# Step 2: Exploring the app

The app is a simple microservice based on Spring Boot for rating conference talks. It provides an API to track the ratings of the talks in real time.

## Storage

### SQL database with the talks

When a rating is submitted, we must verify that the talk for the given ID is present in our database.

Our database of choice is PostgreSQL, accessed with Spring JDBC.

Check `com.example.demo.repository.TalksRepository`.

### Redis

We store the ratings in Redis database with Spring Data Redis.

Check `com.example.demo.repository.RatingsRepository`.

### Kafka

We use ES/CQRS to materialize the events into the state. Kafka acts as a broker and we use Spring Kafka.

Check `com.example.demo.streams.RatingsListener`.

## API

The API is a Spring Web REST controller (`com.example.demo.api.RatingsController`) and exposes two endpoints:

* `POST /ratings { "talkId": ?, "value": 1-5 }` to add a rating for a talk
* `GET /ratings?talkId=?` to get the histogram of ratings of the given talk