dotnet-interview-exercise/README.md

# 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
* PostgreSQL Database (Docker)
* 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
  * Setup of the client.
  * Method for fetching one post from the external API at `https://jsonplaceholder.typicode.com/posts/{id}`.
* In our endpoint handler, 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
$ docker compose exec --env PGPASSWORD=password db psql -h localhost -U user --db mydatabase -c "select * from posts;"

 id | user_id |                                   title                                    |                        body                         |       updated_at
----+---------+----------------------------------------------------------------------------+-----------------------------------------------------+------------------------
  1 |       1 | sunt aut facere repellat provident occaecati excepturi optio reprehenderit | quia et suscipit                                   +| 1970-01-01 00:00:00+00
    |         |                                                                            | suscipit recusandae consequuntur expedita et cum   +|
    |         |                                                                            | reprehenderit molestiae ut ut quas totam           +|
    |         |                                                                            | nostrum rerum est autem sunt rem eveniet architecto |
(1 row)
```

### 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, pull 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 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

### Start database manually

Open a terminal at the repo root and execute

```bash
$ docker compose up -d db

[+] Running 3/3
 ✔ Network dotnet-interview-exercise_default   Created  0.0s
 ✔ Volume "dotnet-interview-exercise_db_data"  Created  0.0s
 ✔ Container dotnet-interview-exercise-db-1    Started  0.1s
```

### Cleanup database

In case something went wrong, wipe and rebuild the database

```bash
$ docker compose down -v

[+] Running 3/3
 ✔ Container dotnet-interview-exercise-db-1   Removed. 0.2s
 ✔ Network dotnet-interview-exercise_default  Removed. 0.2s
 ✔ Volume dotnet-interview-exercise_db_data   Removed. 0.0s

$ docker compose up -d db
[+] Running 3/3
 ✔ Network dotnet-interview-exercise_default   Created  0.0s
 ✔ Volume "dotnet-interview-exercise_db_data"  Created  0.0s
 ✔ Container dotnet-interview-exercise-db-1    Started  0.1s
```