> ... the xml reader, probably has same problem
> ... xml writer truncates long strings via truncation in db_sprintf()

Removed truncation of overlong strings in the xml writer and confirmed that xml reader handles them correctly (always loaded overlong strings correctly).

Both JSON and XML odb dumps now handle strings of unlimited size correctly.

K.O.

The mhttpd json-rpc interface now implements batch requests per
http://www.jsonrpc.org/specification#batch

In the nutshell, instead of a single request, one can send a json array of requests and receive a json 
array of replies.

As a variance from the spec, the midas implementation executes the requests strictly in-order and 
the array of replies corresponds exactly to the array of requests (the spec requires user to use the 
"id" field to match replies to requests, in midas json-rpc, the 1st reply is always to the 1st request,
2nd reply is to the 2nd request and so forth).

See this in action look at resources/example.html and in resources/transition.html

K.O.

Notes on installing midas from scratch. The instruction on midaswiki will be synced with this later.

cd ~/packages
git clone ...
cd midas
make
cd ~
mkdir ~/online
cd ~/online
~/git/midas/darwin/bin/odbinit --env
source env.sh
~/git/midas/darwin/bin/odbinit --exptab
~/git/midas/darwin/bin/odbinit
ls -la
send:online olchansk$ ls -la
total 2376
drwxr-xr-x   15 olchansk  staff      510 Aug  4 15:34 .
drwxr-xr-x+ 244 olchansk  staff     8296 Aug  4 15:33 ..
-rw-r--r--    1 olchansk  staff        0 Aug  4 15:34 .ALARM.SHM
-rw-r--r--    1 olchansk  staff        0 Aug  4 15:34 .ELOG.SHM
-rw-r--r--    1 olchansk  staff        0 Aug  4 15:34 .HISTORY.SHM
-rw-r--r--    1 olchansk  staff        0 Aug  4 15:34 .MSG.SHM
-rw-r--r--    1 olchansk  staff  1183808 Aug  4 15:34 .ODB.SHM
-rw-r--r--    1 olchansk  staff        8 Aug  4 15:34 .ODB_SIZE.TXT
-rw-r--r--    1 olchansk  staff       15 Aug  4 15:34 .SHM_HOST.TXT
-rw-r--r--    1 olchansk  staff       12 Aug  4 15:34 .SHM_TYPE.TXT
-rw-r--r--    1 olchansk  staff        0 Aug  4 15:34 .SYSMSG.SHM
-rw-r--r--    1 olchansk  staff      341 Aug  4 15:33 env.csh
-rw-r--r--    1 olchansk  staff      322 Aug  4 15:33 env.sh
-rw-r--r--    1 olchansk  staff       40 Aug  4 15:34 exptab
-rw-r--r--    1 olchansk  staff      287 Aug  4 15:34 midas.log
send:online olchansk$

odbedit ### works
mhttpd ### bombs, requires SSL certificate https://bitbucket.org/tmidas/midas/issues/57/initial-mhttpd-should-bind-to-localhost
odbedit ### cd /experiment, set "http redirect to https" to no, set "midas https port" to 0
mhttpd ### runs now
connect to http://localhost:8080 ### status page works
restart mhttpd as mhttpd -D
mlogger -D
fetest ### runs, prints time and data
start a run from web page ### works
### fetest generates crazy data rate https://bitbucket.org/tmidas/midas/issues/58/fetest-crazy-data-rate
### go to history, define plot for SLOW/SLOW, see sine wave ### works
### history is written to expt dir, no good, go to "history"
### data files written to expt dir, no good, go to "data"
### midas.log written to data dir, no good (want expt dir)
### elog written to expt dir, go to "elog"
### logger channel config is wrong - gzip compression and crc32c should be enabled by default
### history config is wrong - FILE per-variable history should be enabled by default

K.O.
 

The checks for byte sizes of critical data structures have been added to ensure (enforce) binary compatibility
of midas with itself on different platforms (32-bit and 64-bit intel, on PPC, on ARM, etc).

