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.

Defined Under Namespace

Modules: Version Classes: Client, Server

Constant Summary collapse

VERSION = 
Version::STRING
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 256

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

  state_file = self.class.state_file
  if state_file && File.size?(state_file) then
    next_sequence
  else
    @mac = mac_address
    fail "Cannot determine MAC address from any available interface, tried with #{mac_address}" if @mac == 0
    @sequence = rand 0x10000

    # Ensure the mode is respected, even with a restrictive umask
    File.open(state_file, 'w') { |f| f.chmod(self.class.mode) } if state_file && !File.exist?(state_file)

    if state_file
      open_lock 'wb' do |io|
        write_state io
      end
    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 126

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

.generator ⇒ Object

Returns the UUID generator used by generate. Useful if you need to mess with it, e.g. force next sequence when forking (e.g. Unicorn, Resque):

after_fork do

UUID.generator.next_sequence

end


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# File 'lib/uuid.rb', line 138

def self.generator
  @uuid ||= new
end

.mode ⇒ Object

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


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# File 'lib/uuid.rb', line 114

def self.mode
  @mode
end

.mode=(mode) ⇒ Object 


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# File 'lib/uuid.rb', line 118

def self.mode=(mode)
  @mode = mode
end

.server=(address) ⇒ Object

Call this to use a UUID Server. Expects address to bind to (SOCKET_NAME is a good default)


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# File 'lib/uuid.rb', line 145

def self.server=(address)
  @uuid = Client.new(address) unless Client === @uuid
end

.state_file(mode = 0644) ⇒ Object

Creates an empty state file in #Dir.tmpdir/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 158

def self.state_file(mode = 0644)
  return @state_file unless @state_file.nil?

  @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 = Dir.tmpdir
  end

  @state_file = File.join(state_dir, 'ruby-uuid')

  if !File.writable?(state_dir) || (File.exist?(@state_file) && !File.writable?(@state_file)) then
    @state_file = File.expand_path('.ruby-uuid', '~')
  end

  @state_file
end

.state_file=(path) ⇒ Object

Specify the path of the state file. Use this if you need a different location for your state file.

Set to false if your system cannot use a state file (e.g. many shared hosts).


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# File 'lib/uuid.rb', line 191

def self.state_file=(path)
  @state_file = path
  @mode ||= 0644
end

.validate(uuid) ⇒ Object

Returns true if uuid is in compact, default or urn formats. Does not validate the layout (RFC 4122 section 4) of the UUID.


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# File 'lib/uuid.rb', line 199

def self.validate(uuid)
  return true if uuid =~ /\A[\da-f]{32}\z/i
  return true if
    uuid =~ /\A(urn:uuid:)?[\da-f]{8}-([\da-f]{4}-){3}[\da-f]{12}\z/i
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 283

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

#iee_mac_address ⇒ Object

Uses system calls to get a mac address


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# File 'lib/uuid.rb', line 238

def iee_mac_address
  begin
    Mac.addr.gsub(/:|-/, '').hex & 0x7FFFFFFFFFFF
  rescue
    0
  end
end

#inspect ⇒ Object 


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# File 'lib/uuid.rb', line 354

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

#mac_address ⇒ Object

return iee_mac_address if available, pseudo_mac_address otherwise


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# File 'lib/uuid.rb', line 249

def mac_address
  return iee_mac_address unless iee_mac_address == 0
  return pseudo_mac_address
end

#next_sequence ⇒ Object

Updates the state file with a new sequence number.


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# File 'lib/uuid.rb', line 330

def next_sequence
  if self.class.state_file
    open_lock 'rb+' do |io|
      @mac, @sequence, @last_clock = read_state(io)

      io.rewind
      io.truncate 0

      @sequence += 1

      write_state io
    end
  else
    @sequence += 1
  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

#pseudo_mac_address ⇒ Object

Generate a pseudo MAC address because we have no pure-ruby way to know the MAC address of the NIC this system uses. Note that cheating with pseudo arresses here is completely legal: see Section 4.5 of RFC4122 for details.

This implementation is shamelessly stolen from

https://github.com/spectra/ruby-uuid/blob/master/uuid.rb

Thanks spectra.


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# File 'lib/uuid.rb', line 215

def pseudo_mac_address
  sha1 = ::Digest::SHA1.new
  256.times do
    r = [rand(0x100000000)].pack "N"
    sha1.update r
  end
  str = sha1.digest
  r = rand 14 # 20-6
  node = str[r, 6] || str
  if RUBY_VERSION >= "1.9.0"
    nnode = node.bytes.to_a
    nnode[0] |= 0x01
    node = ''
    nnode.each { |s| node << s.chr }
  else
    node[0] |= 0x01 # multicast bit
  end
  node.bytes.collect{|b|b.to_s(16)}.join.hex & 0x7FFFFFFFFFFF
end