Why is a “heavy” CRON script executed several times?

Question

PHP

Greetings,
CRON hits the database once a minute and looks for the last entry with the pid=0 field .
If it finds, then parsing json is performed with further writing to different database tables of what has been parsed.
I recently noticed such a nuisance -
For example, in parsed json there is such a block:

...
   {
      "pr_id":5,
      "com_total":150,
   }
...

com_total with the value (150) in the parsed object is present only in the singular, com_total is written to a separate table. As a result, sometimes I meet "duplication", that is, two / three / more than 150 times are written to the table, although it should be 1 time. (and far from all records are so duplicated, )
I noticed a pattern that the "heavier" json, i.e. the more processing effort, the more duplication occurs. ~1000-1500 blocks - everything is fine, 2000 - some operations will be performed 2 times, 4000 - 3 times, etc.
If it's some kind of lack of something, then I expected the script to just crash. But no, it is being executed, but somehow very cunningly.
Do not prompt with what can be connected?
Thanks to.

Answer 1

Obviously, the script does not have enough minutes to execute and it processes the same data several times. You need to keep track that the previous run has completed while the new one is starting. If not, then abort the execution of a new launch and do nothing.
And the most correct would be to profile the script. A minute is a very long time for a computer and during this time it can do a lot. Most likely, you have a suboptimal algorithm.

Answer 2

According to the crown, everything you do must be written to some kind of additional log, for example, a text log or a database, and make more tricky conditions in the script. We took record 150 and wrote it into a text box if the script worked successfully at the very end, delete the record. At the beginning of the script, check if the script took 150 and in the text is 150, then something is wrong ...

Answer 3

flock -n /tmp/lockfile -c /path/to/script.sh