In-memory reaper. TtlSet needs tests.

lib/units/device/plugins/heartbeat.js

@@ -0,0 +1,24 @@
+var syrup = require('stf-syrup')
+
+var lifecycle = require('../../../util/lifecycle')
+var wire = require('../../../wire')
+var wireutil = require('../../../wire/util')
+
+module.exports = syrup.serial()
+  .dependency(require('../support/push'))
+  .define(function(options, push) {
+    function beat() {
+      push.send([
+        wireutil.global
+      , wireutil.envelope(new wire.DeviceHeartbeatMessage(
+          options.serial
+        ))
+      ])
+    }
+
+    var timer = setInterval(beat, options.heartbeatInterval)
+
+    lifecycle.observe(function() {
+      clearInterval(timer)
+    })
+  })