Follow the steps below to subscribe to tags and receive realtime tag updates (last value) via Web Sockets. The example will be using Postman.
The following article will demonstrate how to subscribe to a tag to receive real time tag changes via a web socket connection. Postman will be used to demonstrate / simulate the connection.
Connect
In Postman, make a new web socket connection and enter the following address for the Url to connect to.
Url ws://<yourServerAddressHere>:4511/api/subscribe
The hub that serves the connection is managed by the Real Time Historian component. Ensure you use the some port as your Historian is configured to utilise. The default port number is 4511. Also, remember to change the <yourServerAddressHere> to your respective node address(IP/FQDN) where your Historian is running
Press connect to start the web socket connection.
Send "Format" Message
This message must be sent before any others. It informs the Historian Live Streaming Hub of the message protocol and version that will be used.
{
"protocol":"json",
"version":1
}
Paste the message content into the Message box and press the "Send" command
Please note. The Json message is followed by a special character, . The ASCI code for this character is 30.
Send "Subscribe" Message
Send a "Subscribe" message to register for live updates of a tag. The message arguments are Dataset name and Tagname:
{
"arguments": ["<Dataset>", "<Tagname>"],
"target": "Subscribe",
"type": 1
}
Please remember to replace <Dataset> with the data set your tag resides in and the <Tagname> with the name of the tag you want to subscribe to.
Notice the messages appear for every new tag change value.
Send "Unsubscribe" Message
If live updates is no longer required, send an "Unsubscribe" message to deregister from live updates. The message arguments are Dataset name and Tagname:
{
"arguments": ["<Dataset>", "<Tagname>"],
"target": "Unsubscribe",
"type": 1
}
Please remember to replace <Dataset> with the data set your tag resides in and the <Tagname> with the name of the tag you want to subscribe to.