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Re^3: Generate a unique ID

by dHarry (Abbot)
on Nov 15, 2010 at 16:42 UTC ( #871504=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Generate a unique ID
in thread Generate a unique ID

that prevent one copy of the program picking up the wrong files

I solved this once using "semaphore files", if a copy of a program picks up a file it checks if the semaphore is there. If it is, it skips the file, if not it creates one. After processing the file it is removed/moved to another directory and the semaphore file deleted. You need some bookkeeping to make it safe.

It's not clear to me why you need the "uniqueness" at all? If you don't want the slightest chance of a collision I think only the "lastId + 1" remains. You need some sort of locking mechanism of course to control access to the value, e.g. prevent a dirty read. Both timestamps and UUID-like things are not 100% safe. On the other hand, reading through UUID, I would say the chance is probably too small to bother.

Cheers

Harry


Comment on Re^3: Generate a unique ID
Re^4: Generate a unique ID
by BrowserUk (Pope) on Nov 15, 2010 at 17:25 UTC
    It's not clear to me why you need the "uniqueness" at all?

    The requirement came out of a real situation.

    A bug caused the program to crash and leave behind some of its temporary files. I was away from the machine at the time. When I came back, I re-ran the program having made a few changes--including correcting the problem that caused the crash I hadn't noticed happen--and the second phase of the code mixed up the content of the previous run with those of the current run. I spent a long time trying to work out why the final output was completely screwed up before noticing that the previous run had terminated badly whilst reviewing the debug trace in the console buffer.

    Imagine how annoying it would be if your system sort utility mixed up spill files from an interrupted run with those generated by the current run.

    Any mechanism that relies upon correct clean-up of either the files themselves or semaphore files is vulnerable to the same issue.

    For the PID + timestamp to fail, the system would have to issue exactly the same PID at exactly the same time--within the resolution of the timestamp granularity. The combined value suitably encoded (hex or base64) will be used as the prefix supplied to tempname() to generate the names for the temp. files.


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