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

GUID algorithms are usually implemented according to the v4 GUID specification, which is essentially a pseudo-random string. Sadly, these fall into the category of "likely non-unique", from Wikipedia (I don't know why so many people ignore this bit): "... other GUID versions have different uniqueness properties and probabilities, ranging from guaranteed uniqueness to likely non-uniqueness."

The pseudo-random properties of V8's JavaScript Math.random() are TERRIBLE at uniqueness, with collisions often coming after only a few thousand iterations, but V8 isn't the only culprit. I've seen real-world GUID collisions using both PHP and Ruby implementations of v4 GUIDs.

Because it's becoming more and more common to scale ID generation across multiple clients, and clusters of servers, entropy takes a big hit -- the chances of the same random seed being used to generate an ID escalate (time is often used as a random seed in pseudo-random generators), and GUID collisions escalate from "likely non-unique" to "very likely to cause lots of trouble".

To solve this problem, I set out to create an ID algorithm that could scale safely, and make better guarantees against collision. It does so by using the timestamp, an in-memory client counter, client fingerprint, and random characters. The combination of factors creates an additive complexity that is particularly resistant to collision, even if you scale it across a number of hosts:

http://usecuid.org/

Answer 2

GUID stands for Global Unique Identifier

In Brief: (the clue is in the name)

In Detail: GUIDs are designed to be unique; they are calculated using a random method based on the computers clock and computer itself, if you are creating many GUIDs at the same millisecond on the same machine it is possible they may match but for almost all normal operations they should be considered unique.

Answer 3

The hardest part is not about generating a duplicated Guid.

The hardest part is designed a database to store all of the generated ones to check if it is actually duplicated.

From WIKI:

For example, the number of random version 4 UUIDs which need to be generated in order to have a 50% probability of at least one collision is 2.71 quintillion, computed as follows:

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This number is equivalent to generating 1 billion UUIDs per second for about 85 years, and a file containing this many UUIDs, at 16 bytes per UUID, would be about 45 exabytes, many times larger than the largest databases currently in existence, which are on the order of hundreds of petabytes

Answer 4

Theoretically, no, they are not unique. It's possible to generate an identical guid over and over. However, the chances of it happening are so low that you can assume they are unique.

I've read before that the chances are so low that you really should stress about something else--like your server spontaneously combusting or other bugs in your code. That is, assume it's unique and don't build in any code to "catch" duplicates--spend your time on something more likely to happen (i.e. anything else).

I made an attempt to describe the usefulness of GUIDs to my blog audience (non-technical family memebers). From there (via Wikipedia), the odds of generating a duplicate GUID:

• 1 in 2^128
• 1 in 340 undecillion (don’t worry, undecillion is not on the quiz)
• 1 in 3.4 × 10^38
• 1 in 340,000,000,000,000,000,000,000,000,000,000,000,000

Answer 5

If your system clock is set properly and hasn't wrapped around, and if your NIC has its own MAC (i.e. you haven't set a custom MAC) and your NIC vendor has not been recycling MACs (which they are not supposed to do but which has been known to occur), and if your system's GUID generation function is properly implemented, then your system will never generate duplicate GUIDs.

If everyone on earth who is generating GUIDs follows those rules then your GUIDs will be globally unique.

In practice, the number of people who break the rules is low, and their GUIDs are unlikely to "escape". Conflicts are statistically improbable.

Answer 6

The simple answer is yes.

Raymond Chen wrote a great article on GUIDs and why substrings of GUIDs are not guaranteed unique. The article goes in to some depth as to the way GUIDs are generated and the data they use to ensure uniqueness, which should go to some length in explaining why they are :-)