Class: UUID

Inherits:
Object
  • Object
show all
Defined in:
lib/uuid.rb

Overview

Generating UUIDs

Call #generate to generate a new UUID. The method returns a string in one of three formats. The default format is 36 characters long, and contains the 32 hexadecimal octets and hyphens separating the various value parts. The :compact format omits the hyphens, while the :urn format adds the :urn:uuid prefix.

For example:

uuid = UUID.new

10.times do
  p uuid.generate
end

UUIDs in Brief

UUID (universally unique identifier) are guaranteed to be unique across time and space.

A UUID is 128 bit long, and consists of a 60-bit time value, a 16-bit sequence number and a 48-bit node identifier.

The time value is taken from the system clock, and is monotonically incrementing. However, since it is possible to set the system clock backward, a sequence number is added. The sequence number is incremented each time the UUID generator is started. The combination guarantees that identifiers created on the same machine are unique with a high degree of probability.

Note that due to the structure of the UUID and the use of sequence number, there is no guarantee that UUID values themselves are monotonically incrementing. The UUID value cannot itself be used to sort based on order of creation.

To guarantee that UUIDs are unique across all machines in the network, the IEEE 802 MAC address of the machine’s network interface card is used as the node identifier.

For more information see RFC 4122.

Constant Summary collapse

VERSION = 
'2.0.1'
CLOCK_MULTIPLIER =

Clock multiplier. Converts Time (resolution: seconds) to UUID clock (resolution: 10ns)

10000000
CLOCK_GAPS =

Clock gap is the number of ticks (resolution: 10ns) between two Ruby Time ticks.

100000
VERSION_CLOCK =

Version number stamped into the UUID to identify it as time-based.

0x0100
FORMATS =

Formats supported by the UUID generator.

:default

Produces 36 characters, including hyphens separating the UUID value parts

:compact

Produces a 32 digits (hexadecimal) value with no hyphens

:urn

Adds the prefix urn:uuid: to the default format

{
  :compact => '%08x%04x%04x%04x%012x',
  :default => '%08x-%04x-%04x-%04x-%012x',
  :urn     => 'urn:uuid:%08x-%04x-%04x-%04x-%012x',
}
STATE_FILE_FORMAT =

MAC address (48 bits), sequence number and last clock

'SLLQ'

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ UUID

Create a new UUID generator. You really only need to do this once.



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# File 'lib/uuid.rb', line 155

def initialize
  @drift = 0
  @last_clock = (Time.now.to_f * CLOCK_MULTIPLIER).to_i
  @mutex = Mutex.new

  if File.exist?(self.class.state_file) then
    next_sequence
  else
    @mac = Mac.addr.gsub(/:|-/, '').hex & 0x7FFFFFFFFFFF
    fail "Cannot determine MAC address from any available interface, tried with #{Mac.addr}" if @mac == 0
    @sequence = rand 0x10000

    open_lock 'w' do |io|
      write_state io
    end
  end
end

Class Method Details

.generate(format = :default) ⇒ Object

Generates a new UUID string using format. See FORMATS for a list of supported formats.



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# File 'lib/uuid.rb', line 112

def self.generate(format = :default)
  @uuid ||= new
  @uuid.generate format
end

.mode ⇒ Object

The access mode of the state file. Set it with state_file.



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# File 'lib/uuid.rb', line 104

def self.mode
  @mode
end

.state_file(mode = 0644) ⇒ Object

Creates an empty state file in /var/tmp/ruby-uuid or the windows common application data directory using mode 0644. Call with a different mode before creating a UUID generator if you want to open access beyond your user by default.

If the default state dir is not writable, UUID falls back to ~/.ruby-uuid.

State files are not portable across machines.



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# File 'lib/uuid.rb', line 126

def self.state_file(mode = 0644)
  return @state_file if @state_file

  @mode = mode

  begin
    require 'Win32API'

    csidl_common_appdata = 0x0023
    path = 0.chr * 260
    get_folder_path = Win32API.new('shell32', 'SHGetFolderPath', 'LLLLP', 'L')
    get_folder_path.call 0, csidl_common_appdata, 0, 1, path

    state_dir = File.join(path.strip)
  rescue LoadError
    state_dir = File.join('', 'var', 'tmp')
  end

  if File.writable?(state_dir) then
    @state_file = File.join(state_dir, 'ruby-uuid')
  else
    @state_file = File.expand_path(File.join('~', '.ruby-uuid'))
  end

  @state_file
end

Instance Method Details

#generate(format = :default) ⇒ Object

Generates a new UUID string using format. See FORMATS for a list of supported formats.

Raises:

  • (ArgumentError) 


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# File 'lib/uuid.rb', line 176

def generate(format = :default)
  template = FORMATS[format]

  raise ArgumentError, "invalid UUID format #{format.inspect}" unless template

  # The clock must be monotonically increasing. The clock resolution is at
  # best 100 ns (UUID spec), but practically may be lower (on my setup,
  # around 1ms). If this method is called too fast, we don't have a
  # monotonically increasing clock, so the solution is to just wait.
  #
  # It is possible for the clock to be adjusted backwards, in which case we
  # would end up blocking for a long time. When backward clock is detected,
  # we prevent duplicates by asking for a new sequence number and continue
  # with the new clock.

  clock = @mutex.synchronize do
    clock = (Time.new.to_f * CLOCK_MULTIPLIER).to_i & 0xFFFFFFFFFFFFFFF0

    if clock > @last_clock then
      @drift = 0
      @last_clock = clock
    elsif clock == @last_clock then
      drift = @drift += 1

      if drift < 10000 then
        @last_clock += 1
      else
        Thread.pass
        nil
      end
    else
      next_sequence
      @last_clock = clock
    end
  end until clock

  template % [
      clock        & 0xFFFFFFFF,
     (clock >> 32) & 0xFFFF,
    ((clock >> 48) & 0xFFFF | VERSION_CLOCK),
    @sequence      & 0xFFFF,
    @mac           & 0xFFFFFFFFFFFF
  ]
end

#inspect ⇒ Object 



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# File 'lib/uuid.rb', line 243

def inspect
  mac = ("%012x" % @mac).scan(/[0-9a-f]{2}/).join(':')
  "MAC: #{mac}  Sequence: #{@sequence}"
end

#next_sequence ⇒ Object

Updates the state file with a new sequence number.



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# File 'lib/uuid.rb', line 223

def next_sequence
  open_lock 'r+' do |io|
    @mac, @sequence, @last_clock = read_state(io)

    io.rewind
    io.truncate 0

    @sequence += 1

    write_state io
  end
rescue Errno::ENOENT
  open_lock 'w' do |io|
    write_state io
  end
ensure
  @last_clock = (Time.now.to_f * CLOCK_MULTIPLIER).to_i
  @drift = 0
end