History System

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Introduction

The history system is an add-on capability built into the MIDAS data logger mlogger to record useful information in parallel to the data logging. This information is recorded with a time stamp and saved into a history file (in a special format of the form of a database) for later retrieval. One set of files is created per day containing all the requested history events. The history data may be displayed in graphical form using the mhttpd MIDAS webserver utility, giving the user an easy way of seeing how experimental variables have changed with time. The history logging will be in action only if the MIDAS data logger is running, but it is not necessary to have any data logging channel enabled.

There are currently three ways the history data may be saved

MIDAS history 
history is saved into a MIDAS History file. By default ODB subtree history channel 0 is set up to write MIDAS history.
ODBC history  
history is saved using ODBC driver. By default ODB subtree history channel 1 is set up to write ODBC history.
SQLITE history 
history data saved into an SQL database history channel 2 is set up to write SQLITE history.

Location of History Files

MIDAS history is enabled by default, writing history files into the MIDAS data directory path given by the ODB key Data dir. If history data is important, it is recommended to write it to a directory that is backed up or archived. (The MIDAS data directory is usually located on a scratch data disk without backups).

The location of the history data files can be changed by use of the ODB Key History dir.

Types of History Events

There are two basic types of history events, which are defined in different ways:

  • "Frontend" History event composed in the frontend. See Frontend History Event for details.
  • "Virtual" History event composed within the ODB under the specific tree /History/Links (see ODB History tree)

Both these history event definitions take effect when the data logger gets a start run transition. Any modifications made during the run are not applied until the start of the next run.


Frontend History Event

Each Equipment has the capability to generate "history data".

To write a Frontend History Event

In the history data directory you should see history files with name "YYMMDD.hst" corresponding to each day of midas operation. Contents of .hsr files can be examined directly using mhdump or indirectly using mhist or the history display page of mhttpd.

Enabling the history system for an Equipment causes the event contents to be copied to the ODB /Equipment/<equipment-name>/Variables subtree for the use of the History system . The contents of the event will then be completely copied into the history files.


In the case of MIDAS history, the definition of the event will be used as tag names for every element of the event.


The main data path for the frontend History Event is as follows:

   The user code in the frontend equipment Event Readout routines reads the data, placing it into a MIDAS data bank
   In mfe.c, if /Equipment/<equipment-name>/Common/Log history > 0 (i.e. the history system is enabled for this equipment), this data bank is written into ODB (by update_odb()).

The frequency of ODB writes is limited by ODB_UPDATE_TIME (1/sec in mfe.c rev 4298).

   The odb write triggers an odb hot-link into mlogger
   The hot-link calls mlogger.c::log_history(), which calls hs_write() to write the data into the history file. The frequency of the history writes is specified by the number of seconds between writes stored in the ODB key /Equipment/<Equipment-Name>/Common/Log history.
   history.c::hs_write() or history_odbc::hs_write_odbc() writes the data into a history file or into an SQL database.

The internal structure of MIDAS history files is documented in History format .


History Tag names

Tags are only used for MIDAS history events.

The mlogger/mhttpd implementation of /History/Tags has proved troublesome and we are moving away from it.


During the transition period
Implementations starting with Version 4435 of mlogger and mhttpd (i.e. "new" versions) will now work without /History/Tags. The history tags are read directly from the history files themselves. To run in this mode, set ODB key /History/DisableTags to "y". Existing /History/Tags subtree will be automatically deleted.

Two downsides to running like this are

  • it is slower and tags become non-persistent
  • if some frontends have not been running for a while, their variables may vanish from the history panel editor.


New-style History Tags

For the above two reasons, using /History/Tags is still recommended. However, the format of the tags is now changed to simplify management and reduce ODB size.

If you have old-style tags
mlogger will automatically convert the old tags to this new format (this is why you should make a backup of your ODB).
  • using "old" (i.e. pre Version 4435) mlogger with "new" mhttpd is OK: new mhttpd understands both formats of /History/Tags.
  • using "old" mhttpd with "new" mlogger is OK provided the ODB key /History/CreateOldTags is set to "y" before starting mlogger.


For new implementations
the key CreateOldTags will not be present. "New-style" history tags will be created automatically in the Tags subtree.


Old-style History Tags

Prior to versions 4435 of mlogger and mhttpd, old-style history tags were generated. The history variable name for each element of the Frontend History Event was composed as follows, where

  • "equipment-name" is the name of the equipment, and
  • "bankname" is the name of a bank generated by the "equipment-name" equipment :
In the case of a MIDAS format event
If the ODB key /Equipment/equipment-name/Settings/Names <bankname>[ ] IS present,
the history name is composed of the corresponding name found in the Names <bankname>[ ] array.
The size of this array must match the size of the /equipment/equipment-name/Variables/<bankname>[ ] array, or an error will be generated.
Otherwise the history name is composed of the bank name followed by the corresponding index of the element.
       See example .
In the case of a Fixed format event 
  • No array : the names of the individual element under /Equipment/equipment-name/Variables/ are used for the history name composition.
  • With array: if the key /Equipment/equipment-name/Settings/Names[ ] exists, each element of the array is referenced using the corresponding name of the ../Settings/Names[ ] array.