A simple proof to test the Arduino mqtt library to publish digital pin state and control relay using I2C RELAY8 shield (tested on EtherTen from Freetronics).
To test use mosquitto_pub & mosquitto_sub example :
mosquitto_pub -d -m '{"command":"setHigh","pin":"8"}' -t "/devices/ethercard/action"
mosquitto_sub -h 127.0.0.1 -t devices/ethercard/params/#
Results :
devices/ethercard/params/inputPin 6 {state:1}
devices/ethercard/params/outputPin 1 {state:0}