NodeMCU (ESP8266) board controlled over shared hosting database
Use MQTT.

You need:


The PubSubClient.h library
An MQTT server


For MQTT there are some free ones around on the internet (ask Google). Or if you have a Linux server available for use (e.g., a cheap VPS server from one of many many providers online) then you can run your own with mosquitto.

MQTT works kind of how you describe, but it's not making constant GET requests - it opens a single connection to the MQTT server and realtime requests are sent down it to the client, and back from the client to the server.

Near realtime communication.