The device pings the agent every second. If it misses a pong response it turns on the light for at least 10 seconds.

agent_ping.agent.js

device.on("ping", function(ping) {
    device.send("pong", ping);
})

server.log("Agent started")

agent_ping.device.js

const TOLERANCE = 1000;
const LOST = 3000;
const WAKEUP = 1000;
const DELAY = 5000;


//------------------------------------------------------------------------------
imp.configure("Agent ping", [], []);
server.log("Device started") 

//------------------------------------------------------------------------------
turnoff <- time();
pings <- {};

//------------------------------------------------------------------------------
led <- hardware.pin9;
led.configure(DIGITAL_OUT);
led.write(0);

//------------------------------------------------------------------------------
function alert(message) {
    server.log(message)
    led.write(1);
    turnoff = time() + 10;    
}

//------------------------------------------------------------------------------
agent.on("pong", function(pong) {
    local dur = math.abs(hardware.millis() - pong);
    if (pong in pings) {
        delete pings[pong];
        if (dur > TOLERANCE) {
            alert(format("ALERT: %0.02f seconds", dur/1000.0));
        } else {
            // server.log(format("PONG: %0.02f seconds", dur/1000.0));
        }
    } else {
        server.log(format("FOUND: %0.02f seconds", dur/1000.0));
    }
})


//------------------------------------------------------------------------------
function ping() {
    // Check for lost pings
    local lost = [];
    foreach (ping,dummy in pings) {
        local dur = math.abs(hardware.millis() - ping);
        if (dur > LOST) {
            lost.push(ping);
        }
    }
    if (lost.len() > 0) {
        foreach (ping in lost) {
            delete pings[ping];
        }
        alert(format("LOST: %d pings", lost.len()));
    }
    
    // Send the next ping
    local last_ping = hardware.millis();
    pings[last_ping] <- true;
    agent.send("ping", last_ping)
    if (time() >= turnoff) {
        led.write(0);
    }
        
    imp.wakeup(WAKEUP/1000.0, ping);
}
imp.wakeup(DELAY/1000.0, ping);