This has worked well in the past and helped avoid problems and subtle bugs in the transition
from 32-bit to 64-bit machines a few years ago. Of course now 32-bit machines are back
as ARM CPUs and FPGA synthetic CPUs.

Replacing the checks with "computed" values will defeat this purpose because the values may be computed
differently on different machines.

Specifically as proposed by Stefan, sizeof(int) can change depending on the target machine and depending
on the compiler settings.

Of course this needs to be balanced against flexibility to adjust important settings like MAX_CLIENTS and MAX_EVENT_REQUESTS.

I would say the present system is just fine. You can change MAX_CLIENTS, rebuild MIDAS and it will not run (assert failure) giving
you an indication that you are doing something non-trivial that will cause problems if you do it without thinking about it.

For example, one may think nothing of changing midas.h and recompiling MIDAS. But having to change odb.c
may ring the little bell to tell you that you *also* have to rebuild *all* of your frontends. Even one unrebuilt frontend
will corrupt all shared memory and crash everything.

I guess one other way to look at this is as a balance between something a few people do rarely against
a function that protects everybody all the time.

That said, I think the checks should be reworked, instead of an assert failure they should give the error message
and tell the user exactly what number to adjust in the size test. Also some checks are obsolete, there is no longer
need to check the size of many ODB structures (equipment, etc). Once we are done with the db_get_record() rework,
only checks for data structures in shared memory shall remain.

As the bottom line, to change MAX_CLIENTS, you already have to edit midas.h, asking you to also edit odb.c does
not add much to the burden.

P.S. We are thinking how to make all these values dynamically changable, but basically it requires rolling out
a new binary-incompatible version of MIDAS with added bugs. Maybe some day.

K.O.


> The sizeof checks were originally invented by KO to check for binary compatibility between processes attached to the same ODB and event buffers. So if a 
> compiler generates different structure sizes due to different padding, one would see that immediately. I wonder however if the absolute numbers make sense 
> here. We could replace the 16444 by
> 
> NAME_LENGTH + 7*sizeof(INT) + MAX_CLIENTS *(NAME_LENGTH+13*sizeof(INT)+sizeof(float)+2*sizeof(DWORD)+MAX_EVENT_REQUESTS*4*sizeof(INT))
> 
> which makes this value automatically scale when one changes MAX_CLIENTS.
> 
> People of course have to be aware that if one changes MAX_CLIENTS, then all programs connected to the same ODB or event buffer need to be re-compiled 
> and the ODB needs to be re-created from an ASCII file, but at least this would avoid tedious manual calculations.
> 
> Any opinion?
> 
> Stefan
> 
> 
> > Below are the steps we used to increase the maximum number of frontends that we could run.
> > 
> > In midas.h
> > 
> > #define MAX_CLIENTS            64
> > 
> > changed to
> > 
> > #define MAX_CLIENTS            128
> > 
> > In msystem.h:
> > 
> > #define MAX_RPC_CONNECTION     64
> > 
> > changed to
> > 
> > #define MAX_RPC_CONNECTION     128
> > 
> > In odb.c:
> > 
> > assert(sizeof(BUFFER_HEADER) == 16444); 
> > 
> > GUESS: 256*64+60 = 16444, so change 64 to 128
> > 
> > changed to:                                                                                                                         
> > 
> > assert(sizeof(BUFFER_HEADER) == 32828); //256*128+60
> > 
> >  
> > 
> > DATABASE_HEADER = 64 + 64*DATABASE_CLIENT = 64 + 64*8256 = 528448
> > 
> > changed to:
> > 
> > DATABASE_HEADER = 64 + 128*DATABASE_CLIENT = 64 + 128*8256 = 1056832.

> if (sizeof(INT) != 4) then severe_error_and_stop_all_programs()

Quick reply.

