Understanding ULID: A Universally Unique Lexicographically Sortable Identifier

ULID (Universally Unique Lexicographically Sortable Identifier) is an alternative to UUID that combines a timestamp with randomness, offering lexicographic ordering and a lower risk of collisions.

Why not choose UUID

UUID has five versions; version 1 requires a MAC address, version 2 uses UID/GID, version 3/5 rely on MD5/SHA‑1 hashing with a seed, and version 4 is purely random. Even UUID‑4 can collide.

ULID Features

ulid() # 01ARZ3NDEKTSV4RRFFQ69G5FAV

128‑bit compatibility with UUID

1.21e+24 unique ULIDs per millisecond

Lexicographically sortable

Encoded as 26 characters (Crockford Base32)

Case‑insensitive and URL‑safe

Monotonic ordering within the same millisecond

ULID Specification

The binary layout consists of a 48‑bit timestamp (milliseconds since Unix epoch) followed by an 80‑bit random component, both encoded in network byte order.

01AN4Z07BY      79KA1307SR9X4MV3

|----------|    |----------------|
 Timestamp      Randomness
 10 chars        16 chars
 48 bits         80 bits

Components

Timestamp

48‑bit integer representing Unix time in milliseconds

Valid until year 10889

Randomness

80‑bit random value, preferably generated with a cryptographic source

Encoding

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

Use Cases

Replace auto‑increment primary keys without database involvement

Distributed systems where ordered, globally unique IDs are needed

Sharding or partitioning databases by embedded timestamp

Sorting records by creation time without a separate timestamp column

Python Usage

Install the library:

pip install ulid-py

Create a new ULID:

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

Convert from an existing UUID:

>> import ulid, uuid
>>> value = uuid.uuid4()
>>> ulid.from_uuid(value)

Create from a timestamp or custom randomness, and retrieve the timestamp and randomness parts via timestamp() and randomness() methods.

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