"Ron Teitelbaum" <Ron at USMedRec.com> writes:
> I need a unique image identifier.  Does anyone have any suggestions?
> 
> The identifier must be unique so that if someone copies an image to another
> machine I get a new id.  It must also be unique by instance, so that if two
> images or the same image is launched multiple times I should get unique
> identifiers.   The id should be local and non transferable plus it should
> never persist, since I would like to use this id to prevent movement between
> images of objects like in spoon, magma, glorp, squeakELib, croquet, or any
> other distributed objects scheme.  


UUID's are a sort of perpetual motion machine for computer science.
They are nice to have, but none of them work, despite the large number
of designs that are tantalizingly close.  It is better to talk about
plain old identifiers, ID's.  They are never universally unique, so
the first U should go; they are always *somewhat* unique, and so the
second U is redundent.  It's just about identifiers.

The "universally" part seems possible for some people, so let me dwell
on it a minute.  One problem is, if you have no restrictions on the
execution environment, then you can have no grounds for ensuring
uniqueness, either.  Anything that you might be using for uniqueness,
might get duplicated if people are running your code on a virtual
machine such as VMWare.

To address just one example, consider using a random number generator
from the network.  First, you can never really know that two images
are really calling the same generator server.  The Internet is a
mysterious thing when you factor in firewalls, caches, and emulation.
Further, if you somehow do know you are calling the same server, why
not save yourself the headaches and have the server hand out plain old
sequential ID numbers?


Okay, enough bashing of UUID's.  For your problem, here are two
possible avenues for (non-universally unique) ID's to use.

1. An IP plus a process ID (which might be an IP port number) plus an
image-wide unique number.  Such an ID is unique relative to a
particular IP network, but of course can be duplicated--just think
about all the computers in the world with address 192.168.1.1.

2. You might want to use something like http URL's, with a hostname
and a list of strings as the identifier.


These are just ideas, though.  In the end, you need to spend some time
thinking about the more specific problem.  Internet email's
Message-ID's are a farce, but POP's UIDL's are simple and work.  The
difference is that Message-ID's pretended to be universally unique,
while UIDL's are only unique relative to a particular mail server.

-Lex