# dotnet-interview-exercise

This exercise is designed to test your skills in C# and .NET.

## Technologies used in this exercise

* .NET 8.0 / C#
* Minimal ASP.NET Core Web API
* Entity Framework Core
* SQLite3 Database (simplified for portability)
* HttpClient for downstream REST API calls
* ???

## Prerequisites

* Latest [.NET 8 SDK](https://dotnet.microsoft.com/en-us/download/dotnet/8.0)
* Visual Studio Code
* Docker Desktop and docker compose plugin
* **Disable Copilot completions** (that would make it too easy...)
* Be ready for sharing your screen in Zoom
* Launch settings have been configured to run and debug the web application

## Exercise

### Phase 1

Note: For all API and schemas, see <https://jsonplaceholder.typicode.com/>.

* Implement the missing code in `JsonPlaceholderClient.cs`
  * Setup of the client.
  * Method for fetching one post from the external API at `https://jsonplaceholder.typicode.com/posts/{id}`.
* In our endpoint handler in `Program.cs`, fetch the post with the ID provided.
* Store the post received in the database using the `Post` entity, but leave out `UpdatedAt` for now.
* Use <http://localhost:8080/swagger/index.html> for testing.

```mermaid
sequenceDiagram
    title Overview
    actor B as Swagger UI
    participant S as Service
    participant E as External Service
    participant D as Database
    B->>S: GetPostById
    S<<->>E: GetPostById
    S->>D: Store Post
    S->>B: Return Post
```

**Success Criteria: Verify that the post has been saved with the database with:**

```bash
$ dotnet run --project show-posts/

| Id | UserId |                                   Title                                    |                                                                              Body                                                                              |          UpdatedAt          |
| -- | ------ | -------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------- |
| 1  | 1      | sunt aut facere repellat provident occaecati excepturi optio reprehenderit | quia et suscipit suscipit recusandae consequuntur expedita et cum reprehenderit molestiae ut ut quas totam nostrum rerum est autem sunt rem eveniet architecto | 1/1/1970 12:00:00 AM +00:00 |
```

### Phase 2

* Check whether we already have the post in the database.
* If we do, return the post from the database, but without the internal `updated_at` timestamp.
* If we don't, fetch the post from the external API and store it in the database, and return it.
* **Success Criteria: Check the service logs to see if the post was read from the database, or whether the client made a call to the downstream service.**

### Phase 3

* Set the `UpdatedAt` property of the `Post` entity to the current date and time when storing it in the database.
* If the post in the database has been updated less than 1 minute ago, return the one from the database.
* If the post in the database is older than 1 minute (extra short value for testing), fetch the post from the external API again and update it in the database before returning.
* **Success Criteria: Check the service logs to see if the post was read from the database, or whether the client made a call to the downstream service.**

### Phase 4

* Think about how we could improve resiliency in downstream API calls.
* For setup, merge in the changes from the `phase4` branch.
  * It adds chaos-injecting code in `Program.cs` to simulate random failures in downstream API calls.
  * Execute a couple of requests with increasing ids until an error happens.
* Implement basic resiliency measures to handle these kinds of failures.
* **Success Criteria: Check the service logs to see the resiliency measures in action.**

### Phase 5

* Now increase the chaos outcomes rate of 503 to 95% to simulate a partial outage. What is the outcome on our endpoint?
* Improve the error response returned by handling the downstream error accordingly.
* **Success Criteria: Service should return the appropriate error response that warrants retrying the call from the client side.**

### Bonus Phase 6 (discussion only)

* The current DB schema solution has big drawbacks. What are they?
* How would you implement schema migrations that avoid these issues?
* What do you need to consider in the setup for proper schema migrations?

### Bonus Phase 7 (discussion only)

* Imagine the following deployment scenario:
  * The service is deployed to and running in the Elastic Container Service (ECS) in Amazon AWS
  * The downstream service requires authentication
* How would you store and retrieve the required credentials?
* What options do you see to also require authentication on our little service?

## Troubleshooting

### Cleanup database

In case something went wrong or you want to start over, wipe the database by resetting the SQLite file.

```bash
git restore service/service.db
```