spec

Universally Unique Lexicographically Sortable Identifier UUID can be suboptimal for many use-cases because: It isn't the most character efficient way of encoding 128 bits of randomness 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 Instead, herein is proposed ULID: ulid() // 01ARZ3NDEKTSV4RRFFQ69G5FAV 128-bit compatibility with UUID 1.21e+24 unique ULIDs per millisecond 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) Implementations in other languages From ourselves and the community! Language Author Binary Implementation C++ suyash ✓ C# mcb2001 Clojure theikkila Objective-C ricardopereira Crystal SuperPaintman Dart isoos ✓ Dart GuepardoApps Delphi matinusso D enckse D (dub) extrawurst Erlang savonarola Elixir Homepolish ✓ Elixir merongivian Elixir omisego ✓ Elixir (Ecto) dcuddeback ✓ F# lucasschejtman Factor Alexander Ilin ✓ Go oklog ✓ Haskell steven777400 ✓ Java huxi ✓ Java azam Java Lewiscowles1986 JavaScript ulid JavaScript aarondcohen ✓ Julia ararslan Kotlin GuepardoApps Lua Tieske .NET RobThree ✓ .NET fvilers Nim adelq OCaml stripedpajamas Perl 5 bk ✓ PHP Lewiscowles1986 PHP robinvdvleuten PowerShell PetterBomban Python mdipierro Python ahawker ✓ Python mdomke ✓ R hrbrmstr Ruby rafaelsales Rust mmacedoeu ✓ Rust dylanhart ✓ Scala petitviolet SQL-Microsoft rmalayter ✓ Swift simonwhitehouse Swift yaslab ✓ Tcl dbohdan Specification Below is the current specification of ULID as implemented in ulid/javascript. Note: the binary format has not been implemented in JavaScript as of yet. 01AN4Z07BY 79KA1307SR9X4MV3 |----------| |----------------| Timestamp Randomness 48bits 80bits Components Timestamp 48 bit integer UNIX-time in milliseconds Won't run out of space 'til the year 10889 AD. Randomness 80 bits Cryptographically secure source of randomness, if possible Sorting The left-most character must be sorted first, and the right-most character sorted last (lexical order). The default ASCII character set must be used. Within the same millisecond, sort order is not guaranteed Canonical String Representation ttttttttttrrrrrrrrrrrrrrrr where t is Timestamp (10 characters) r is Randomness (16 characters) Encoding Crockford's Base32 is used as shown. This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse. 0123456789ABCDEFGHJKMNPQRSTVWXYZ Monotonicity When generating a ULID within the same millisecond, we can provide some guarantees regarding sort order. Namely, if the same millisecond is detected, the random component is incremented by 1 bit in the least significant bit position (with carrying). For example: import { monotonicFactory } from 'ulid' const ulid = monotonicFactory() // Assume that these calls occur within the same millisecond ulid() // 01BX5ZZKBKACTAV9WEVGEMMVRZ ulid() // 01BX5ZZKBKACTAV9WEVGEMMVS0 If, in the extremely unlikely event that, you manage to generate more than 280 ULIDs within the same millisecond, or cause the random component to overflow with less, the generation will fail. import { monotonicFactory } from 'ulid' const ulid = monotonicFactory() // Assume that these calls occur within the same millisecond ulid() // 01BX5ZZKBKACTAV9WEVGEMMVRY ulid() // 01BX5ZZKBKACTAV9WEVGEMMVRZ ulid() // 01BX5ZZKBKACTAV9WEVGEMMVS0 ulid() // 01BX5ZZKBKACTAV9WEVGEMMVS1 ... ulid() // 01BX5ZZKBKZZZZZZZZZZZZZZZX ulid() // 01BX5ZZKBKZZZZZZZZZZZZZZZY ulid() // 01BX5ZZKBKZZZZZZZZZZZZZZZZ ulid() // throw new Error()! Overflow Errors when Parsing Base32 Strings Technically, a 26-character Base32 encoded string can contain 130 bits of information, whereas a ULID must only contain 128 bits. Therefore, the largest valid ULID encoded in Base32 is 7ZZZZZZZZZZZZZZZZZZZZZZZZZ, which corresponds to an epoch time of 281474976710655 or 2 ^ 48 - 1. Any attempt to decode or encode a ULID larger than this should be rejected by all implementations, to prevent overflow bugs. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Prior Art Partly inspired by: http://instagram-engineering.tumblr.com/post/10853187575/sharding-ids-at-instagram https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html