Understanding ULID: A Universally Unique Lexicographically Sortable Identifier and Its Python Usage

ULID (Universally Unique Lexicographically Sortable Identifier) is introduced as an alternative to UUID, offering both timestamp‑based ordering and high‑entropy randomness, with a 128‑bit representation that is URL‑safe and lexicographically sortable.

Why not choose UUID? UUID has five versions; version 1 and 2 rely on MAC addresses, version 3 and 5 use hash algorithms that can cause fragmentation, and version 4 is purely random. Even UUID‑4 can collide, whereas ULID combines millisecond‑precision timestamps with 1.21e+24 possible random values per millisecond, eliminating collision risk and producing a more readable string.

ULID Features

128‑bit compatibility with UUID

1.21e+24 unique IDs per millisecond

Lexicographic (alphabetical) sorting

Encoded as 26 characters (vs. 36 for UUID)

Uses Crockford's Base32 for efficiency and readability

Case‑insensitive and URL‑safe (no special characters)

Monotonic ordering within the same millisecond

ULID Specification

The identifier consists of a 48‑bit timestamp (milliseconds since Unix epoch) and an 80‑bit random component. The timestamp provides ordering up to the year 10889, and the random part ensures uniqueness.

Composition

Timestamp

48‑bit integer representing Unix time in milliseconds

Randomness

80‑bit random number, preferably generated with cryptographic quality

Sorting

Characters are ordered left‑to‑right using the default ASCII set; within the same millisecond, ordering is not guaranteed.

Encoding

Uses Crockford's Base32 alphabet (0123456789ABCDEFGHJKMNPQRSTVWXYZ), which omits I, L, O, and U to avoid confusion.

Binary Layout

The 128 bits are encoded as sixteen octets in network byte order (big‑endian).

Application Scenarios

Replace auto‑increment primary keys in databases, removing the need for DB‑side key generation.

In distributed environments, substitute UUIDs with globally unique, millisecond‑ordered IDs.

Use the embedded timestamp for time‑based sharding or partitioning.

Sort records by ULID instead of a separate created_at column when millisecond precision is sufficient.

Python Usage

Install the library:

pip install ulid-py

Create a new ULID:

import ulid
ulid.new()  # → ULID('01BJQE4QTHMFP0S5J153XCFSP9')

Generate a ULID from an existing UUID:

import ulid, uuid
value = uuid.uuid4()
ulid.from_uuid(value)  # → ULID('09GF8A5ZRN9P1RYDVXV52VBAHS')

Create a ULID from a specific timestamp:

import datetime, ulid
ulid.from_timestamp(datetime.datetime(1999, 1, 1))  # → ULID('00TM9HX0008S220A3PWSFVNFEH')

Create a ULID from custom randomness bytes:

import os, ulid
randomness = os.urandom(10)
ulid.from_randomness(randomness)  # → ULID('01BJQHX2XEDK0VN0GMYWT9JN8S')

ULID objects provide methods:

u = ulid.new()
u.timestamp()   # returns the 48‑bit timestamp component
u.randomness()   # returns the 80‑bit randomness component

Source code is available at github.com/ahawker/ulid .