What does the data look like when it is generated by the sensor? The sensor sends JSON-format data over the internet and each reading looks like the following:

{
  "sensor_id": 1,
  "time": 1511935948000,
  "value": 21.89
} 

Here, we can see the following:

• The sensor_id field is the unique identifier of the sensor that has emitted the record.
• The time field is the time of the reading in milliseconds since the epoch, that is, 00:00:00 on January 1, 1970.
• The value field is the actual metric value emitted by the sensor.

This type of JSON payload is generated every minute by all the sensors in the system. Since all sensors are registered in the system, the server-side system has the associated metadata with each sensor. Let's look at the sensor-related metadata that is available to us on the server side in a database.