Today, for fixed size data types one should use uint32_t & co, see
stdint.h
https://en.wikipedia.org/wiki/C_data_types#stdint.h
https://en.wikipedia.org/wiki/C99 (scroll down and click to open "implementation -> compiler support"

The other popular convention is "u32" used by the Linux kernel, you will see it in the linux kernel drivers.

If I remember right, WORD and DWORD grow legs from the 16-bit Motorolla 68xxx processors,
VxWorks and the VME bus. At some point the data buses were 16-bit wide and that we the WORD.

(I do not think UNIX ever used the WORD/DWORD names, i.e. MacOS has int32_t and u_int32_t).

K.O.

Support for building for ucLinux was added to MIDAS. I use the emcraft toolchain and userland on 
some kind of embedded ARM CPU that does not have an MMU. See the Makefile for details. The 
main difference of ucLinux is lack of fork(), which cannot be done without an MMU. Not everything 
works, but at the least I can run a frontend and connect to an experiment on a remote host 
computer (mserver connection). K.O.

multithreaded access to odb was implemented back in 2013-2014. but recently a bug surfaced - 
there was a race condition in the odb locking code against cm_watchdog(). Somehow this only 
affected the mserver for the DRAGON experiment at TRIUMF. This is now fixed on the branch 
feature/midas-2017-10. (this branch collects all the code that needs additional testing before 
merging into develop and becoming the next release of midas).
K.O.

I the agdaq system I see memory corruption in the mlogger. There were at least two bugs: one 
memory allocation error in mxml and one incorrect memset() in mlogger.cxx. The mxml bug is fixed 
in the mxml repository, mlogger.cxx bug is fixed in the midas-2017-10 branch.

I suggest that all update mxml to the latest version: (without waiting for the new midas release)
https://bitbucket.org/tmidas/mxml/commits/branch/master

K.O.

> Currently one has to set the required flag in the ODB (e.g., /Programs/Logger/Required) to "y" for the program 
> to appear on the "Programs page" and being able to start and stop the program easily.
> 
> However, if one wants to run with the "Prevent start on required progs" in /Experiment enabled, all the 
> programs in the "Programs page" need to be running and one cannot have one of them stopped while still 
> taking a run.
> 
> It would be nice to separate these two functionalities: Have a flag that makes the program appear on the 
> "Programs page" and have a flag that controls the "Prevent start on required frogs" functionality.

I agree. All the programs should be always visible on the "programs" page, there should be /Programs/xxx/hidden to 
hide them, and /Programs/xxx/required should be used for "Prevent start on required progs".

K.O.

Hi, Frederick, this is my personal opinion on the slow controls hv classes, I have
used them a couple of times and I found them full of little buglets like this,
plus some incomplete functions, plus some missing features, plus it is all
written in C trying to do object oriented programming. On the balance my opinion
is that it is less work to write a high voltage control program in C++ from scratch
using the regular midas frontend infrastructure compared to having to understand
the hv class driver, write the missing bits, fix the little buglets, debug
the crashes in the C string handling, and what not. (For example I had to debug
mysterious failures to pass float and double values through the C stdarg interface,
there are more fun things to do out there).

K.O.

> bug in init
> -----------
> 
> I used the lv.c class driver, combined with a custom device driver, to control 
> our Keithley2611B source meter. This to set negative voltage on Si detectors.
> 
> In the 'init' routing, the class driver sets the hv:
> 
>   hv_info->demand_mirror[i] = MIN(hv_info->demand[i], hv_info->voltage_limit[i]);
> 
> This fails for negative voltage, as it sets the (negative) voltage limit, instead 
> of the demand voltage. A simple 'fabs' solves this.
> 
> suggestion for 'idle'
> ---------------------
> 
> I let the device do the ramping, not the driver. This also means I have to reset 
> the state of the device (current limit) after ramping. The easiest way to to 
> this, is using CMD_IDLE of the device driver. This is currently not done in the 
> hv.c class driver.

