UUID

V7

UUID v7 is monotonic within a process and ordered by time, making it ideal for databases and distributed systems.

id, err := uuid.MakeV7()

Field	Bit Positions	Byte Positions	Length (bits)	Description
unix_ts_ms	0–47	uuid[0:6]	48	48-bit big-endian unsigned integer representing milliseconds since the Unix Epoch.
ver	48–51	uuid[6] (high nibble)	4	4-bit version field, fixed to 0111 (7).
rand_a	52–63	uuid[6] (low nibble) – uuid[7]	12	12-bit field for additional time precision or monotonic counter within a millisecond.
var	64–65	uuid[8] (high 2 bits)	2	2-bit variant field, fixed to 10 to conform with RFC 4122.
rand_b	66–127	uuid[8:16]	62	62-bit cryptographically secure random field (from crypto/rand).

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V4

UUID v4 is completely random. It provides no ordering or time-based semantics — only uniqueness with extremely low collision probability (1 in 2^122).

id, err := uuid.MakeV4()

Field	Bit Positions	Byte Positions	Length (bits)	Description
ver	48–51	uuid[6] (high nibble)	4	4-bit version field, fixed to 0100 (4).
var	64–65	uuid[8] (high 2 bits)	2	2-bit variant field, fixed to 10 to conform with RFC 4122.
rand	All others	uuid[0:6], uuid[7:8], uuid[9:16]	122	122 cryptographically secure random bits (from crypto/rand).

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Errors

When the Go runtime could not read random data from crypto/rand, it usually means:

• You're running in a locked-down environment (like Docker or a VM) without /dev/urandom
• The system is out of entropy (common on headless servers or cloud instances)
• /dev/urandom is missing, unreadable, or blocked by a security policy
• You're on an old or broken OS with no proper random number generator

The uuid package require cryptographically secure randomness. If crypto/rand fails, none of the UUID generation will be possible, unless you provide your own Random io.reader in the generators.