Understanding ULID: Features, Specification, and Python Usage Compared to UUID

ULID (Universally Unique Lexicographically Sortable Identifier) is a 128‑bit identifier that combines a millisecond‑precision timestamp with 80 bits of randomness, offering lexicographic ordering and URL‑safe encoding.

UUID has five versions; version 4 (random) is most common but can still collide. Unlike UUID, ULID embeds time, providing 1.21×10²⁴ unique values per millisecond and eliminating collision risk while remaining 128‑bit compatible.

Key ULID characteristics include:

Compatibility with the 128‑bit size of UUID.

1.21×10²⁴ unique IDs per millisecond.

Lexicographic (dictionary) ordering.

Encoded as a 26‑character Crockford Base32 string (no ambiguous characters, case‑insensitive, URL‑safe).

Monotonic ordering within the same millisecond when using the default algorithm.

Specification

Timestamp : 48‑bit integer representing UNIX time in milliseconds (valid until year 10889).

Randomness : 80‑bit random value, preferably generated with cryptographic quality.

Sorting : Characters are ordered lexicographically; the leftmost character is the most significant.

Encoding : Crockford Base32 alphabet (0123456789ABCDEFGHJKMNPQRSTVWXYZ) without I, L, O, U.

Binary layout (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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Typical application scenarios include replacing auto‑increment primary keys, generating globally unique keys without database coordination, sharding databases by embedded timestamp, and ordering records when millisecond precision is sufficient.

Python usage (ulid‑py)

pip install ulid-py

Creating a new ULID:

>>> import ulid
>>> ulid.new()

Creating a ULID from an existing UUID:

>>> import ulid, uuid
>>> value = uuid.uuid4()
>>> value
UUID('0983d0a2-ff15-4d83-8f37-7dd945b5aa39')
>>> ulid.from_uuid(value)

Creating a ULID from a specific timestamp:

>>> import datetime, ulid
>>> ulid.from_timestamp(datetime.datetime(1999, 1, 1))

Creating a ULID from raw randomness:

>>> import os, ulid
>>> randomness = os.urandom(10)
>>> ulid.from_randomness(randomness)

Once you have a ULID object, you can retrieve its components:

>>> u = ulid.new()
>>> u.timestamp()
>>> u.randomness()

For more information, see the ULID project on GitHub: https://github.com/ahawker/ulid .