Question 1

What is a UUID?

Accepted Answer

A UUID (Universally Unique Identifier) is a 128-bit identifier standardized in RFC 4122, formatted as 8-4-4-4-12 hexadecimal digits, designed to be globally unique without a central authority.

Question 2

What is the difference between UUID v1 and v4?

Accepted Answer

UUID v1 is time-based and includes a timestamp and MAC address, making it traceable. UUID v4 uses 122 bits of cryptographic randomness, offering better privacy and no traceability.

Question 3

What makes a UUID unique?

Accepted Answer

UUID uniqueness comes from either cryptographic randomness (v4), hashing (v3/v5), or time+MAC address (v1). RFC 4122 also reserves specific bits for version and variant, reducing the collision space.

Question 4

What is the nil UUID?

Accepted Answer

The nil UUID is 00000000-0000-0000-0000-000000000000 — all zeros. It is used as a placeholder or null value and is not considered a valid random UUID.

Question 5

What is the probability of UUID collision?

Accepted Answer

For UUID v4, generating 1 billion per second would take ~85 years to reach a 50% probability of a single collision. In practice, collisions are negligible.

Question 6

Should I use UUID as a database primary key?

Accepted Answer

UUID v4 works but its randomness causes B-tree fragmentation and slower writes. UUID v7 or ULID, which are time-ordered, are better for primary keys when insert performance matters.

Question 7

Why does a UUID have hyphens and 36 characters?

Accepted Answer

RFC 4122 specifies the 8-4-4-4-12 hyphenated format. This groups fields logically: time_low, time_mid, time_hi_and_version, clock_seq, and node, totaling 32 hex chars + 4 hyphens = 36 characters.

Question 8

How is UUID version detected?

Accepted Answer

The 13th character of a UUID (first char of the 3rd group) encodes the version number (1–5). The 17th character encodes the variant bits, indicating RFC 4122 compliance.

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