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Initial exercise setup.

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* Instructions for manually starting the database.
* Each phase now has a success criteria with how to verify.
* Troubleshooting section for working with the Docker DB container.
* All phases of the exercise are described in README.md (with as little advanced hints as possible).
* DFD to illustrate phase 1
* Debugging starts the database container and applies schema migrations (no setup required).
* Launch settings for VS Code, VS 2022 and Rider (IntelliJ IDEA).
* Swagger UI is available at <http://localhost:8080/swagger/index.html> for testing.
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Simon Dellenbach
2025-06-26 11:32:01 +02:00
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README.md
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# 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
```