Historical Read

To read the historical values of a variable, the client must use HISTORICALREAD_REQUEST request.

Historical Read Request

Field		Description
Header
	MessageType	Must be HISTORICALREAD_REQUEST.
	ClientHandler	See Message Format.
Body
	Variable	The variable to read
	StartTime	Beginning of period to read in ISO 8601 format. Example: “2015-09-06T09:03:21Z”
	Endtime	End of period to read in ISO 8601 format. Example: “2015-09-06T09:03:21Z”

Historical Read Response

Field					Description
Header
	MessageType				Must be HISTORICALREAD_RESPONSE.
	ClientHandler				See Message Format.
Body
	@DataValues				The history data
		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: “2015-09-06T09:03:21.237123”
		ServerTimestamp			The time of the value given by the server in ISO 8601 format. Example: “2015-09-06T09:03:21.237123”

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.
BadSessionClosed	The connection with OPC UA server is lost.

Example in Python

import websocket
import json

msg = {
    'Header': {
      'MessageType':'HISTORICALREAD_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": "READ_RESPONSE",
                  #    "ClientHandle": "1"
                  # },
                  # "Body": {
                  #    "Value": {
                  #        "Type": "Boolean",
                  #        "Body": true
                  #    },
                  #    "SourceTimestamp": "2019-07-26T11:10:20Z",
                  #    "ServerTimestamp": "2019-07-26T11:10:20Z"
                  # }
                  #}