MQTT

MQTT Essentials
https://www.hivemq.com/blog/mqtt-essentials/

Publish a Message From Android on MQTT
https://pupli.net/2017/10/06/publish-a-message-from-android-on-mqtt/

MQTT Topics & Best Practices
https://pupli.net/2017/11/25/mqtt-topics-best-practices/

Maximum simultaneous connections on a mosquitto broker
https://pupli.net/2017/12/13/maximum-simultaneous-connections-on-a-mosquitto-broker/