Re: Change PVA structure dynamically during IOC operation

[Andrew Johnson via Tech-talk <tech-talk at aps.anl.gov>]

On 9/28/22 1:25 PM, Kasemir, Kay via Tech-talk wrote:

> I need to change a PVA structure dynamically during IOC operation. Looking at pvxs::ioc::server, I do not see an update function that allows that. Does anyone know if this is possible?

I don't think that's possible.

If you check the protocol documentation, https://github.com/epics-base/pvAccessCPP/wiki/protocol, you'll find that the overall structure information is exchanged once when a client connects. From then on, only a compact bitmap that indicates what changed is sent, plus the modified values.

To change the structure, the protocol would have to send the complete new structure.

Arrays can change the number of elements, but the overall structure stays the same because the protocol was optimized for this.

PVA does support unions though, and an "any" type, so assuming you can design all the versions of the structure that you will need up front (and that the pvxs implementation supports them) you should be able to use them to make changes to your PVA structure without having to withdraw and re-add the PV. I believe a union will be more efficient than using an "any".

- Andrew

-- 
Complexity comes for free, Simplicity you have to work for.