How can ethereum be used to send messages between connected machines?

Question

I want to know how I can send messages between accounts and contracts.

The idea is that I have three Raspberry Pi devices. Each one is connected to an object (the first to a temperature sensor, the second to a LED and the third one to an LCD).

I want to create an autonomous system in which these objects can communicate through the Ethereum network.

How can I use Ethereum to send messages between connected machines?

Answer 1

For temporary messages, I'd use Events.

They can be used to send individual instructions to the devices.

For persistent data like the current temperature, you can use storage variables.

For example, this could turn the LED on if the temperature raises above 60 degrees.

contract IOT {

    uint public temperature;
    address sensor;
    address led;

    event Instruction( address device, string instruction)

    function IOT (address _sensor, address LED ){
        sensor = _sensor;
        led = LED;
    }

    function updateTemp(uint temp){
        if(msg.sender != sensor) throw;
        temperature = temp;
        if (temperature > 60) {
            Instruction(led, "ON");
        {
        else Instruction (led,"OFF");
    }
}

The client would simply watch for events with their address. The device could also read the temperature directly, but I wanted to show some logic.

Answer 2

Communication between parties on the Ethereum network, if the messages are temporarily and shouldn't be stored for ever, can be done over the Whisper protocol.

If you want to give this a whirl and are using geth make sure to start your nodes with the --shh flag. This will enable whisper.

You can find more information about the Whisper API here. A working chat example can be found here.