I've been doing for fun the TryRiot Backend Challenge which wants to :

1. Create an expressJS app which will have multiple endpoints :

   • GET /metrics/{key}/sum (returns the sum of a given key over the past hour)
   • POST /metrics/{key} (post a new key with a given value)

   In order to build this API, I'll store the data directly in the app. The data will be stored in an object activeUsers. Here's an example :

   POST ​/metrics/​active_visitors { ​"value"​ = ​4​ }
   // activeUsers : {active_visitors: [{value: 4, time: currentTime}]} 
   POST ​/metrics/​active_visitors { ​"value"​ = 6​ }
   // activeUsers : {active_visitors: [{value: 4, time: currentTime}, {value: 6, time: currentTime}]} 
   

   For each new value added for a metric, we store the time as well.

   Why are we storing the time ?

   Because we use a cron job which is executed every 10seconds that filters if there are any values that need to be deleted from the object (currentTime - timeStored).

   What packages did I use ?

    - express
    - node-cron (to run the method every 10seconds)
    - moment (to compute the difference between two times)
   

2. Build a SQL database schema based on this screenshot:

   The diagram is available here