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InfluxDB
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You could use MQTT as a form of communication for all the devices, and make all your messages retained, so whenever some device updates the status, all subscribed devices will receive the new message, and because the messages are retained any new device will imediatly receive the retained messages, essencially syncing all current and future devices. Technically you will have an MQTT server, or better know as broker in this context, so a RPi would a good ideia, but an ESP8266 might be sufficient, I've beber tried it though A quick search on Google lead me to this link: "GitHub - martin-ger/uMQTTBroker: MQTT Broker library for ESP8266 Arduino" https://github.com/martin-ger/uMQTTBroker .