HNAggregator

Development Notes

The main logic that periodically retrieves the 50 top stories from Hacker News is located in lib/hn_aggregator/worker.ex. This GenServer is supervised by the HNAggregator.Supervisor with a one_for_one strategy.

The code that interacts with the HN API is located in lib/hn_aggregator/hn_client.ex. This module uses the Tesla HTTP client and performs all the requests in parallel using supervised Tasks.

Code for the HTTP API and WebSocket endpoint is located under lib/hn_aggregator/api/. The HTTPController module defines the functions that handle the requests dispatched by the router. The WebSocketHandler module implements the cowboy_websocket behaviour and uses the Registry in order to dispatch the stories to the clients connected to the websocket endpoint.

Public APIs

HTTP

The HTTP API provides the following endpoints:

localhost:4040/stories: lists the top stories with pagination (e.g: /stories?page=3)
localhost:4040/stories/:id: retrieves a single top story (e.g: /stories/33843069)

WebSocket

When connecting to ws://localhost:4040/ws the top stories are sent To test this, the following Javascript code can be input in the browser console:

socket = new WebSocket("ws://localhost:4040/ws")
socket.addEventListener("message", (event) => {
  console.log(event.data)
})

Every 5 minutes the top stories list will be refreshed.

Essential Constraints

All major components of the application (Router, worker, and Registry) are supervised by the HNAggregator.Supervisor. Other internal components such as the Tasks used when calling the HN API are supervised by an ad-hoc supervisor.

Data is always kept in memory. This application uses ETS in order to enable multiple processes to read the stored data concurrently. The ETS table is protected so the Worker is the only process that can write data to the table.

Nice To Haves

Non blocking operations: Accomplished.
Testing: Not implemented. (Due to personal issues I only had one day to implement the application and could not achieve this requirement)
Type and function specifications: Accomplished
Skip Phoenix scaffold: No framework was used. For the API only the following libraries were used: cowboy, plug, and plug_cowboy.
Release strategy: The project supports mix release for generating release.
Isolation strategy: A Dockerfile is included although it does not use mix release since there seems to be a problem with the base image missing some dependencies. It uses mix run --no-halt to run the application.

In order to run the application with Docker perform the following steps:

docker build -t hn_aggregator .
docker run -p 4040:4040 hn_aggregator

manelli / hn_aggregator