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Ask HN: We just had an actual UUID v4 collision...

read original get UUID v4 Collision Detector → more articles
Why This Matters

This incident highlights a rare and unexpected UUID v4 collision, challenging the assumption that such collisions are practically impossible due to the vast address space. It underscores the importance of understanding the limitations of seemingly unique identifiers in critical systems, prompting developers to consider additional safeguards. For consumers and the tech industry, it emphasizes the need for vigilance even with robust randomness sources in large-scale applications.

Key Takeaways

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.

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