Programming language: Go
License: Apache License 2.0
Tags: UUID    
Latest version: v2.0.2

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Universally Unique Lexicographically Sortable Identifier

Project status Build Status Go Report Card Coverage Status go.dev reference Apache 2 licensed

A Go port of ulid/javascript with binary format implemented.


A GUID/UUID can be suboptimal for many use-cases because:

  • It isn't the most character efficient way of encoding 128 bits
  • UUID v1/v2 is impractical in many environments, as it requires access to a unique, stable MAC address
  • UUID v3/v5 requires a unique seed and produces randomly distributed IDs, which can cause fragmentation in many data structures
  • UUID v4 provides no other information than randomness which can cause fragmentation in many data structures

A ULID however:

  • Is compatible with UUID/GUID's
  • 1.21e+24 unique ULIDs per millisecond (1,208,925,819,614,629,174,706,176 to be exact)
  • Lexicographically sortable
  • Canonically encoded as a 26 character string, as opposed to the 36 character UUID
  • Uses Crockford's base32 for better efficiency and readability (5 bits per character)
  • Case insensitive
  • No special characters (URL safe)
  • Monotonic sort order (correctly detects and handles the same millisecond)


This package requires Go modules.

go get github.com/oklog/ulid/v2


ULIDs are constructed from two things: a timestamp with millisecond precision, and some random data.

Timestamps are modeled as uint64 values representing a Unix time in milliseconds. They can be produced by passing a time.Time to ulid.Timestamp, or by calling time.Time.UnixMilli and converting the returned value to uint64.

Random data is taken from a provided io.Reader. This design allows for greater flexibility when choosing trade-offs, but can be a bit confusing to newcomers.

If you just want to generate a ULID and don't (yet) care about details like performance, cryptographic security, etc., use the ulid.Make helper function. This function calls time.Now to get a timestamp, and uses a source of entropy which is process-global, pseudo-random, and monotonic.


More advanced use cases should utilize ulid.New.

entropy := rand.New(rand.NewSource(time.Now().UnixNano()))
ms := ulid.Timestamp(time.Now())
println(ulid.New(ms, entropy))

Care should be taken when providing a source of entropy.

The above example utilizes math/rand.Rand, which is not safe for concurrent use by multiple goroutines. Consider alternatives such as x/exp/rand. Security-sensitive use cases should always use cryptographically secure entropy provided by crypto/rand.

Performance-sensitive use cases should avoid synchronization when generating IDs. One option is to use a unique source of entropy for each concurrent goroutine, which results in no lock contention, but cannot provide strong guarantees about the random data, and does not provide monotonicity within a given millisecond. One common performance optimization is to pool sources of entropy using a sync.Pool.

Monotonicity is a property that says each ULID is "bigger than" the previous one. ULIDs are automatically monotonic, but only to millisecond precision. ULIDs generated within the same millisecond are ordered by their random component, which means they are by default un-ordered. You can use ulid.MonotonicEntropy or ulid.LockedMonotonicEntropy to create ULIDs that are monotonic within a given millisecond, with caveats. See the documentation for details.

If you don't care about time-based ordering of generated IDs, then there's no reason to use ULIDs! There are many other kinds of IDs that are easier, faster, smaller, etc. Consider UUIDs.

Commandline tool

This repo also provides a tool to generate and parse ULIDs at the command line.

go install github.com/oklog/ulid/v2/cmd/ulid@latest


Usage: ulid [-hlqz] [-f <format>] [parameters ...]
 -f, --format=<format>  when parsing, show times in this format: default, rfc3339, unix, ms
 -h, --help             print this help text
 -l, --local            when parsing, show local time instead of UTC
 -q, --quick            when generating, use non-crypto-grade entropy
 -z, --zero             when generating, fix entropy to all-zeroes


$ ulid
$ ulid -z
$ ulid 01D78XZ44G0000000000000000
Sun Mar 31 03:51:23.536 UTC 2019
$ ulid --format=rfc3339 --local 01D78XZ44G0000000000000000


Below is the current specification of ULID as implemented in this repository.



