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Entry  05 Aug 2019, Stefan Ritt, Info, Precedence of equipment/common structure 
    Reply  06 Aug 2019, Thomas Lindner, Info, Precedence of equipment/common structure 
       Reply  06 Aug 2019, Stefan Ritt, Info, Precedence of equipment/common structure 
          Reply  06 Aug 2019, Stefan Ritt, Info, Precedence of equipment/common structure 
    Reply  09 Aug 2019, Konstantin Olchanski, Info, Precedence of equipment/common structure 
       Reply  13 Aug 2019, Stefan Ritt, Info, Precedence of equipment/common structure 
Message ID: 1643     Entry time: 06 Aug 2019     In reply to: 1642     Reply to this: 1644
Author: Stefan Ritt 
Topic: Info 
Subject: Precedence of equipment/common structure 
Hi Thomas,

the change only affects Eqipment/<name>/common not the Equipment/<name>/Settings. 

The Common subtree is still hot-linked into the frontend, so when running things can be changed if needed. This mainly concerns the readout period of periodic events. 
Sometimes you want to change this quickly without restarting the frontend. Changing the other settings are kind of dangerous. If you change the ID of an event on the fly
you won't be able to analyze your data. So having this read-only in the ODB might be a good idea (you still need it in the ODB for the status page), except for the values
you want to change (like the readout period). 

Let's see what other people have to say.


> Hi Stefan,
> This change does not sound like a good idea to me.  I think that this change will cause just as much confusion as before; probably more since you are changing established behaviour.
> It is common that MIDAS frontends usually have a Settings directory in the ODB where details about the frontend behaviour are set.  The Settings directory might get initialized from strings in the frontend code, but after initialization the Settings in the ODB have precedence and define how the frontend will behave.  Indeed, most of my custom webpages are designed to control my frontend programs through their Settings ODB tree.
> So you have created a situation where Frontend/Settings in the ODB has precedence and is the main place for changing frontend behaviour; but Frontend/Common in the ODB is essentially meaningless and will get overwritten the next time the frontend restarts.  That seems likely to confuse people. 
> If you really want to make this change I suggest that you delete the Frontend/Common directory entirely; or make it read-only so that people aren't fooled into changing it.
> Thomas 
> > Today I fixed a long-annoying problem. We have in each front-end an equipment structure 
> > which defined the event id, event type, readout frequency etc. This is mapped to the ODB 
> > subtree
> > 
> > /Equipment/<name>/Common
> > 
> > In the past, the ODB setting took precedence over the frontend structure. We defined this 
> > like 25 years ago and I forgot what the exact reason was. It causes however many people 
> > (including myself) to fall into this trap: You change something in the front-end EQUIPMENT 
> > structure, you restart the front-end, but the new setting does not take effect since the 
> > (old) ODB value took precedence. After some debugging you find out that you have to both 
> > change the EQUIPMENT structure (which defines the default value for a fresh ODB) and the 
> > ODB value itself.
> > 
> > So I changed it in the current develop tree that the front-end structure takes precedence. 
> > You still have a hot-link, so if you want to change anything while the front-end is running 
> > (like the readout period), you can do that in the ODB and it takes effect immediately. But 
> > when you start the front-end the next time, the value from the EQUIPMENT structure is 
> > taken again. So please be aware of this new feature.
> > 
> > Happy BC day,
> > Stefan
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