TinyMqtt

ESP 8266 is a very capable Mqtt Broker and Client

Here are the features

mqtt client can Works without WiFi (local mode) in a unique ESP Thus, publishes and subscribes are possible and allows minimal (degraded) function of a single module.
broker can connect to another broker and becomes then a proxy for clients that are connected to it.
zeroconf, this is a strange but very powerful mode where each ESP is a a broker and scans the local network. After a while one ESP becomes the 'master' and all ESP are connected together. The master can die whithout breaking the system.