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': ''
# }