> 
> The original idea behind the hv driver is that all voltages in the ODB and the class driver are
> positive. If you have a negative power supply, then the voltage is inverted at the device
> driver level. That's why you have MIN and MAX in the class driver.
> 

This rings a bell. I used the hv class driver to write a frontend for the L1440 mainframe (negative voltage),
on ODB it will be positive values, when writing to the device I had to add a minus sign,
and when reading back they came back negative and I had to add an fabs() in the comparison
between readback and demand.

Persons with bipolar power supplies need not apply.

K.O.

It has been reported that the current midas release candidate does not build on el5 linux (SL/RHEL/CentOS-5).

According to Red Hat, el5 is end-of-life, last SL 5 (SL5.11) was done in 2014, so this linux is very old. Also as it happens, I do not have access to any 
el5 machines to check if midas builds or runs (but this can be fixed).

https://www.scientificlinux.org/downloads/sl-versions/sl5/
https://access.redhat.com/support/policy/updates/errata

On the midas web page (https://midas.triumf.ca) we do not explicitly state which versions of which linux we definitely support. Most other open-
source projects only support current major linux distributions, hardly anybody supports end-of-life linuxes such as el5. Some projects do not even 
support recent linuxes still widely in use (ROOT6 does not build on stock el6 and there is no KDE5 for el7).

So back to midas. Support for different operating systems comes down to:

1) C/C++ language support. We still use el6 (GCC 4.4.7), so use of c++-11 language features should be avoided
2) operating system features support:
a) sysv semaphores (sysv shared memory no longer used, cannot be used on macos)
aa) (macos also is missing parts of the sysv semaphore api, such as "wait for lock, with timeout", we are using an ugly work-around)
b) posix shared memory with mprotect() & co
c) posix mutexes, including recursive-type mutexes (this seems to be the problem on el5)
d) bsd networking (need to migrate from select() to poll() and from gethostbyname() to getaddrinfo() & co (for IPv6 support))

Not all of these operating system functions are required for all of midas. Running mhttpd and mlogger requires
pretty much everything. Running just a frontend connected to midas through the mserver requires the least features,
just the networking is enough, I think.

Obviously we cannot support midas in perpetuity on all versions of all operating systems, once I do not have
access to a machine, I cannot even check that midas builds and that it runs the basic functions.

Instead, we could provide a "feature reduced" build of midas (makefile target) that includes "just enough" of midas
to (say) run a frontend, maybe even odbedit. We already have some provisions for this, but no obvious documented
way actually doing it.

So back to el5.

How important it is to support very old operating systems?
How many people still use el5?
How about old  versions of Ubuntu? Macos?

If you use anything older than el6, can you speak up,
(and if possible say why you cannot migrate to an up-to-date linux).

K.O.

We are readying a new release of midas and it is almost here except for a few buglets on the new html status page.

The current release candidate branch is "feature/midas-2017-10" and if you have problems with the older versions
of midas, I recommend that you try this release candidate to check if your problem is already fixed. If the problem
still exists, please file a bug report on this forum or on the bitbucket issue tracker
https://bitbucket.org/tmidas/midas/issues?status=new&status=open

Highlights of the new release include
- new and improved web pages done in html and javascript
- many bug fixes and improvements for json and json-rpc support, including improvements in handling of long strings in odb
- locked (protected) operation of odb, where odb shared memory is not writable outside of odb operations
- improved multithead support for odb
- fixes for odb corruption when odb becomes 100% full

For the next release we hope to switch midas from C to fully C++ (building everything with C++ already works). To support el6 we avoid use of 
c++11 language constructs.

K.O.

> [mhttpd,ERROR] [mhttpd.cxx:563:rread,ERROR] Cannot read file '/root', read of 
> 4096 returned -1, errno 21 (Is a directory)

On some linux systems, "/root" exists, it is a directory used as the home directory 
of user "root" (~root is /root; traditional UNIX has ~root as /).

open() of a directory succeeds on some UNIX systems, on some of them,
read() also works, but on other systems one is supposed
to use opendir() and readdir().

