Is a GUID unique 100% of the time?

Question

Is a GUID unique 100% of the time?Will it stay unique over multiple threads?

Answer 1

GUIDs (also called UUIDs) are not really unique, but the overall space of possible GUIDs (2¹²⁸) is large enough that there shouldn't be collisions between them. (eg, the same exact GUID can be probabalistically not expected to be created more than once before the heat death of the universe).

The specifics of the algorithms used to generate GUIDs/UUIDs may cause them to be slightly more likely to collide in specific circumstances (eg, many in the same clock, on the same computer), but they are intended to be generated at different times, by different systems.

Even with that said the GUID/UUID spaces is large enough, you could dedicate every core on your CPU, all of its ram, and terabytes of hard disk to do nothing but generate and catalog GUIDs, and you will probably not see a collision until our sun starts expanding, though there still is a chance it could happen.

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TL;DR, you can expect a GUID to be unique 99.99....% of the time, regardless of what thread or system creates it.

In the rare case there is a GUID collision, the collided GUIDs likely exist in different systems, and the collision won't actually matter, or ever be known.