[][src]Crate rusty_ulid

ULID - Universally Unique Lexicographically Sortable Identifier

UUID can be suboptimal for many uses-cases because:

  • It isn't the most character efficient way of encoding 128 bits of randomness
  • UUID v1/v2 is impractical in many environments, as it requires access to a unique, stable MAC address
  • UUID v3/v5 requires a unique seed and produces randomly distributed IDs, which can cause fragmentation in many data structures
  • UUID v4 provides no other information than randomness which can cause fragmentation in many data structures

Instead, herein is proposed ULID:


  • 128-bit compatibility with UUID
  • 1.21e+24 unique ULIDs per millisecond
  • Lexicographically sortable!
  • Canonically encoded as a 26 character string, as opposed to the 36 character UUID
  • Uses Crockford's base32 for better efficiency and readability (5 bits per character)
  • Case insensitive
  • No special characters (URL safe)
  • Monotonic sort order (correctly detects and handles the same millisecond)


Below is the current specification of ULID as implemented in this crate.

 01AN4Z07BY      79KA1307SR9X4MV3

|----------|    |----------------|
 Timestamp          Randomness
   48bits             80bits



  • 48 bit integer
  • UNIX-time in milliseconds
  • Won't run out of space until +10889-08-02T05:31:50.655Z.


  • 80 bits
  • Cryptographically secure source of randomness, if possible


The left-most character must be sorted first, and the right-most character sorted last (lexical order). The default ASCII character set must be used. Within the same millisecond, sort order is not guaranteed

Canonical String Representation


t is Timestamp (10 characters)
r is Randomness (16 characters)


Crockford's Base32 is used as shown. This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse.



When generating a ULID within the same millisecond, we can provide some guarantees regarding sort order. Namely, if the same millisecond is detected, the random component is incremented by 1 bit in the least significant bit position (with carrying).

If, in the extremely unlikely event that, you manage to generate more than 280 ULIDs within the same millisecond, or cause the random component to overflow with less, the generation will fail.

Overflow Errors when Parsing Base32 Strings

Technically, a 26-character Base32 encoded string can contain 130 bits of information, whereas a ULID must only contain 128 bits. Therefore, the largest valid ULID encoded in Base32 is 7ZZZZZZZZZZZZZZZZZZZZZZZZZ, which corresponds to an epoch time of 281474976710655 or 2 ^ 48 - 1.

Any attempt to decode or encode a ULID larger than this should be rejected by all implementations, to prevent overflow bugs.

Binary Layout and Byte Order

The components are encoded as 16 octets. Each component is encoded with the Most Significant Byte first (network byte order).

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                      32_bit_uint_time_high                    |
|     16_bit_uint_time_low      |       16_bit_uint_random      |
|                       32_bit_uint_random                      |
|                       32_bit_uint_random                      |


pub use crate::crockford::DecodingError;



Contains functions for encoding and decoding of crockford Base32 strings.



The ULID data type.



Returns new ULID bytes.


Returns a new ULID string.