group cache in Cometvisu

[ManfredA]

Hello;
I am trying to use the knxd as a backend of Cometvisu and I read in a document that :
"Note: To run the CometVisu it is important to activate the group cache (i.e. the parameter -c)."
Before that I run the Cometvisu whithout the "-c" argument..
I want to know the meaning of group cache, and what happen if it is disabled or enabled?

Best Regards.

[Chris M.]

When you open the CometVisu in a browser it must get all the states (e.g. which lights are on, the temperatures, ...).

During startup without cache:
To get such a state a read request can be sent. With many states it will flood the bus with requests an answers thus delay any action there are as well as take a while till the CometVisu is displaying the values. This will happen each time a client opens (PC, smartphone, ...)

During startup with cache:
The states are read out of the cache immediately and only those not in the cache will be read from the bus. This will cause hardly any till no bus traffic.

But also when the CometVisu is running the cache is needed:
When a new data of interest is appearing on the bus it is sent immediately to the client (that's done via the Comet pattern - the reason why the CometVisu has its name). Once the client has received that data it will also immediately open a new read request to the server again.
When the knxd has its cache active this request will check there whether new data of interest happened during that last request and the new request.
When the knxd is running without the cache this information is lost and will never appear on the browser. So you are running the risk that states shown on the screen are not matching reality any more.