> Let's improve the midas history system...

After implementing 2 prototypes, one aspect of the new design is starting to firm up enough to write it down (I do so in a mock FAQ format).

Q. I ran an experiment at triumf, returned home and now I have a bunch of midas history files (*.hst) on my laptop. How do I export these history 
data to some useful format?
A. Run "mhdump *.hst | import_to_sql.perl" or "mh2ttree -o history.root *.hst" (export to mysql or ROOT TTree respectively). (TBW: 
import_to_sql.perl and mh2ttree)

Q. I have all these midas history files (*.hst), how do I look at them with mhttpd?
A. Follow these steps:
1) setup a blank experiment (no frontends, no analyzer, no mlogger), make sure you can run odbedit and mhttpd.
2) put (symlink) the history files into the history (data) directory
3) run "mhdump -t *.hst > tags.cmd"
4) run "odbedit -c @tags.cmd"
5) start mhttpd, go to the "history" page, setup history plots
6) look at history plots as usual

As always, all the cool stuff is happening behind the scenes:

- in step (3) and (4) we create ODB entries for all events and tags in the history files:
/history/tags/2 = "Trigger"   <--- declare event 2 "Trigger" (was equipment "Trigger" while we were taking data)
/history/tags/2:Rate = 1       <--- declare tag "Rate" as an array of one element
/history/tags/2:Scalers = 10 <--- declare tag "Scalers" as an array of 10 elements
... and so forth for each event and tag that ever existed in the history files.

When running a live experiment, the /history/tags entries are created by the mlogger.

- in step (5), the history plot setup page reads the names of history events and tags from /history/tags. The existing code for extracting the 
names of events and tags from the /equipment tree goes away. The variables part of history plots are saved the same way as now, i.e. 
"Trigger:Rate" and "Trigger:Scalers[3]" - existing plot definitions continue working as before.

- in step (6), to plot the variable named "Trigger:Scalers[3]", the mhttpd code again reads /history/tags to find out that "Trigger" corresponds to 
event id 2 and "Scalers" is a valid array (of size 10). This is enough to call hs_read() with the correct arguments to read the existing .hst files - the 
existing code will even regenerate the .idx and .def history files.

How do existing experiments migrate to the new code? It is all automatic, no user actions needed. For writing history files, there are no changes. 
For reading history files, the "new mhttpd" expects to find /history/tags, which will be created automatically by the "new mlogger".

I am presently cleaning up the implementation of this idea in mhttpd and in the mlogger (only those 2 files are affected- 2 functions in mhttpd.c 
and 1 function in mlogger.c) and after some testing it will be ready for commiting to midas svn.

The next step would be changes in mlogger.c for recording the history for each variable separately (each variable gets it's own event id). I have 
this implemented, but interaction with mhttpd is still in flux and I may want to run the new code at CERN for a few months before I deem it stable 
enough for general use.

K.O.