Experimental Physics and
Industrial Control System

Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov> · Sun, 28 Nov 2021 19:31:55 -0800

On 11/25/21 10:07 PM, Kuldeep Joshi via Tech-talk wrote:
> Hello,
> I understand that the PVAccess protocol sends only the data that changed. 
> I was seeing some demo codes in phoebus using PVAccess https://github.com/ControlSystemStudio/phoebus/blob/master/app/display/model/src/main/resources/examples/table/server.py <https://github.com/ControlSystemStudio/phoebus/blob/master/app/display/model/src/main/resources/examples/table/server.py>. where the whole structure is updated.

With P4P change tracking is mostly automatic.

https://mdavidsaver.github.io/p4p/values.html#change-tracking

> In my case the top level PVA object is epics:nt/NTTable:1.0 .I wanted to incrementally add the data to the table.

However, I don't think this will help you much in the case of
NTTable as the PVA delta encoding operations on whole fields.
So an array field, like those which represent each columns of
a NTTable, will always be sent in their entirety.

So eg. if the contents of a column don't change, then there is
no need to re-send.  However, if one cell is changed, added,
or removed, then the entire column needs to be re-sent.

For this reason I would suggest thinking in terms of publishing
a new set of rows with each subscription/monitor update, and not
of appending as this will necessitate inefficient duplication
over the network.

> How do I do so?
> 
> PS: I am new to PVAccess
> 
> Regards,
> Kuldeep

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System