HTTP GET only works once

[PatSpring]

Web UI Builder:

I have a device that responds to HTTP GET messages.

I made a simple vi that uses the HTTP GET String function connected to a string constant and WireShark to monitor Ethernet traffic.

When the vi is first run an HTTP message is sent and replied to.

If the vi is run again the message is not transmitted.

If the HTTP GET message is changed the new message will be transmitted. But not twice.

If LabVIEW Web UI Builder is closed and restarted the message will again be sent just once when run.

Why is the message only being sent once? It's like all outgoing messages are being tracked and only sent once.

[malcolmsmith]

I think this is standard (but not always desirable) behavior of the networking stack that Web UI Builder uses. (On Windows, it's basically the built in/  Internet Explorer networking stack). After the first GET request with the same URL, if the server doesn't send no-cache headers, a cached result will be returned.

If you control the server that you're connecting to (and can add HTTP headers), try adding a no-cache header to the GET response. ("Cache-Control: no-cache" is the first one I'd try).

If you don't control the server you're connecting to / that doesn't work, another workaround is to append a query string parameter with a random number or timestamp. With this approach, the GET URL is always changing, so the caching behavior is disabled and the request should always go to the server.

(Something like this appended to the end of the URL:

or, "&n=[timeInTicks]" if you're already using other query parameters)

[PatSpring]

I think it's caching also. I did a little reading about it and that makes sense.

The random number/time trick doesn't work because all the previous GET's are being cached.

Thanks for taking the time to reply! 🙂