Hi, what you try should have worked. Perhaps your symlink is wrong and should say "/External/..." (with a leading slash). The "links period" would have
worked same as equipment/common/history period - as a rate limiter.

Anyhow, I suggest another way to do the same - create a fake equipment - the logger does
not care if the equipment is real or not and if you write into /eq/fake/variables from a proper frontend
or from a script. To hide the fake equipment from the status page, set /eq/fake/common/hidden to "true".

This will work for sure.

K.O.




> Hi,
> I am trying to set up virtual history events following
> https://midas.triumf.ca/MidasWiki/index.php/History_System#Virtual_History_Event 
> 
> Trying it the first way, using the following setup:
> Key name                        Type    #Val  Size  Last Opn Mode Value
> ---------------------------------------------------------------------------
> Links                           DIR
>     dirlink -> External/dir     KEY     1     12    >99d 0   RWD  <subdirectory>
> 
> 
> Key name                        Type    #Val  Size  Last Opn Mode Value
> ---------------------------------------------------------------------------
> External                        DIR
>     dir                         DIR
>         foo                     FLOAT   1     4     16s  0   RWD  12.5
> 
> 
> Then I get the following error message:
> ==================== History link "dirlink", ID 28150  =======================
> [Logger,ERROR] [mlogger.cxx:4942:open_history,ERROR] History event dirlink has
> no variables in ODB
> 
> 
> Trying the second way, I set up the following:
> Key name                        Type    #Val  Size  Last Opn Mode Value
> ---------------------------------------------------------------------------
> Links                           DIR
>     dir                         DIR
>         testlink -> External/foo
>                                 FLOAT   1     4     8m   0   RWD  5.2
> 
> Key name                        Type    #Val  Size  Last Opn Mode Value
> ---------------------------------------------------------------------------
> External                        DIR
>     foo                         FLOAT   1     4     6m   0   RWD  5.2
> 
> 
> Starting mlogger in verbose mode yields the following error:
> ==================== History link "dir", ID 28150  =======================
> [Logger,ERROR] [mlogger.cxx:4935:open_history,ERROR] History link
> /History/Links/dir/testlink is invalid
> Error in history system, aborting startup.
> 
> I'm not sure if this is a bug or just a case of PEBCAK.
> 
> Finally, to set the update period, do I need entries in /history/links periods
> with the tag name? Is there a way to only write them in the history file when
> they change? I want to use the virtual history events for measurements I get
> from external scripts, some periodic, some manual.
> 
> Thanks