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Published: Jan 24, 2019 License: GPL-3.0 Imports: 9 Imported by: 0



Package xid is a globally unique id generator suited for web scale

Xid is using Mongo Object ID algorithm to generate globally unique ids: https://docs.mongodb.org/manual/reference/object-id/

  • 4-byte value representing the seconds since the Unix epoch,
  • 3-byte machine identifier,
  • 2-byte process id, and
  • 3-byte counter, starting with a random value.

The binary representation of the id is compatible with Mongo 12 bytes Object IDs. The string representation is using base32 hex (w/o padding) for better space efficiency when stored in that form (20 bytes). The hex variant of base32 is used to retain the sortable property of the id.

Xid doesn't use base64 because case sensitivity and the 2 non alphanum chars may be an issue when transported as a string between various systems. Base36 wasn't retained either because 1/ it's not standard 2/ the resulting size is not predictable (not bit aligned) and 3/ it would not remain sortable. To validate a base32 `xid`, expect a 20 chars long, all lowercase sequence of `a` to `v` letters and `0` to `9` numbers (`[0-9a-v]{20}`).

UUID is 16 bytes (128 bits), snowflake is 8 bytes (64 bits), xid stands in between with 12 bytes with a more compact string representation ready for the web and no required configuration or central generation server.


  • Size: 12 bytes (96 bits), smaller than UUID, larger than snowflake
  • Base32 hex encoded by default (16 bytes storage when transported as printable string)
  • Non configured, you don't need set a unique machine and/or data center id
  • K-ordered
  • Embedded time with 1 second precision
  • Unicity guaranted for 16,777,216 (24 bits) unique ids per second and per host/process

Best used with xlog's RequestIDHandler (https://godoc.org/github.com/rs/xlog#RequestIDHandler).




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var ErrInvalidID = errors.New("xid: invalid ID")

ErrInvalidID is returned when trying to unmarshal an invalid ID


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type ID

type ID [rawLen]byte

ID represents a unique request id

func FromString

func FromString(id string) (ID, error)

FromString reads an ID from its string representation

func New

func New() ID

New generates a globaly unique ID

func (ID) Counter

func (id ID) Counter() int32

Counter returns the incrementing value part of the id. It's a runtime error to call this method with an invalid id.

func (ID) Machine

func (id ID) Machine() []byte

Machine returns the 3-byte machine id part of the id. It's a runtime error to call this method with an invalid id.

func (ID) MarshalText

func (id ID) MarshalText() ([]byte, error)

MarshalText implements encoding/text TextMarshaler interface

func (ID) Pid

func (id ID) Pid() uint16

Pid returns the process id part of the id. It's a runtime error to call this method with an invalid id.

func (*ID) Scan

func (id *ID) Scan(value interface{}) (err error)

Scan implements the sql.Scanner interface.

func (ID) String

func (id ID) String() string

String returns a base32 hex lowercased with no padding representation of the id (char set is 0-9, a-v).

func (ID) Time

func (id ID) Time() time.Time

Time returns the timestamp part of the id. It's a runtime error to call this method with an invalid id.

func (*ID) UnmarshalText

func (id *ID) UnmarshalText(text []byte) error

UnmarshalText implements encoding/text TextUnmarshaler interface

func (ID) Value

func (id ID) Value() (driver.Value, error)

Value implements the driver.Valuer interface.

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