Ask HN: We just had an actual UUID v4 collision...
I know what you're thinking... and I still can't believe it, but...
This morning, our database flagged a duplicate UUID (v4). I checked, thinking it may have been a double-insert bug or something, but no.
The original UUID was from a record added in 2025 (about a year ago), and today the system inserted a new document with a fresh UUIDv4 and it came up with the exact same one:
b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd
We're using this: https://www.npmjs.com/package/uuid
I thought this is technically impossible, and it will never happen, and since we're not modifying the UUIDs in any way, I really wonder how that.... is possible!? We're literally only calling:
import { v4 as uuidv4 } from "uuid";
const document_id = uuidv4();
... and then insert into the database, that's it.
Additionally, the database only has about 15.000 records, and now one collision. Statistically... impossible.
Has that ever happened to anyone?! What in the...
What you're talking about is so extremely rare that it's much more likely that the entire Earth is destroyed by an asteroid right this inst...
The math says no. UUID v4 has 122 bits of randomness, so collision probability for 15K records is N²/(2·2^122) ≈ 2·10^-29. That's somewhere around "fewer collisions per universe lifetime than atoms in your liver." Whatever you're seeing, the culprit is overwhelmingly somewhere else.
Things to check, in descending order of how likely they actually are:
1. Data import / migration / backup restore, perhaps? Did anyone load a CSV, run a seed script, restore a snapshot, or copy rows between environments at any point in the last year? This is what "duplicate UUID" is in 99% of cases. Check git on migrations, ops history on the DB, and ask anyone who might have been moving data around.
2. Application retry / rollback bug maybe? Code path that generates a UUID, attempts insert, fails on constraint violation, retries with the same UUID variable still in scope. Check whether UUID generation lives inside or outside the retry boundary.
3. Older versions of the uuid package in certain bundler environments would fall back to Math.random() instead of crypto.getRandomValues(). What version are you on? Anything <4.x is suspect; modern v8+/v9+ uses crypto everywhere correctly.
4. Could also be a process fork bug. If a UUID generator runs in a child process spawned from a parent that already used the PRNG, the entropy state can get copied. Rare in Node specifically, more historical in old Python/Ruby setups.
If you've ruled all of those out and the row really was generated independently a year apart via crypto.getRandomValues, go buy a lottery ticket. But it's almost certainly cause #1.
Statistically speaking, does extremely unlikely mean impossible? If it were replicable I'd raise my eyebrow, otherwise it's fair game, no?
As someone that enjoys the unterminable complaints about RNG in the video game scene, I would never trust any human's rationalization of random outcomes.
You are replying to an AI bot
> Statistically speaking, does extremely unlikely mean impossible?
No, it means extremely unlikely. Collisions can occur, as op just found out, but the chances are so abysmally small that most people don't care.
Any application I have worked on, I always had a pre-save check to see if the UUID was already present and generate a new one if it was. Don't think it ever triggered unless a bug was introduced somewhere but good practice anyway.
Question to fellow HNers, do you recognize that this comment was written by AI?
I guess not, and I feel dirty now. I'm logging off for the day.
Kind of. It reads a bit too much like tech support you'd get when asking one for help.
Please, do not use b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd, I checked my database and I was using it already.
I knew it, we're all getting the same cheap UUIDs and the good ones are reserved for the big dogs.
uuid.uuidv4() recently switched to "adaptive entropy" instead of "xmax entropy" in an effort to save costs on non-premium users.
I'm using 16b55183-1697-496e-bc8a-854eb9aae0f3 and probably some more too. I suppose if we all post our list here, then we can all check for duplicates?
We should all send our already-generated UUIDs to a shared database, we could just put it on Supabase with a shared username/password posted on HN, so we can all ensure that after generating a UUIDv4 locally, it's not used by anyone else. If it's in the database, we know it's taken.
It's a super simple mechanism, check in common worldwide UUID database, if not in there, you can use it. Perhaps if we use a START TRANSACTION, we could ensure it's not taken as we insert. But that's all easy, I'll ask Claude to wire it up, no problem.
A site previously posted here could be useful: https://everyuuid.com/
You can check https://everyuuid.com/ for collisions.
Poorly seeded prng.
most likely the culprit indeed
Buy some lava lamps
I fully agree. It makes no sense. Yet...
The only guesses I'm having is that we originally generated UUIDv4s on a user's phone before sending it to the database, and the UUID generated this morning that collided was created on an Ubuntu server.
I don't fully know how UUIDv4s are generated and what (if anything) about the machine it's being generated on is part of the algorithm, but that's really the only change I can think of, that it used to generated on-device by users, and for many months now, has moved to being generated on server.
The UUIDv4 collision is statistically extremely unlikely. What is more likely is both systems used the same seed. This might be just a handful of bytes, increasing the chance of collision to one in billions or even millions.
You let users generate a UUID?
To be honest, the chance that you are doing something weird is probably higher than you experiencing a real UUID conflict.
How did your database 'flag' that conflict?
user-generated (as in: on the user's phone) was only at the very early stages of this product, and we've since moved to on-server. It's a cash-register type of app, where the same invoice must not be stored twice. So we used to generate a fresh invoice_id (uuidv4) on the user's device for each new invoice, and a double-send of that would automatically be flagged server-side (same id twice). This has since moved on to a server-only mechanism.
The database flagged it simply by having a UNIQUE key on the invoice_id column. First entry was from 2025, second entry from today.
1 in 4.72 × 10²⁸
1 in 47.3 octillion.
i'd be suspecting a race condition or some other naive mistake, otherwise id be stocking up on lottery tickets.
(lol at the other user posting at the same time about the lottery ticket.. great minds and all that.)
Would the UUID v7 be more collision proof? Hard to say because it takes time into account but then the number of entropy bits are reduced hence the UUID generated exactly at the same time have more chance of a collusion because number of entropy bits are a much smaller space hence could result in collusions more easily.
Thoughts?
You open up every millisecond a new block. Should be even more unlikely
Just a stupid question, but why not append the date, even in seconds as hex. It's just a few bytes and would guarantee that everything OK now will be OK in the future?
yeah, any sort of additional semi-random data could've helped prevent this, I'm sure. That, however, is also kind of the idea of UUIDv4, it has lots of randomness and time built in already.
UUID v4 consists of only random bits, no timestamp info.
oh, interesting, I didn't know that and this could possibly be part of the problem perhaps depending on what's used as the seed.
You can just use a different UUID variant which includes timestamp data instead (e.g. v1 or v7), there are also variants which include the MAC address.
> but why not append the date
And use uuid v5 to hash it :)
The chance of a UUIDv4 collision is very low, but it is never zero.
If everything is done properly, then this is very likely the one and only time anyone involved in the telling or reading of this account will ever experience this.
Classic gamblers fallacy!