Enable onChange Feature on TagHub.md

Enable onChange Feature on TagHub

Document Version: V1.0

Change Log

Version	Date	Content
1.0	2022-02-10	Document created

Purpose

This document demonstrates how to use tpFunc/Function to extend current TagHub with onChange tag featrues.

Note: tpFunc required ThingsPro Edge V2.2.1+

---

Install and Setup

1. Login to your ThingsPro Edge SSH console with default path (/home/moxa).

2. Switch role to su

   sudo su
   

3. Download onChangeTag.tar and unpack it

   wget https://tpe2.azureedge.net/Python3/onChangeTag.tar
   
   tar -xvf onChangeTag.tar
   

   This command shall create onChangeTag. folder, and contains package.json and index.py files.

   root@Moxa:/home/moxa/onChangeTag# ls -l
   total 3
   -rwxrwxrwx 1 root root 1195 Feb 13 14:23 index.py
   -rwxrwxrwx 1 root root  645 Feb 13 14:23 package.json
   

4. Understand package.json

   {
     "name": "onChangeTag",
     "enabled": true,
     "trigger": {
       "driven": "dataDriven",
       "dataDriven": {
         "tags": {
           "system": {
             "status": [
               "cpuUsage"
             ]
           }
         },
         "events": {}
       },
       "timeDriven": {
         "mode": "boot",
         "intervalSec": 1,
         "cronJob": ""
       }
     },
     "expose": {
       "tags": [
         {
           "prvdName": "virtual",
           "srcName": "onChange",
           "tagName": "cpuUsage",
           "dataType": "double",
           "access": "rw"
         }
       ]
     },
     "executable": {
       "language": "python"
     },
     "params": {}
   }
   

   • This function configured with data-driven pattern, and declare below tags to subscribe
     • tag(system/status/cpuUsage)
   • It is free to add others tags under subscribe list. Once the value of subscribed tags be updated, tpFunc will call the method, onChangeTag() , which defined in index.py.
   • For each subscribe tags, it is make sense to define associated "onChange" Tag, such as
     • tag(system/status/cpuUsage) associated with tag(virtual/onChange/cpuUsage)
   • Please aware that "onChange" tag shall has same dataType with it's origin, and with "read/write" access attribute.

5. Understand index.py

   from thingspro.edge.tag_v1 import tag as tpeTAG
   
   cacheValue = {}
   cacheValue["cpuUsage"] = None
   
   def onChangeTag(_type, data):    
       publisher = tpeTAG.Publisher()
       
       if (_type == "tag"):
           if (data["prvdName"]=="system") and (data["srcName"] == "status") and (data["tagName"] == "cpuUsage"):
               if (cacheValue["cpuUsage"] == None) or (cacheValue["cpuUsage"] != data["dataValue"]):   
                   print("Cache value of virtual/onChange/cpuUsage : " + str(cacheValue["cpuUsage"]))             
                   newTag = {
                       'prvdName': "virtual",
                       'srcName': "onChange",
                       'tagName': "cpuUsage",            
                       'dataValue': data["dataValue"],
                       'dataType' : "double",
                       'ts': data["ts"]
                       }    
                   publisher.publish(newTag)                
                   print("updated virtual/onChange/cpuUsage by new vlaue: " + str(data["dataValue"]))
                   cacheValue["cpuUsage"] = data["dataValue"]
   

   • cacheValue is python dict object, which be used to store onChange Tags value in RAM.

   • onChangeTag() method, shall be with same named as 'function name' in package.json.

   • onChangeTag() compares incoming Tag value with cacheValue, and publish Tag value to onChange Tag if them are different, like below diagram.

     

Add to tpFunc and Run

1. Add it to tpFunc

   cd /home/moxa
   sudo su
   tpfunc add onChangeTag
   

2. Verify there is no error on creation

   root@Moxa:/home/moxa# tpfunc ls
   +-------------+--------+------------+---------------------------+---------+-------+
   |    NAME     | ENABLE |    MODE    |        LASTUPTIME         |  STATE  | ERROR |
   +-------------+--------+------------+---------------------------+---------+-------+
   | onChangeTag | true   | dataDriven | 2022-02-13T14:51:54+08:00 | running |       |
   +-------------+--------+------------+---------------------------+---------+-------+
   

Tips

When onChangeTag function running, other applications or functions could subscribe these onChange Tags, when needed, to reduce complexity of these applications implementation.