See https://community.openhab.org/t/device-status-online-monitor/135397

device_online_monitor.yaml

uid: ssalonen:device-online-monitor
label: Monitor device online status
description: Monitor device online status using combination of thing status and item state
configDescriptions:
  - name: INPUT_ITEM
    type: TEXT
    context: item
    label: Input item to monitor
    required: true
    description: Item to monitor for UNDEF changes
  - name: INPUT_THING
    type: TEXT
    context: thing
    label: Input thing to monitor
    required: true
    description: Thing to monitor for OFFLINE/ONLINE
  - name: ONLINE_STATUS_ITEM
    type: TEXT
    context: item
    label: Item to output the detected online status
    required: true
    description: Item to output the detected online status
  - name: GRACE_PERIOD_SECS
    type: INTEGER
    label: Grace period after startup, in seconds
    required: false
    defaultValue: 0
    description: Grace period after startp before deeming the device status offline
  - name: CRON
    type: TEXT
    label: Cron expression specifying period for checking inital status
    required: false
    defaultValue: 0 0 0/2 * * ? *
    description: Cron expression specifying period for checking inital status
triggers:
  - id: "1"
    configuration:
      startlevel: 100
    type: core.SystemStartlevelTrigger
  - id: "2"
    configuration:
      cronExpression: "{{CRON}}"
    type: timer.GenericCronTrigger
  - id: "3"
    configuration:
      thingUID: "{{INPUT_THING}}"
    type: core.ThingStatusChangeTrigger
  - id: "4"
    configuration:
      itemName: "{{INPUT_ITEM}}"
    type: core.ItemStateUpdateTrigger
conditions: []
actions:
  - inputs: {}
    id: "6"
    configuration:
      type: application/javascript;version=ECMAScript-2021
      script: >-
        /* global Java, event, items, actions */
        (function () {
          const THING_ID = '{{INPUT_THING}}'
          const INPUT_ITEM_NAME = '{{INPUT_ITEM}}'
          const RAW_ITEM_NAME = '{{ONLINE_STATUS_ITEM}}'
          const GRACE_PERIOD_SECS = parseInt('{{GRACE_PERIOD_SECS}}', 10)

          function calculateUptimeSeconds () {
            const ManagementFactory = Java.type('java.lang.management.ManagementFactory')
            const runtime = ManagementFactory.getRuntimeMXBean()

            const startedSecs = runtime.getStartTime() / 1000
            return startedSecs
          }

          const uptimeSecs = calculateUptimeSeconds()

          // We know that the trigger must be time interval based (cron) or "system started" when
          // we do not have any thing status info or item state information in the event
          const cronOrStarted = typeof event === 'undefined' || (event.statusInfo === undefined && event.itemState === undefined)
          const thingTurnedOffline = typeof event !== 'undefined' && event.statusInfo !== undefined && event.statusInfo.status.toString() === 'OFFLINE'
          const thingTurnedOnline = typeof event !== 'undefined' && event.statusInfo !== undefined && event.statusInfo.status.toString() === 'ONLINE'
          const itemUpdatedUndef = typeof event !== 'undefined' && event.itemState !== undefined && event.itemState.toString() === 'UNDEF'
          const itemUpdatedNonUndef = typeof event !== 'undefined' && event.itemState !== undefined && event.itemState.toString() !== 'UNDEF'

          // handle each case separately
          if (cronOrStarted) {
            if (((GRACE_PERIOD_SECS === 0) || (uptimeSecs > GRACE_PERIOD_SECS)) &&
                    (items.getItem(RAW_ITEM_NAME).state.toString() === 'NULL')) {
              // Init with NULL on startup unless we can consider the thing/state clearly "offline".
              // Further updates are based on thing status updates and state updates
              const inputItemStateStr = items.getItem(INPUT_ITEM_NAME).state.toString()
              const consideredNonAlive = (actions.Things.getThingStatusInfo(THING_ID).getStatus().toString() === 'OFFLINE' 
                || inputItemStateStr === 'UNDEF' || inputItemStateStr === 'NULL')
              const aliveState = consideredNonAlive ? 'OFF' : 'NULL'
              console.debug(INPUT_ITEM_NAME + ' / ' + THING_ID + ' initialization (' +
                        (consideredNonAlive ? 'considered non-alive' : 'considered as indeterminate state') +
                        ') -> alive state = ' + aliveState + ' (' + RAW_ITEM_NAME + ')')
              items.getItem(RAW_ITEM_NAME).postUpdate(aliveState)
            } else {
              console.debug(INPUT_ITEM_NAME + ' / ' + THING_ID + ' initialization: completed. Not updating active status (' 
                + RAW_ITEM_NAME + ')')
            }
          } else if (thingTurnedOffline) {
            console.debug(THING_ID + ' turned OFFLINE (' + event.statusInfo.description.toString() 
                + ') -> Considering non-alive (' + RAW_ITEM_NAME + ')')
            items.getItem(RAW_ITEM_NAME).postUpdate('OFF')
          } else if (thingTurnedOnline) {
            const inputItemStateStr = items.getItem(INPUT_ITEM_NAME).state.toString()
            const inputItemValid = inputItemStateStr !== 'UNDEF' && inputItemStateStr !== 'NULL'
            console.debug(THING_ID + ' turned ONLINE and input item state (' + inputItemStateStr + ') is ' +
                    (inputItemValid ? 'valid' : 'non-valid') +
                    ' -> Considering ' + (inputItemValid ? 'alive' : 'non-alive') + ' (' + RAW_ITEM_NAME + ')')
            const aliveState = inputItemValid ? 'ON' : 'OFF'
            items.getItem(RAW_ITEM_NAME).postUpdate(aliveState)
          } else if (itemUpdatedUndef) {
            console.debug(INPUT_ITEM_NAME + ' updated with UNDEF -> Considering non-alive' + ' (' + RAW_ITEM_NAME + ')')
            items.getItem(RAW_ITEM_NAME).postUpdate('OFF')
          } else if (itemUpdatedNonUndef) {
            console.debug(INPUT_ITEM_NAME + ' updated with non-UNDEF state -> Considering alive' + ' (' + RAW_ITEM_NAME + ')')
            items.getItem(RAW_ITEM_NAME).postUpdate('ON')
          } else {
            console.error('Unexpected condition')
          }
        })()


    type: script.ScriptAction