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From: Andy (atompkins_at_[hidden])
Date: 2007-02-28 10:26:10

<Oliver.Kowalke_at_[hidden]> wrote in

> Hello Andy,
>> My implementation of version 4 (random-number-based) guids
>> may not be unique in space and time. It is based on
>> boost::mt19937 to generate random numbers and it is seeded
>> with std::time(0). Cryptographic-quality random numbers
>> would reduce the probability of repeated values.
>> Using version 5 (name-based) guid can be used to ensure that
>> guids are unique in space (guids generated on several
>> computers at the same time) and time. Generate a guid for
>> each computer as the namespace_guid and pass a counter as the
>> name to the create function.
> Because creating uuid v5 seams too expensive for me I could let one
> special computer (in my project == master) generate guids (unique in
> space).
> So I've make a decision wether uuid v4 or v1 should be used in order
> to be uinque in time?!
> Or do you have another suggestion?

I have not done any profiling/timing on any of the functions that create
a guid. A note about multiple threads: all 3 versions are thread safe,
but the random-number-based (version 4) and the time-based (version 1)
function need a mutex to ensure that only one thread is creating a guid
at a time. The name-based (version 5) does not need a mutex.

As to decide between time-based (version 1) and random-number-based
(version 4) for which would be unique in time, I would expect the time-
based (version 1) to be more unique in time since only the node is
generated randomly and only the first time a guid is generated (until the
program is restarted). Whereas the random-number-based (version 4) is
all just random data.

> Regards,Oliver
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