Determine what the calling program is

[dn]

On 19/04/2021 01.46, Jason Friedman wrote:
> I should state at the start that I have a solution to my problem. I am
> writing to see if there is a better solution.
> 
> I have a program that runs via crontab every five minutes. It polls a
> Box.com folder for files and, if any are found, it copies them locally and
> performs a computation on them that can exceed five minutes. It pushes the
> results back up to Box. (Box.com ensures that only complete files are
> visible when I poll.) Files are dropped into this Box.com folder rarely,
> but to ensure a good customer experience I do not want to set my crontab to
> run less frequently. My hardware cannot support multiple simultaneous
> computations.
> 
> I have written a piece of code to detect if more than 1 instance of my
> program is running, and I put this code into a separate module (support.py)
> so that other programs can use it.


In a similar situation, one of my teams used an (OpSys) environment
variable (available in both *nux and MS-Win).
- when the application starts, it checks for the variable
- if exists, stops running, else may proceed

During code review (when I noticed this tactic) I was slightly
surprised, because back when I was young (men were men, and knights were
bold, ...), we used file-stubs.


However, such systems face two complementary, potential-problems:
'single-instance' (which is being addressed), and 'blocking-instance'.

If there is a risk that the long-running computations may fail into a
never-ending loop, the system effectively dies (but silently!) and
source files (ie at Box.com) may accumulate without receiving attention.

Accordingly, the above-mentioned environment-variable was filled with a
time-stamp. Then a second step in the check-routine reviewed the time
since the 'blocking' instance started, in order to log or raise suitable
alerts if things went awry.

YMMV!


An alternative, if the system already uses a database, is to keep a
local record in the DB of all the files lodged at box.com. This can
include a note that each file has/not been processed (plus any other
stats or logging you may deem appropriate). A third state would be 'in
process'. Now, at start-up, the application can quickly check to see if
there is any file in that state...
-- 
Regards,
=dn