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Entry  26 Apr 2020, Yu Chen (SYSU), Forum, Questions and discussions on the Frontend ODB tree structure. 
    Reply  26 Apr 2020, Stefan Ritt, Forum, Questions and discussions on the Frontend ODB tree structure. 
Message ID: 1889     Entry time: 26 Apr 2020     In reply to: 1888
Author: Stefan Ritt 
Topic: Forum 
Subject: Questions and discussions on the Frontend ODB tree structure. 
Dear Yu Chen,

in my opinion, you can follow two strategies:

1) Follow the EQ_SLOW example from the distribution, and write some device driver for you hardware to control. There are dozens of experiments worldwide which use that scheme and it works ok for lots of 
different devices. By doing that, you get automatically things like write cache (only write a value to the actual device if it really has chande) and dead band (only write measured data to the ODB if it changes more 
than a certain value to suppress noise). Furthermore, you get events created automatically and history for all your measured variables. This scheme might look complicated on the first look, and it's quite old-
fashioned (no C++ yet) but it has proven stable over the last ~20 years or so. Maybe the biggest advantage of this system is that each device gets its own readout and control thread, so one slow device cannot 
block other devices. Once this has been introduced more than 10 years ago, we saw a big improvement in all experiments.

2) Do everything by yourself. This is the way you describe below. Of course you are free to do whatever you like, you will find "special" solutions for all your problems. But then you move away from the "standard 
scheme" and you loose all the benefits I described under 1) and of course you are on your own. If something does not work, it will be in your code and nobody from the community can help you.

So choose carefully between 1) and 2).

Best regards,

> Dear MIDAS developers and colleagues,
>     This is Yu CHEN of School of Physics, Sun Yat-sen University, China, working in the PandaX-III collaboration, an experiment under development to search the neutrinoless double 
> beta decay. We are working on the DAQ and slow control systems and would like to use Midas framework to communicate with the custom hardware systems, generally via Ethernet 
> interfaces. So currently we are focusing on the development of the FRONTEND program of Midas and have some questions and discussions on its ODB structure. Since I’m still not 
> experienced in the framework, it would be precious that you can provide some suggestions on the topic.
>     The current structure of the frontend ODB tree we have designed, together with our understanding on them, is as follows:
>       /Equipment/<frontend_name>/
>         ->  /Common/: Basic controls and left unchanged.
>         ->  /Variables/: (ODB records with MODE_READ) Monitored values that are AUTOMATICALLY updated from the bank data within each packed event. It is done by the update_odb() 
> function in mfe.cxx.
>         ->  /Statistics/: (ODB records with MODE_WRITE) Default status values that are AUTOMATICALLY updated by the calling of db_send_changed_records() within the mfe.cxx.
>         ->  /Settings/: All the user defined data can be located here. 
>             -> /stat/: (ODB records with MODE_WRITE) All the monitored values as well as program internal status. The update operation is done simultaneously when 
> db_send_changed_records() is called within the mfe.cxx.
>             -> /set/: (ODB records with MODE_READ) All the “Control” data used to configure the custom program and custom hardware.
>     For our application, some of the our detector equipment outputs small amount of status and monitored data together with the event data, so we currently choose not to use EQ_SLOW 
> and 3-layer drivers for the readout. Our solution is to create two ODB sub-trees in the /Settings/ similar to what the device_driver does. However, this could introduce two 
> troubles:
>     1) For /Settings/stat/: To prevent the potential destroy on the hot-links of /Variables/ and /Statistics/ sub-trees, all our status and monitored data are stored separately in 
> the /Settings/stat/ sub-tree. Another consideration is that some monitored data are not directly from the raw data, so packaging them into the Bank for later showing in /Variables/ 
> could somehow lead to a complicated readout() function. However, this solution may be complicated for history loggings. I have find that the ANALYZER modules could provide some 
> processes for writing to the /Variables/ sub-tree, so I would like to know whether an analyzer should be used in this case.
>     2) For /Settings/set/: The “control” data (similar to the “demand” data in the EQ_SLOW equipment) are currently put in several /Settings/set/ sub-trees where each key in 
> them is hot-linked to a pre-defined hardware configuration function. However, some control operations are not related to a certain ODB key or related to several ODB keys (e.g. 
> configuration the Ethernet sockets), so the dispatcher function should be assigned to the whole sub-tree, which I think can slow the response speed of the software. What we are 
> currently using is to setup a dedicated “control key”, and then the input of different value means different operations (e.g. 1 means socket opening, 2 means sending the UDP 
> packets to the target hardware, et al.). This “control key” is also used to develop the buttons to be shown on the Status/Custom webpage. However, we would like to have your 
> suggestions or better solutions on that, considering the stability and fast response of the control.
>     We are not sure whether the above understanding and troubles on the Midas framework are correct or they are just due to our limits on the knowledge of the framework, so we 
> really appreciate your knowledge and help for a better using on Midas. Thank you so much!
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