Lennart Poettering wrote: >On Do, 25.04.19 13:14, Przemek Klosowski (przemek.klosowski@xxxxxxxx) wrote: >> That leaves the invocation IDs---the UUIDs need to be random to be truly >> Universally Unique, but a limited entropy system is implicitly isolated, so >> maybe the limited UUIDs could be seen as Universal in its very small >> Universe. What is the time duration of the original invocation IDs? What are >> the negative implication of the initial UUIDs being less random than the >> subsequent ones? > >Invocation IDs are useful for globally pinpointing a specific service >invocation. If the UUIDs would stop to be truly random then they'd >stop being universally unique and thus stop being useful. It's perfectly possible for a number to be unique without being random. As an example, you could hash the machine ID, which is supposedly unique in space, and the system clock, which is unique in time. That makes the hash unique in both space and time. Produce invocation IDs by counting up from that value, or by hashing repeatedly. That way you wouldn't need entropy for invocation IDs at every boot, only during installation. Such values would of course be somewhat predictable, but according to what you've said in this thread, invocation IDs don't need to be unpredictable. You've only said that you want them unique. (Of course one needs to be aware that collisions are not impossible, only improbable. That's equally true for hashes and random numbers.) Björn Persson
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