MacOS is of course a BSD system (not SysV like Solaris, not Linux), so things
are different yet again. I think MacOS does not have a /root.

In any case, IMO, mhttpd has no business serving the contents of /root,
or serving any files outside of the mhttpd user $HOME directory. (but also
should not serve files from ~user/.ssh, or any other "secret" files, good
luck making a complete axhuastive list of all secret files that should not be
served).

K.O.


> 
> and in the browser I get a popup which tries to save a file called 'root'.
> 
> I track this down to the following: in mhttpd, interprete (line 18046) it is 
> check if a custom page file exists (ss_file_exist) and if yes, it tries to 'load' 
> it. Now, at this stage the variable dec_path contains '/root'.
> 
> Here now what goes wrong: ss_file_exist tries to open the given path, and if a 
> valid file descriptor is returned it assumes the file exists. This is not 
> perfectly correct since it also will get a valid file descriptor is path is an 
> accessible directory!
> 
> Now for whatever reason, on RHEL/Fedora '/root' will return a valid file 
> descriptor, but not on macOS and Ubuntu. Others I haven't tested. A possible fix 
> would be to check explicitly if path is a directory and if yes return 0 in 
> ss_file_exist (see attached diff).
> 
> Perhaps there is cleaner way to deal with this issue?! 

The ROOT I/O workshop was held on June 20th at CERN. A few things of interest in MIDAS land:

- LZ4 is now used as default compression (replacing gzip-1)
- JSON class streamer is finally implemented (XML streamer updated/reworked)
- recursive read-write lock class implemented
- do not see any special mention of Javascript I/O or jsroot, but jsroot git repo seems to be quite active

Of these the recursive read-write lock is most interesting - using something similar would improve ODB performance
and presumably fix the existing lock fairness problems.

https://root.cern.ch/doc/master/TReentrantRWLock_8hxx_source.html
https://indico.cern.ch/event/715802/contributions/2942560/attachments/1670191/2680682/ROOT_IO_June_Workshop_v2.pdf
https://github.com/root-project/jsroot

K.O.

> Invalid tag 0 'Comment' in event 21 'Run Parameters': cannot do history for 
> TID_STRING data, sorry!

The original MIDAS history API does not have provisions for storing TID_STRING data,
it is a very unfortunate limitation that has been with us for a very long time.

If I ever get around to rewrite the MIDAS history API, I will definitely add support for TID_STRING data.

But not today.

K.O.

P.S. Support for arbitrary binary blobs is also possible, but this will make the midas history
a kind of "a daq inside the daq" thing, probably we do not want to go this direction.

K.O.

Concurrence.

Normally, MIDAS data events are saved to ODB (via RO_ODB into /eq/xxx/variables) to make them go into the midas history (/eq/xxx/common/history > 0).

If you do not want events to go into the history, but still want them saved to ODB, it should work (as long as ODB itself
is big enough), but you may run into other problems, specifically ODB free space fragmentation, when no matter how big ODB is, there is never
enough continuous free space for saving a large event. If it happens you will also see random "odb full" errors.

K.O.



