomerRepository, CustomerRepository>()`), a fresh `CustomerRepository` is injected too.
+
+8. **Repository Access**: The service calls a method on the repository, for example:
+   ```
+   public Customer GetCustomerById(int id)
+   {
+       return _customerRepository.FindById(id);
+   }
+   ```
+   The repository method handles database logic (e.g., using Entity Framework or direct SQL calls) and returns the requested data model (e.g., a `Customer` object).
+   ```
+---
+
+## 5. Repository → Returns Data to the Service
+
+9. The repository fetches the data from the **database** or data store.  
+   - If using EF Core, it might look like:
+     ```
+     public Customer FindById(int id)
+     {
+         return _dbContext.Customers.FirstOrDefault(c => c.Id == id);
+     }
+     ```
+10. The repository returns the found `Customer` (or `null` if not found) to the service method.
+
+11. The **service** may do additional processing on that data—such as formatting, filtering, or applying business rules.
+
+---
+
+## 6. Service → Returns Result to the Controller
+
+12. The service returns the final result (e.g., `Customer` object) back to the controller action.  
+   - In the example, `_customerService.GetCustomerById(id)` gives the controller the result.
+
+13. The controller **formats** the final HTTP response. For instance:
+    - If `customer == null`, the controller may return `NotFound()`.
+    - Otherwise, it returns `Ok(customer)`.
+
+---
+
+## 7. Controller Action Completes → HTTP Response Sent to Client
+
+14. The action method finishes. ASP.NET Core takes the returned result (an `IActionResult`) and **serializes** it into an HTTP response.  
+   - If the controller returned `Ok(customer)`, the framework sends a `200 OK` status code, along with the JSON-serialized `customer` object in the response body.
+
+15. The **HTTP response** goes back over the network to the client (e.g., the browser or API caller).
+
+---
+
+## Notes on the Transient Lifetime in This Flow
+
+- **Transient Services**: Whenever a class (such as a controller) **needs** an `ICustomerService` (registered via `AddTransient`), the DI container will instantiate a **new** `CustomerService`.  
+  - This means each time a request arrives and a new controller instance is created, the service is a brand-new instance with no shared state from previous requests.
+- **Scoped Services**: If you used a scoped lifetime (e.g., `AddScoped`), a single instance of the service would be reused **throughout the same HTTP request**. Once the request completes, that instance is released. 
+- **Singleton Services**: If you used a singleton lifetime (e.g., `AddSingleton`), a single instance would be created the first time it’s needed and then reused **across all requests** (until the application restarts).
+
+In this deep dive, we focus on **AddTransient**, so each request or each injection gets its own fresh instance of the service (and possibly also the repository, if that’s registered as transient too). This is particularly helpful if the service or repository is **stateless** or if you want to ensure no accidental sharing of data between requests.
+
+---
+
+## Summary of the Typical Control Flow
+
+1. **Request** arrives at the application.  
+2. **Routing** locates the correct **Controller** action method.  
+3. Controller is **constructed** with **transient** services injected by DI.  
+4. The **action method** executes, calling methods on the injected **service**.  
+5. The **service** (also created fresh if transient) calls the **repository** or other dependencies.  
+6. The **repository** accesses the **database** (or external data source) and returns results to the service.  
+7. The **service** finalizes any business logic and sends the result back to the controller.  
+8. The **controller** formats the HTTP response (e.g., `Ok(...)`, `NotFound()`, etc.).  
+9. **ASP.NET Core** serializes the result and sends the **HTTP response** to the client.
+
+This flow ensures that each layer handles its own responsibilities:  
+- **Controllers** for request/response handling,  
+- **Services** for business or domain logic,  
+- **Repositories** for data access.
+
+---