In many modern software systems—especially distributed architectures—we need identifiers that are globally unique. Traditional identifiers like auto-incrementing integers often fall short in terms of scalability or uniqueness across machines. Two prominent alternatives are the well-known UUIDs and the newer ULIDs. This article explains what ULIDs are, how they differ from UUIDs, and what best practices you should follow when using them.
What is a ULID?
A ULID (Universally Unique Lexicographically Sortable Identifier) is a 128-bit identifier designed to combine uniqueness (like a UUID) with lexicographically sortable ordering based on creation time. The structure is typically:
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The first 48 bits encode the timestamp (milliseconds since Unix epoch)
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The remaining 80 bits encode random data (or pseudo-random)
This means that two ULIDs generated at different times will sort in order of their creation. Also, ULIDs are typically encoded in a base32 form (Crockford’s alphabet) and produce a 26-character string (rather than 36 characters like a typical UUID string).
This timestamp + randomness combo gives ULIDs some attractive properties: sortable by generation time, reasonably compact string representation, and still largely unique in distributed contexts.
How ULIDs differ from UUIDs
Here are the key distinctions between ULIDs and UUIDs:
1. Sortability (lexicographical order):
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UUIDs (especially version 4, which is purely random) do not guarantee any time‐order correlation between identifiers. Their bits are mostly random, so sorting by UUID string doesn’t correspond to creation time.
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ULIDs embed a timestamp at the start, so sorting their string representation does correspond to creation time (at millisecond granularity).
2. String length / encoding:
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Typical UUID string representation: 36 characters (32 hex digits + 4 hyphens) for the canonical form.
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ULID string representation: Usually 26 characters (base32 encoding) without special punctuation. This makes them more compact and URL‐friendly.
3. Timestamp information leakage:
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Some UUID versions (like v1) embed timestamp + MAC address and so leak metadata; others (v4) are purely random.
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ULIDs embed the timestamp explicitly, so the creation time (to millisecond) is encoded in the identifier itself. That may or may not be acceptable depending on your privacy/security requirements.
4. Adoption & ecosystem:
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UUIDs are very widely supported, with many languages, frameworks, databases having built-in support (RFC 4122 etc.).
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ULIDs are newer, less standardised, and support may be less universal.
5. Use‐case fit & performance considerations:
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For systems where uniqueness is paramount and ordering doesn’t matter, UUIDs (particularly v4) remain a solid choice.
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For systems where ordering by time (or insertion order) is helpful (e.g., sorted indexes, time-series, sharding by time) ULIDs bring benefits. For example, they reduce index fragmentation in databases because new IDs arrive roughly in ascending order.
When to choose ULIDs vs UUIDs
Choose ULID when:
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You want identifiers that are unique and roughly ordered by creation time (which can help indexing, querying by range, sharding).
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You care about compact string size and URL/filename-friendly representation (no hyphens, fewer characters).
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You can tolerate that the timestamp (to millisecond precision) is exposed.
Choose UUID when:
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You need the broadest compatibility, library support, and standardisation.
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You don’t care about insertion order or sortability of the ID.
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You want to avoid embedding any metadata (like timestamp) in the identifier itself.
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You prefer a version with stronger randomness/less predictability in distributed contexts.
Best Practices for Using ULIDs
Here are practical guidelines and considerations when adopting ULIDs in your system:
1. Use a well-tested library
Since ULID generation involves timestamp + randomness + encoding (base32), it’s safer to rely on a mature library for your language/platform rather than roll your own. Choose one that handles edge cases (same-millisecond collisions, monotonic increments, time zone/clock issues).
2. Monotonic generation within the same millisecond
If multiple ULIDs are generated within the same millisecond on the same node, you risk collisions or at least loss of lexicographical strict ordering. Many ULID libraries provide “monotonic” generation mode: when a second ID is generated in the same millisecond, the random part is incremented rather than purely random. This preserves ordering and avoids duplicates in that scenario.
3. Consider clock skew and timestamp precision
Because ULIDs rely on the current timestamp (in milliseconds), if your system clock moves backwards (clock skew) or you have multiple nodes whose clocks are not synchronised, the sortability guarantee may break. Make sure system clocks are well synchronised (e.g., via NTP). Also, if your system generates extremely high volumes of events such that many IDs in one millisecond, you may hit limitations of the randomness space (80 bits) or monotonic algorithm.
4. Understand the implications of timestamp exposure
Since the timestamp is encoded in the ULID, anyone seeing the ID may infer the creation time of the object. If your application considers the creation time of an entity to be sensitive, a ULID might leak more metadata than you desire. In such cases you may prefer fully random IDs (UUID v4) or additional obfuscation.
5. Storage & indexing considerations
Because ULIDs sort by creation time, they can reduce index fragmentation in databases compared to completely random IDs. That means insertion order in primary keys or clustered indexes may align with the natural time order and improve performance. However, if you do heavy deletes or re‐inserts, you should still monitor index health.
6. Consistent encoding/representation
Decide early whether you store ULIDs as strings (26 character base32) or as binary (16 bytes) in your database. Ensure consistency across services and serialization formats. Avoid mixing formats (e.g., uppercase vs lowercase, hyphens) because that may break sorting or equality logic.
7. Global uniqueness in distributed systems
While ULIDs are designed to work without central coordination, if you have multiple machines generating IDs at very high throughput, you must still be mindful of collisions (especially if nodes generate many IDs in the same millisecond). Many ULID spec implementations handle this reasonably well, but you should still monitor generation rates and consider fallback strategies for exceptionally high load.
8. Versioning & future compatibility
Note that ULID specification is less rigid than UUIDs; there is no universally enforced RFC like RFC 4122 for UUIDs. Be aware of the particular ULID implementation version you use and its compatibility when mixing with other systems or libraries. Some systems are moving to other identifiers (e.g., UUID v7) that share characteristics with ULID.
Summary
In summary:
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ULIDs offer an attractive alternative to UUIDs when you care about the ordering of identifiers by creation time, compactness, and ease of sorting.
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They differ from UUIDs in that they embed a timestamp, are lexicographically sortable, and typically have a shorter string representation.
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However, ULIDs come with trade-offs: timestamp exposure, slightly more complexity in implementation/monotonic handling, and less mature ecosystem support compared to UUIDs.
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When you adopt ULIDs, follow best practices around monotonic generation, clock synchronization, encoding consistency, and assessing your uniqueness and security requirements.