> Two options:
> 
> 1) Do NOT send your events into the ODB. This is controlled via the flag RO_ODB in your frontend setting. For simple experiments with small events, it might make sense to copy each 
> event into the ODB for debugging, but if you have large events, this does not make sense. Use the "mdump" utility to check your events instead.
> 
> 2) Increase the size of the ODB. See the first FAQ here: https://midas.triumf.ca/MidasWiki/index.php/FAQ
> 
> Stefan
> 
> 
> > Dear expert,
> >       I'm developing a frontend and I'm getting this kind of error at each event:
> > 
> > 10:14:56.564 2018/05/04 [Sample Frontend,ERROR] [odb.c:5911:db_set_data1,ERROR]
> > online database full
> > 
> > If I run the mem command in odbedit I get the result at the end of this post.
> > 
> > Notice that I need to use an event size which is significantly larger than the
> > default one. I don't know if it is relevant for this error. I have in the ODB:
> > 
> >   /Experiment/MAX_EVENT_SIZE = 900000000
> > 
> > and in the frontend code:
> > 
> >   /* maximum event size produced by this frontend */
> >   INT max_event_size = 300000000;
> > 
> >   /* maximum event size for fragmented events (EQ_FRAGMENTED) */
> >   INT max_event_size_frag = 5 * 1024 * 1024;
> > 
> >   /* buffer size to hold events */
> >   INT event_buffer_size = 600000000;
> > 
> > Events seem to be properly stored in the output files, but I'm afraid I could
> > get some other problem.
> > 
> > Thank you for your help,
> >          Francesco
> > 
> > -------------------------------------------------------------------------
> > 
> > Database header size is 0x21040, all following values are offset by this!
> > Key area  0x00000000 - 0x0007FFFF, size 524288 bytes
> > Data area 0x00080000 - 0x00100000, size 524288 bytes
> > 
> > Keylist:
> > --------
> > Free block at 0x00000B58, size 0x00000008, next 0x000053E0
> > Free block at 0x000053E0, size 0x00000008, next 0x00006560
> > Free block at 0x00006560, size 0x00079AA0, next 0x00000000
> > 
> > Free Key area: 498352 bytes out of 524288 bytes
> > 
> > Data:
> > -----
> > Free block at 0x000847F0, size 0x0007B810, next 0x00000000
> > 
> > Free Data area: 505872 bytes out of 524288 bytes
> > 
> > Free: 498352 (95.1%) keylist, 505872 (96.5%) data

> I'm going to develop MIDAS DAQ for COMET experiment.
> I'm thinking to distribute the load of event building to different PCs.
> I attach a schematics of one of the examples of the design.

Your schematic is reminiscent of the T2K/ND280 structure where the MIDAS DAQ
was split into several separate MIDAS instances (separate "experiments": the FGD, the TPC, 
the slow controls, etc).

They were joined together by the "cascade" equipment which provided a path
for the data events to flow from subsidiary midas instances to the main system (the one
with the final mlogger). It also provided a reverse path for run control, where starting
a run in the main experiment also started the run in all the subsidiary experiments.

This cascade frontend was never included in the midas distribution (an oversight),
but I still have the code for it somewhere.

How many "frontend PC" components do you envision? (10, 100, 1000?).

In T2K/ND280, each subsidiary experiment had it's own ODB which made sense
because e.g. the FGD and the TPC were quite different and were managed by different
groups.

But for you it probably makes sense to have one common ODB. This means a MIDAS
structure where ODB is located on the main computer ("event builder PC"),
all others connect to it via the mserver and midas rpc.

But you will need to have the MIDAS shared event buffers on each "frontend PC" to be local,
which means the bm_xxx() functions have run locally instead of throuhg the mserver rpc.
This is not how midas works right now, but it could be modified to do this.

On the other hand, you do not have to use midas to write the "frontend pc" code. Today's
C++ provides enough features - threads, locks, mutexes, shared memories, event queues,
etc so you can write the whole sub-event builder as one monolithic c++ program
and use midas only to send the data to the main event builder. (plus midas rpc to handle
run control). In this scheme, technically, this "frontend pc" program would
be a multithreaded midas frontend.

K.O.

> Asume you have a variable foo and a link bar -> foo. When you go to the ODB in
> mhttpd, click "Delete" and select bar, it actually deletes foo. bar stays,
> stating "<cannot resolve link>". Trying the same in odbedit with rm gives the
> expected result (bar is gone, foo is still there).
> 
> I'm on the develop branch.

I think I can confirm this. Created a bug report on bitbucket:

https://bitbucket.org/tmidas/midas/issues/148/mhttpd-odb-editor-deletes-wrong-symlink

K.O.

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