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

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 3

It should not happen. However, when .NET is under a heavy load, it is possible to get duplicate guids. I have two different web servers using two different sql servers. I went to merge the data and found I had 15 million guids and 7 duplicates.

Answer 4

I experienced a duplicate GUID.

I use the Neat Receipts desktop scanner and it comes with proprietary database software. The software has a sync to cloud feature, and I kept getting an error upon syncing. A gander at the logs revealed the awesome line:

"errors":[{"code":1,"message":"creator_guid: is already taken","guid":"C83E5734-D77A-4B09-B8C1-9623CAC7B167"}]}

I was a bit in disbelief, but surely enough, when I found a way into my local neatworks database and deleted the record containing that GUID, the error stopped occurring.

So to answer your question with anecdotal evidence, no. A duplicate is possible. But it is likely that the reason it happened wasn't due to chance, but due to standard practice not being adhered to in some way. (I am just not that lucky) However, I cannot say for sure. It isn't my software.

Their customer support was EXTREMELY courteous and helpful, but they must have never encountered this issue before because after 3+ hours on the phone with them, they didn't find the solution. (FWIW, I am very impressed by Neat, and this glitch, however frustrating, didn't change my opinion of their product.)

Answer 5

I have experienced the GUIDs not being unique during multi-threaded/multi-process unit-testing (too?). I guess that has to do with, all other tings being equal, the identical seeding (or lack of seeding) of pseudo random generators. I was using it for generating unique file names. I found the OS is much better at doing that :)

Trolling alert

You ask if GUIDs are 100% unique. That depends on the number of GUIDs it must be unique among. As the number of GUIDs approach infinity, the probability for duplicate GUIDs approach 100%.

Answer 6

The Answer of "Is a GUID is 100% unique?" is simply "No" .

• If You want 100% uniqueness of GUID then do following.

  1. generate GUID
  2. check if that GUID is Exist in your table column where you are looking for uniquensess
  3. if exist then goto step 1 else step 4
  4. use this GUID as unique.