Can we fetch real time data using PI WEB API?
Definitely! You can use the stream controller to get the value at the current time.
Here is the function you would use: https://techsupport.osisoft.com/Documentation/PI-Web-API/help/controllers/stream/actions/getvalue.html
To complement Paul Martin's answer, you can use StreamSet Controller to query data for multiple unrelated items in a single call. Another option is using the Batch Controller which allows you to bundle calls.
Another way is to use the HTML5 web sockets to fetch real time data which also send messages about modifications or deletions of previous values.
Also do take note that this is a Community Technology Preview (CTP)
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