Is a GUID unique 100 of the time

Question

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

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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.

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In a more general sense  this is known as the  birthday problem  or  birthday paradox   Wikipedia has a pretty good overview at   Wikipedia - Birthday Problem  In very rough terms  the square root of the size of the pool is a rough approximation of when you can expect a 50  chance of a duplicate  The article includes a probability table of pool size and various probabilities  including a row for 2 128  So for a 1  probability of collision you would expect to randomly pick 2 6 10 18 128-bit numbers  A 50  chance requires 2 2 10 19 picks  while SQRT 2 128  is 1 8 10 19   Of course  that is just the ideal case of a truly random process  As others mentioned  a lot is riding on the that random aspect - just how good is the generator and seed  It would be nice if there was some hardware support to assist with this process which would be more bullet-proof except that anything can be spoofed or virtualized  I suspect that might be the reason why MAC addresses time-stamps are no longer incorporated

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Yes  a GUID should always be unique   It is based on both hardware and time  plus a few extra bits to make sure it s unique   I m sure it s theoretically possible to end up with two identical ones  but extremely unlikely in a real-world scenario     Here s a great article by Raymond Chen on Guids    https   blogs msdn com oldnewthing archive 2008 06 27 8659071 aspx

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The Answer of  Is a GUID is 100  unique   is simply  No      If You want 100  uniqueness of GUID then do following    generate GUID check if that GUID is Exist in your table column where you are looking for uniquensess if exist then goto step 1 else step 4 use this GUID as unique

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Is a GUID unique 100  of the time    Not guaranteed  since there are several ways of generating one  However  you can try to calculate the chance of creating two GUIDs that are identical and you get the idea  a GUID has 128 bits  hence  there are 2128 distinct GUIDs     much more than there are stars in the known universe  Read the wikipedia article for more details

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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

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Eric Lippert has written a very interesting series of articles about GUIDs      There are on the order 230 personal computers in the world  and of   course lots of hand-held devices or non-PC computing devices that have   more or less the same levels of computing power  but lets ignore   those   Let s assume that we put all those PCs in the world to the   task of generating GUIDs  if each one can generate  say  220 GUIDs per   second then after only about 272 seconds -- one hundred and fifty   trillion years -- you ll have a very high chance of generating a   collision with your specific GUID  And the odds of collision get   pretty good after only thirty trillion years     GUID Guide  part one GUID Guide  part two GUID Guide  part three

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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   enter image description here  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

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From http   www guidgenerator com online-guid-generator aspx  What is a GUID  GUID  or UUID  is an acronym for  Globally Unique Identifier   or  Universally Unique Identifier    It is a 128-bit integer number used to identify resources  The term GUID is generally used by developers working with Microsoft technologies  while UUID is used everywhere else  How unique is a GUID  128-bits is big enough and the generation algorithm is unique enough that if 1 000 000 000 GUIDs per second were generated for 1 year the probability of a duplicate would be only 50   Or if every human on Earth generated 600 000 000 GUIDs there would only be a 50  probability of a duplicate

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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  -

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While each generated GUID is not guaranteed to be unique  the total number of unique keys  2128 or 3 4  1038  is so large that the probability of the same number being generated twice is very small  For example  consider the observable universe  which contains about 5  1022 stars  every star could then have 6 8  1015 universally unique GUIDs   From Wikipedia   These are some good articles on how a GUID is made  for  NET  and how you could get the same guid in the right situation  https   ericlippert com 2012 04 24 guid-guide-part-one  https   ericlippert com 2012 04 30 guid-guide-part-two  https   ericlippert com 2012 05 07 guid-guide-part-three

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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

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None seems to mention the actual math of the probability of it occurring   First  let s assume we can use the entire 128 bit space  Guid v4 only uses 122 bits    We know that the general probability of NOT getting a duplicate in n picks is       1-1 2128  1-2 2128     1- n-1  2128    Because 2128 is much much larger than n  we can approximate this to       1-1 2128 n n-1  2   And because we can assume n is much much larger than 0  we can approximate that to       1-1 2128 n 2 2   Now we can equate this to the  acceptable  probability  let s say 1        1-1 2128 n 2 2   0 01   Which we solve for n and get      n   sqrt 2  log 0 01   log  1-1 2128     Which Wolfram Alpha gets to be 5 598318    1019  To put that number into perspective  lets take 10000 machines  each having a 4 core CPU  doing 4Ghz and spending 10000 cycles to generate a Guid and doing nothing else  It would then take  111 years before they generate a duplicate

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If you are scared of the same GUID values then put two of them next to each other    Guid NewGuid   ToString     Guid NewGuid   ToString      If you are too paranoid then put three

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As a side note  I was playing around with Volume GUIDs in Windows XP  This is a very obscure partition layout with three disks and fourteen volumes       Volume 23005604-eb1b-11de-85ba-806d6172696f    F       Volume 23005605-eb1b-11de-85ba-806d6172696f    G       Volume 23005606-eb1b-11de-85ba-806d6172696f    H       Volume 23005607-eb1b-11de-85ba-806d6172696f    J       Volume 23005608-eb1b-11de-85ba-806d6172696f    D       Volume 23005609-eb1b-11de-85ba-806d6172696f    P       Volume 2300560b-eb1b-11de-85ba-806d6172696f    K       Volume 2300560c-eb1b-11de-85ba-806d6172696f    L       Volume 2300560d-eb1b-11de-85ba-806d6172696f    M       Volume 2300560e-eb1b-11de-85ba-806d6172696f    N       Volume 2300560f-eb1b-11de-85ba-806d6172696f    O       Volume 23005610-eb1b-11de-85ba-806d6172696f    E       Volume 23005611-eb1b-11de-85ba-806d6172696f    R                                                                                                -- 6f   o                                             ---- 69   i                                           ------ 72   r                                         -------- 61   a                                       ---------- 6d   m   It s not that the GUIDs are very similar but the fact that all GUIDs have the string  mario  in them  Is that a coincidence or is there an explanation behind this   Now  when googling for part 4 in the GUID I found approx 125 000 hits with volume GUIDs   Conclusion  When it comes to Volume GUIDs they aren t as unique as other GUIDs

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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

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MSDN       There is a very low probability that the value of the new Guid is all zeroes or equal to any other Guid

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For more better result the best way is to append the GUID with the timestamp  Just to make sure that it stays unique   Guid NewGuid   ToString     DateTime Now ToString

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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

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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

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Guids are statistically unique   The odds of two different clients generating the same Guid are infinitesimally small  assuming no bugs in the Guid generating code    You may as well worry about your processor glitching due to a cosmic ray and deciding that 2 2 5 today   Multiple threads allocating new guids will get unique values  but you should get that the function you are calling is thread safe   Which environment is this in

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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

[language-agnostic] Is a GUID unique 100% of the time?

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