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KSUID vs ULID — Comparing Time-Sortable ID Formats

KSUID (K-Sortable Unique Identifier), created by Segment.io, and ULID (Universally Unique Lexicographically Sortable Identifier) are both designed to solve the same fundamental problem: generating unique identifiers that sort chronologically. However, they take meaningfully different approaches to bit allocation, timestamp resolution, and encoding. KSUID uses a 160-bit structure consisting of a 32-bit timestamp with second-level precision and a 128-bit random payload, encoded as a 27-character Base62 string. ULID uses a 128-bit structure with a 48-bit millisecond-precision timestamp and 80 bits of randomness, encoded as a 26-character Crockford Base32 string.

The difference in random payload size is one of the most important distinctions. KSUID's 128-bit random component provides the same collision-resistance guarantee as an entire UUID v4, even within the same second. ULID's 80 bits of randomness per millisecond are still more than sufficient for the vast majority of workloads, but systems generating extremely high volumes of IDs within tight time windows may benefit from KSUID's larger entropy pool. On the other hand, ULID's millisecond-precision timestamp provides much finer-grained ordering, which is valuable for event-sourcing systems or logs where sub-second ordering matters.

From an integration standpoint, ULID's 128-bit size makes it directly compatible with UUID storage columns in most databases, which simplifies adoption in systems already using UUID. KSUID's 160-bit size does not fit into a standard UUID column and typically requires a CHAR(27) or BINARY(20) column, which can complicate migrations. ULID also has broader community adoption and library availability across languages, while KSUID benefits from battle-tested production use at Segment, where it was designed to handle billions of events per day.

Side-by-Side Comparison

Propertyksuidulid
Bit Length160128
Output Length2726
Encodingbase62Crockford base32
SortableYesYes
TimestampedYesYes
MonotonicNoYes
Crypto RandomYesYes

ksuid Pros & Cons

Pros

  • 128 bits of randomness per second provides collision resistance equivalent to UUID v4, making it extremely safe for high-throughput distributed systems
  • Base62 encoding produces URL-safe, case-sensitive strings without special characters, fitting naturally into REST APIs and file names
  • Battle-tested in production at Segment.io for handling billions of events daily, giving confidence in real-world reliability at scale
  • Custom epoch starting in 2014 extends the usable timestamp range to well beyond 100 years, avoiding the 2038 problem entirely

Cons

  • 160-bit size does not fit into standard UUID database columns, requiring CHAR(27) or BINARY(20) storage and preventing drop-in UUID replacement
  • Second-level timestamp precision means IDs created within the same second are only ordered by their random component, not by exact creation time
  • Smaller library ecosystem compared to ULID, with fewer implementations available in less mainstream programming languages

ulid Pros & Cons

Pros

  • 48-bit millisecond-precision timestamp provides fine-grained chronological ordering, valuable for event-sourcing, logs, and real-time analytics
  • 128-bit total size is binary-compatible with UUID columns, allowing ULID to be stored in existing UUID infrastructure without schema changes
  • Crockford Base32 encoding is case-insensitive and avoids ambiguous characters, reducing errors in manual transcription or verbal communication
  • Monotonic generation mode guarantees strict ordering for IDs created within the same millisecond, eliminating same-timestamp ordering ambiguity

Cons

  • 80 bits of randomness per millisecond, while sufficient for most workloads, provides less collision resistance than KSUID or UUID v4 for extremely high-volume systems
  • Crockford Base32 is less space-efficient than Base62, resulting in a 26-character string despite having fewer total bits than KSUID
  • No single canonical implementation or governing organization, leading to occasional inconsistencies between library behaviors across languages

Verdict

Choose ULID when you need millisecond-precision ordering, compatibility with existing UUID storage columns, or case-insensitive encoding. Choose KSUID when maximum collision resistance is paramount and you can accommodate the 160-bit size, particularly in high-throughput event pipelines where second-level precision is acceptable.

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Frequently Asked Questions

Which format has better database performance, KSUID or ULID?

Both offer similar B-tree indexing benefits because they are time-sorted, leading to sequential inserts rather than random page splits. ULID has a slight practical edge because its 128-bit size fits into native UUID columns with optimized storage engines, whereas KSUID requires a string or custom binary column.

Can KSUID and ULID be used in the same system?

Yes, but it adds complexity. Since they have different sizes (160-bit vs 128-bit) and encodings (Base62 vs Base32), you would need separate parsing logic and storage columns. In practice, it is better to standardize on one format per system or bounded context.

Is KSUID or ULID better for distributed microservices?

Both are well-suited for distributed systems because neither requires coordination between nodes. ULID is generally preferred for microservices because its 128-bit size simplifies storage and interoperability, while KSUID is favored when you need the extra collision resistance of 128 random bits.

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