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Entry  10 Mar 2022, Gennaro Tortone, Bug Report, Python ODB watch 
    Reply  16 Mar 2022, Ben Smith, Bug Report, Python ODB watch 
       Reply  21 Mar 2022, Stefan Ritt, Bug Report, Python ODB watch 
Message ID: 2356     Entry time: 16 Mar 2022     In reply to: 2353     Reply to this: 2357
Author: Ben Smith 
Topic: Bug Report 
Subject: Python ODB watch 
> It seems that the second write operation "overlaps" the first one...

Hi Gennaro,

In principle the same issue can happen in C++ code, but is much less likely as the callbacks get executed  more quickly (partly due to C++/python in general, and partly because the python code does some extra work to make the interface more user-friendly). The C++ code at the end of this message adds a 100ms sleep to the callback and can result in output like this when you do quick edits of "Test[0-19]" in odbedit.

Element 1 is 0
Element 2 is 0
Element 3 is 0
Element 4 is 0
Element 5 is 0
Element 6 is 0
Element 7 is 1
Element 8 is 1
Element 9 is 1

I agree that this can be a really nasty source of bugs if you need to react to every change. I'll add a warning to the python docstrings, but I can't think of a way to make this more robust at the midas level - I think we'd need some sort of ODB "snapshot" system...

#include "midas.h"

void watch_fn(HNDLE hDB, HNDLE hKey, int index, void *info) {
  DWORD data = 0;
  INT buf_size = sizeof(data);
  db_get_data_index(hDB, hKey, &data, &buf_size, index, TID_DWORD);
  printf("Element %d is %u\n", index, data);

int main() {
  HNDLE hDB, hClient, hTestKey;

  std::string host, expt;
  cm_get_environment(&host, &expt);
  cm_connect_experiment(host.c_str(), expt.c_str(), "test_odb", nullptr);
  cm_get_experiment_database(&hDB, &hClient);

  static const DWORD numValues = 20;
  DWORD data[numValues] = {};
  db_set_value(hDB, 0, "Test", data, sizeof(DWORD) * numValues, numValues, TID_DWORD);
  db_find_key(hDB, 0, "Test", &hTestKey);
  db_watch(hDB, hTestKey, watch_fn, nullptr);

  printf("Press any key to exit loop...\n");

  while (!ss_kbhit()) {

  db_delete_key(hDB, hTestKey, FALSE);

  return 0;
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