History System

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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 (see History drivers) 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.


New history configuration defines history channels similar to logger channels. Each history channel creates a history interface class (class MidasHistoryInterface). The mlogger can use this interface to write data into the history, mhttpd, mhist & co can use this interface to read data from the history. The interface is defined in history.h

History drivers

The MIDAS distribution contains these drivers:

MIDAS history
the traditional MIDAS history, data is stored in the .hst binary files. (format is documented here mhformat, see also mhist and mhdump)
By default ODB subtree history channel 0 is set up to write MIDAS history.
ODBC history
stores history data in SQL database using the ODBC abstraction layer. MySQL is fully supported, Pgsql support is partially implemented, support for arbitrary ODBC driver is easy to add.
By default ODB subtree history channel 1 is set up to write ODBC history.
SQLITE history
stores data in SQLITE3 .sqlite3 files. The SQL table format is similar to ODBC history, performance is similar, no database server required.
history data saved into an SQL database history channel 2 is set up to write SQLITE history.

More drivers can be easily added by the user (create history_xxx.cxx, add constructor to hs_get_history(), create the ODB subtree /Logger/History/x/channel).

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

History files will be saved into the history data directory.

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 the key Log history > 0 (i.e. the history system is enabled for this equipment), this data bank is written into ODB (by update_odb()).
  • This 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 Log history.
  • history.c::hs_write() or history_odbc::hs_write_odbc() writes the data into a history file or into an SQL database.

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.

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

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.

Old-style History Tags

Prior to versions 4435 of mlogger and mhttpd, old-style history tags were generated. See

for details.