  • 48 bits
  • UNIX-time in milliseconds
  • Won't run out of space till the year 10889 AD


  • 80 bits
  • User defined entropy source.
  • Monotonicity within the same millisecond with ulid.Monotonic


Crockford's Base32 is used as shown. This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse.


Binary Layout and Byte Order

The components are encoded as 16 octets. Each component is encoded with the Most Significant Byte first (network byte order).

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                      32_bit_uint_time_high                    |
|     16_bit_uint_time_low      |       16_bit_uint_random      |
|                       32_bit_uint_random                      |
|                       32_bit_uint_random                      |

String Representation

 01AN4Z07BY      79KA1307SR9X4MV3
|----------|    |----------------|
 Timestamp           Entropy
  10 chars           16 chars
   48bits             80bits
   base32             base32


go test ./...


On a Intel Core i7 Ivy Bridge 2.7 GHz, MacOS 10.12.1 and Go 1.8.0beta1

BenchmarkNew/WithCryptoEntropy-8      2000000        771 ns/op      20.73 MB/s   16 B/op   1 allocs/op
BenchmarkNew/WithEntropy-8            20000000      65.8 ns/op     243.01 MB/s   16 B/op   1 allocs/op
BenchmarkNew/WithoutEntropy-8         50000000      30.0 ns/op     534.06 MB/s   16 B/op   1 allocs/op
BenchmarkMustNew/WithCryptoEntropy-8  2000000        781 ns/op      20.48 MB/s   16 B/op   1 allocs/op
BenchmarkMustNew/WithEntropy-8        20000000      70.0 ns/op     228.51 MB/s   16 B/op   1 allocs/op
BenchmarkMustNew/WithoutEntropy-8     50000000      34.6 ns/op     462.98 MB/s   16 B/op   1 allocs/op
BenchmarkParse-8                      50000000      30.0 ns/op     866.16 MB/s    0 B/op   0 allocs/op
BenchmarkMustParse-8                  50000000      35.2 ns/op     738.94 MB/s    0 B/op   0 allocs/op
BenchmarkString-8                     20000000      64.9 ns/op     246.40 MB/s   32 B/op   1 allocs/op
BenchmarkMarshal/Text-8               20000000      55.8 ns/op     286.84 MB/s   32 B/op   1 allocs/op
BenchmarkMarshal/TextTo-8             100000000     22.4 ns/op     714.91 MB/s    0 B/op   0 allocs/op
BenchmarkMarshal/Binary-8             300000000     4.02 ns/op    3981.77 MB/s    0 B/op   0 allocs/op
BenchmarkMarshal/BinaryTo-8           2000000000    1.18 ns/op   13551.75 MB/s    0 B/op   0 allocs/op
BenchmarkUnmarshal/Text-8             100000000     20.5 ns/op    1265.27 MB/s    0 B/op   0 allocs/op
BenchmarkUnmarshal/Binary-8           300000000     4.94 ns/op    3240.01 MB/s    0 B/op   0 allocs/op
BenchmarkNow-8                        100000000     15.1 ns/op     528.09 MB/s    0 B/op   0 allocs/op
BenchmarkTimestamp-8                  2000000000    0.29 ns/op   27271.59 MB/s    0 B/op   0 allocs/op
BenchmarkTime-8                       2000000000    0.58 ns/op   13717.80 MB/s    0 B/op   0 allocs/op
BenchmarkSetTime-8                    2000000000    0.89 ns/op    9023.95 MB/s    0 B/op   0 allocs/op
BenchmarkEntropy-8                    200000000     7.62 ns/op    1311.66 MB/s    0 B/op   0 allocs/op
BenchmarkSetEntropy-8                 2000000000    0.88 ns/op   11376.54 MB/s    0 B/op   0 allocs/op
BenchmarkCompare-8                    200000000     7.34 ns/op    4359.23 MB/s    0 B/op   0 allocs/op

Prior Art

*Note that all licence references and agreements mentioned in the ulid README section above are relevant to that project's source code only.