# Universally Unique Lexicographically Sortable Identifier
[](http://travis-ci.org/oklog/ulid)
[](https://goreportcard.com/report/oklog/ulid)
[](https://coveralls.io/github/oklog/ulid?branch=master)
[](https://godoc.org/github.com/oklog/ulid)
[](https://raw.githubusercontent.com/oklog/ulid/master/LICENSE)
A Go port of [alizain/ulid](https://github.com/alizain/ulid) with binary format implemented.
## Background
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)
## Install
```shell
go get github.com/oklog/ulid
```
## Usage
An ULID is constructed with a `time.Time` and an `io.Reader` entropy source.
This design allows for greater flexibility in choosing your trade-offs.
Please note that `rand.Rand` from the `math` package is *not* safe for concurrent use.
Instantiate one per long living go-routine or use a `sync.Pool` if you want to avoid the potential contention of a locked `rand.Source` as its been frequently observed in the package level functions.
```go
func ExampleULID() {
t := time.Unix(1000000, 0)
entropy := ulid.Monotonic(rand.New(rand.NewSource(t.UnixNano())), 0)
fmt.Println(ulid.MustNew(ulid.Timestamp(t), entropy))
// Output: 0000XSNJG0MQJHBF4QX1EFD6Y3
}
```
## Commandline tool
This repo also provides a tool to generate and parse ULIDs at the command line.
Installation:
```shell
go get github.com/oklog/ulid/cmd/ulid
```
Usage:
```shell
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
```
Examples:
```shell
$ ulid
01D78XYFJ1PRM1WPBCBT3VHMNV
$ ulid -z
01D78XZ44G0000000000000000
$ ulid 01D78XZ44G0000000000000000
Sun Mar 31 03:51:23.536 UTC 2019
$ ulid --format=rfc3339 --local 01D78XZ44G0000000000000000
2019-03-30T20:51:23.536PDT
```
## Specification
Below is the current specification of ULID as implemented in this repository.
### Components
**Timestamp**
- 48 bits
- UNIX-time in milliseconds
- Won't run out of space till the year 10895 AD
**Entropy**
- 80 bits
- User defined entropy source.
- Monotonicity within the same millisecond with [`ulid.Monotonic`](https://godoc.org/github.com/oklog/ulid#Monotonic)
### Encoding
[Crockford's Base32](http://www.crockford.com/wrmg/base32.html) is used as shown.
This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse.
```
0123456789ABCDEFGHJKMNPQRSTVWXYZ
```
### 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
```
## Test
```shell
go test ./...
```
## Benchmarks
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
- [alizain/ulid](https://github.com/alizain/ulid)
- [RobThree/NUlid](https://github.com/RobThree/NUlid)
- [imdario/go-ulid](https://github.com/imdario/go-ulid)