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
• Visual Studio Code with C# Dev Kit extension
  • Also works with Visual Studio 2022 and JetBrains Rider.
• 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.

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:

$ 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.

git restore service/service.db