Monitoring

OpcUaWebServer provides a subscription model. The client can subscribe to a variable by using MONITORSTART_REQUEST. After that the server sends the data of the variable as MONITORUPDATE_MESSAGE only when it changes. Finally, the client must stop monitoring the value and send MONITORSTOP_REQUEST request.

Monitor Start Request

Field		Description
Header
	MessageType	Must be MONITORSTART_REQUEST.
	ClientHandler	See Message Format.
Body
	Variable	The variable to read.

Monitor Start Response

Field		Description
Header
	MessageType	Must be MONITORSTART_RESPONSE.
	ClientHandler	See Message Format.
Body
	[Status]	The OPC UA status if it is not Success.

Monitor Start Status Codes

Status Code	Description
BadInternalError	The server failed to process the request due to internal error.
BadAttributeInvalid	The server failed decode the body of the message.
BadNodeIdUnknown	The variable name isn’t found in the server configuration.

Monitor Update Message

Field			Description
Header
	MessageType		Must be MONITORUPDATE_MESSAGE.
	ClientHandler		See Message Format.
Body
	Value
		Body	The value of the variable.
		Type	The type of the variable.
	[Status]		The OPC UA status of the variable if it is not Success.
	SourceTimestamp		The time of the value given by the source in ISO 8601 format. Example: “2015-09-06T09:03:21Z”
	ServerTimestamp		The time of the value given by the server in ISO 8601 format. Example: “2015-09-06T09:03:21Z”

Monitor Stop Request

Field		Description
Header
	MessageType	Must be MONITORSTOP_REQUEST.
	ClientHandler	See Message Format.
Body
	Variable	The variable to read.

Monitor Stop Response

Field		Description
Header
	MessageType	Must be MONITORSTOP_RESPONSE.
	ClientHandler	See Message Format.
Body
	[Status]	The OPC UA status if it is not Success.

Monitor Stop Status Codes

Status Code	Description
BadInternalError	The server failed to process the request due to internal error.
BadAttributeInvalid	The server failed decode the body of the message.
BadNoEntryExists	The variable name isn’t found in the server configuration.

Example in Python

import websocket
import json

msg = {
    'Header': {
      'MessageType':'MONITORSTART_REQUEST',
      'ClientHandle':'1'
    },
    'Body': { 'Variable' : 'Boolean'}
 }

ws = websocket.create_connection('ws://127.0.0.1:8081')
ws.send(json.dumps(msg))
resp = ws.recv()
json.loads(resp)  #=> {
                  # 'Header': {
                  #   'MessageType': 'MONITORSTART_RESPONSE',
                  #   'ClientHandle': '1'},
                  #  'Body': ''
                  # }

resp = ws.recv()
json.loads(resp)  #=> {
                  # "Header": {
                  #    "MessageType": "MONITORUPDATE_MESSAGE",
                  #    "ClientHandle": "1"
                  # },
                  # "Body": {
                  #    "Value": {
                  #        "Type": 1,
                  #        "Body": true
                  #    },
                  #    "SourceTimestamp": "2019-07-26T11:10:20Z",
                  #    "ServerTimestamp": "2019-07-26T11:10:20Z"
                  # }
                  #}
msg = {
    'Header': {
      'MessageType':'MONITORSTOP_REQUEST',
      'ClientHandle':'1'
    },
    'Body': { 'Variable' : 'Boolean'}
}

ws.send(json.dumps(msg))
resp = ws.recv()
json.loads(resp)  #=> {
                  # 'Header': {
                  #   'MessageType': 'MONITORSTOP_RESPONSE',
                  #   'ClientHandle': '1'},
                  #  'Body': ''
                  # }