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Re^5: Generate a unique ID

by afoken (Monsignor)
on Nov 15, 2010 at 20:48 UTC ( #871577=note: print w/replies, xml ) Need Help??

in reply to Re^4: Generate a unique ID
in thread Generate a unique ID

So, if you can't use (or don't trust) the random number generator, re-implement what an SQL sequence does. As long as your job is restricted to a single machine, and your OS supports at least advisory locks, that should not be too hard. This is very similar to a robust web page visitor counter script (one that does not damage the counter when called in parallel).

You need a file that contains the current sequence number, all access to that file is protected by locks, so that at any time exactly one process can read and increment the sequence number. The thread Trying to understand flock contains some tips.

If you have to work with different machines and networked filesystems (NFS, CIFS, AFS, ...), don't bet on working locks. Implement the sequence number generator as a dumb TCP/IP server on a high port (>1024), that can handle only one client. Use a (properly locked) counter file on a local disk. Run it on exactly one machine. Make all instances of your program query that server for an individual sequence number (simply by connecting and reading one line). Using TCP sockets automatically makes sure that there can be only one server per network address and port. If you want to be paranoid, use the "lock the DATA handle" trick to prevent multiple instances.


Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

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[Corion]: You'll only ever need that option if you have a long-running query whose results are not cached by your DB already, and in those cases I presume that the programmer will want to maintain the temporary tables themselves - I wouldn't know when to drop ...
[Corion]: ... the temporary tables, and also don't have a good idea on how to create unique table names for them
[hippo]: OIC. In that case leave it out but invite feature requests and see if any of the users suggest it. :-)
[Corion]: Talking about this makes me realize that it's likely only a half useful idea. But it still would be convenient to have as an option...
[Corion]: hippo: Hmmm - yeah, I could document it and wait for code implementing that option to show up ;-D

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