A ULID (Universally Unique Lexicographically Sortable Identifier) is a modern alternative to UUIDs. While UUIDs are excellent for unique identification, they lack predictable sorting and are visually difficult to read. ULIDs solve these problems by providing globally unique identifiers that are also time-sortable and more compact.
ULIDs are becoming popular in event-driven architectures, databases, logging systems, microservices, and distributed applications where both uniqueness and ordering matter.
Why ULIDs Were Created
UUIDs, especially version 4 (random), do not sort chronologically. This is problematic in systems that rely on timestamp ordering, such as log pipelines, message queues, or event storage. Additionally, UUIDs:
- Are lengthy and hard to visually scan
- Do not preserve creation order
- Are not URL-friendly
ULIDs were introduced as a modern solution to these shortcomings, providing both uniqueness and lexicographical sorting.
ULID Structure
A ULID string consists of 26 characters and looks like this:
01F8MECHZX3TBDSZ7XRADM79XV
It is composed of two main parts:
- 48-bit Timestamp — represents the time in milliseconds since Unix epoch
- 80-bit Random Component — ensures uniqueness even when many ULIDs are generated in the same millisecond
The Base32 encoding used in ULIDs makes them shorter and easier to handle than UUIDs.
Why ULIDs Are Sortable
Because the timestamp is placed at the beginning of the ULID, sorting ULIDs lexicographically (alphabetically) results in perfectly chronological ordering. This is extremely valuable for:
- Time-series databases
- Event sourcing
- Log indexing
- User activity tracking
- Audit trails
Unlike UUIDs, ULIDs give developers the best of both worlds: randomness and sortable structure.
ULID vs UUID — Key Differences
Here is how ULIDs improve upon UUIDs:
- Sortable: UUID-v4 is not sortable, ULID is
- Length: ULIDs are shorter (26 chars vs UUID’s 36 chars)
- Readability: ULIDs avoid dashes and are more compact
- Entropy: ULIDs include 80 bits of randomness, similar to UUIDv4
- URL-safe: ULIDs contain only alphanumeric characters
This makes ULIDs ideal for systems that require sequencing or where UUIDs may be too bulky.
Where ULIDs Are Used
ULIDs are becoming increasingly popular across cloud architectures and distributed systems. Some common use cases include:
- Database primary keys (PostgreSQL, MongoDB, DynamoDB)
- Message queues and event logs
- Tracking transactions or user actions
- Versioning and ordering events
- Generating IDs in serverless environments
Developers love ULIDs because they solve ordering problems without requiring additional indexing or timestamp fields.
Are ULIDs Truly Unique?
Yes. With 80 bits of randomness, the probability of collision is extremely low—similar to UUIDv4. Even if thousands of ULIDs are generated in the same millisecond, randomness ensures they remain unique.
Conclusion
ULIDs represent an evolution of unique identifiers, offering the predictability and readability that UUIDs often lack. They are compact, URL-safe, sortable, and ideal for distributed systems where chronological order is important. As more developers recognize their benefits, ULIDs are rapidly becoming a preferred alternative to UUIDs in modern applications.
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