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Message ID: 2283     Entry time: 11 Oct 2021
Author: Stefan Ritt 
Topic: Info 
Subject: Modification in the history logging system 
A requested change in the history logging system has been made today. Previously, history values were
logged with a maximum frequency (usually once per second) but also with a minimum frequency, meaning
that values were logged for example every 60 seconds, even if they did not change. This causes a problem.
If a frontend is inactive or crashed which produces variables to be logged, one cannot distinguish between
a crashed or inactive frontend program or a history value which simply did not change much over time.
The history system was designed from the beginning in a way that values are only logged when they actually
change. This design pattern was broken since about spring 2021, see for example this issue:

Today I modified the history code to fix this issue. History logging is now controlled by the value of 
common/Log history in the following way:

* Common/Log history = 0 means no history logging
* Common/Log history = 1 means log whenever the value changes in the ODB
* Common/Log history = N means log whenever the value changes in the ODB and 
  the previous write was more than N seconds ago

So most experiments should be happy with 0 or 1. Only experiments which have fluctuating values due to noisy 
sensors might benefit from a value larger than 1 to limit the history logging. Anyhow this is not the preferred 
way to limit history logging. This should be done by the front-end limiting the updates to the ODB. Most of the 
midas slow control drivers have a “threshold” value. Only if the input changes by more then the threshold are 
written to the ODB. This allows a per-channel “dead band” and not a per-event limit on history logging 
as ‘log history’ would do. In addition, the threshold reduces the write accesses to the ODB, although that is
only important for very large experiments.

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