Module: UUID

Defined in:: lib/uuid.rb

Overview

Generating UUIDs

Call UUID.new to generate and return 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:

10.times do
  p UUID.new
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, use the IEEE 802 MAC address of the machine’s network interface card as the node identifier. Network interface cards have unique MAC address that are 47-bit long (the last bit acts as a broadcast flag). Use ipconfig (Windows), or ifconfig (Unix) to find the MAC address (aka physical address) of a network card. It takes the form of six pairs of hexadecimal digits, separated by hypen or colon, e.g. ‘08-0E-46-21-4B-35’

For more information see RFC 4122.

Configuring the UUID generator

The UUID generator requires a state file which maintains the MAC address and next sequence number to use. By default, the UUID generator will use the file uuid.state contained in the current directory, or in the installation directory.

Use UUID.config to specify a different location for the UUID state file. If the UUID state file does not exist, you can create one manually, or use UUID.config with the options :sequence and :mac_addr.

A UUID state file looks like:

---
last_clock: "0x28227f76122d80"
mac_addr: 08-0E-46-21-4B-35
sequence: "0x1639"

–

Time-based UUID

The UUID specification prescribes the following format for representing UUIDs. Four octets encode the low field of the time stamp, two octects encode the middle field of the timestamp, and two octets encode the high field of the timestamp with the version number. Two octets encode the clock sequence number and six octets encode the unique node identifier.

The timestamp is a 60 bit value holding UTC time as a count of 100 nanosecond intervals since October 15, 1582. UUIDs generated in this manner are guaranteed not to roll over until 3400 AD.

The clock sequence is used to help avoid duplicates that could arise when the clock is set backward in time or if the node ID changes. Although the system clock is guaranteed to be monotonic, the system clock is not guaranteed to be monotonic across system failures. The UUID cannot be sure that no UUIDs were generated with timestamps larger than the current timestamp.

If the clock sequence can be determined at initialization, it is incremented by one. The clock sequence MUST be originally (i.e. once in the lifetime of a system) initialized to a random number to minimize the correlation across systems. The initial value must not be correlated to the node identifier.

The node identifier must be unique for each UUID generator. This is accomplished using the IEEE 802 network card address. For systems with multiple IEEE 802 addresses, any available address can be used. For systems with no IEEE address, a 47 bit random value is used and the multicast bit is set so it will never conflict with addresses obtained from network cards.

UUID state file

The UUID state is contained in the UUID state file. The file name can be specified when configuring the UUID generator with UUID.config. The default is to use the file uuid.state in the current directory, or the installation directory.

The UUID state file is read once when the UUID generator is first used (or configured). The sequence number contained in the UUID is read and used, and the state file is updated to the next sequence number. The MAC address is also read from the state file. The current clock time (in 100ns resolution) is stored in the state file whenever the sequence number is updated, but is never read.

If the UUID generator detects that the system clock has been moved backwards, it will obtain a new sequence in the same manner. So it is possible that the UUID state file will be updated while the application is running. ++

Constant Summary collapse

VERSION =

'1.0.2'

PACKAGE =

"uuid"

STATE_FILE = Default state file.

"uuid.state"

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.

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

0x0100

FORMATS = Formats supported by the UUID generator.

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

FORMATS_LENGTHS = Length (in characters) of UUIDs generated for each of the formats.

{:compact=>32, :default=>36, :urn=>45}

ERROR_INVALID_SEQUENCE = :nodoc:

"Invalid sequence number: found '%s', expected 4 hexdecimal digits"

ERROR_NOT_A_SEQUENCE = :nodoc:

"Not a sequence number: expected integer between 0 and 0xFFFF"

ERROR_INVALID_MAC_ADDR = :nodoc:

"Invalid MAC address: found '%s', expected a number in the format XX-XX-XX-XX-XX-XX"

INFO_INITIALIZED = :nodoc:

"Initialized UUID generator with sequence number 0x%04x and MAC address %s"

ERROR_INITIALIZED_RANDOM_1 = :nodoc:

"Initialized UUID generator with random sequence number/MAC address."

ERROR_INITIALIZED_RANDOM_2 = :nodoc:

"UUIDs are not guaranteed to be unique. Please create a uuid.state file as soon as possible."

IFCONFIG_PATTERN = :nodoc:

/[^:\-](?:[0-9A-Za-z][0-9A-Za-z][:\-]){5}[0-9A-Za-z][0-9A-Za-z][^:\-]/

REGEXP = Regular expression to identify a 36 character UUID. Can be used for a partial match.

/[[:xdigit:]]{8}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{12}/

REGEXP_FULL = Regular expression to identify a 36 character UUID. Can only be used for a full match.

/^[[:xdigit:]]{8}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{4}[:-][[:xdigit:]]{12}$/

@@mutex =

Mutex.new

@@last_clock =

nil

@@logger =

nil

@@state_file =

nil

Class Method Summary collapse

.config(options) ⇒ Object

Configures the UUID generator.
.new(format = nil) ⇒ Object

Generates and returns a new UUID string.
.setup ⇒ Object

Create a uuid.state file by finding the IEEE 802 NIC MAC address for this machine.
.uuid ⇒ Object

Generates and returns a new UUID string.

Class Method Details

.config(options) ⇒ `Object`

Configures the UUID generator. Use this method to specify the UUID state file, logger, etc.

The method accepts the following options:

:state_file – Specifies the location of the state file. If missing, the default is uuid.state
<tt>:logger<tt> – The UUID generator will use this logger to report the state information (optional).
:sequence – Specifies the sequence number (0 to 0xFFFF) to use. Required to create a new state file, ignored if the state file already exists.
:mac_addr – Specifies the MAC address (xx-xx-xx-xx-xx) to use. Required to create a new state file, ignored if the state file already exists.

For example, to create a new state file:

UUID.config :state_file=>'my-uuid.state', :sequence=>rand(0x10000), :mac_addr=>'0C-0E-35-41-60-65'

To use an existing state file and log to STDOUT:

UUID.config :state_file=>'my-uuid.state', :logger=>Logger.new(STDOUT)

:call-seq:

UUID.config(config)

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

def self.config options
  options ||= {}
  @@mutex.synchronize do
    @@logger = options[:logger]
    next_sequence options
  end
end

.new(format = nil) ⇒ `Object`

Generates and returns a new UUID string.

The argument format specifies which formatting template to use:

:default – Produces 36 characters, including hyphens separating the UUID value parts
:compact – Produces a 32 digits (hexadecimal) value with no hyphens
:urn – Aadds the prefix urn:uuid: to the :default format

For example:

print UUID.new :default

or just

print UUID.new

:call-seq:

UUID.new([format])  -> string

# File 'lib/uuid.rb', line 182

def new format = nil
  # Determine which format we're using for the UUID string.
  template = FORMATS[format || :default] or
      raise RuntimeError, "I don't know the format '#{format}'"

  # 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
    # Initialize UUID generator if not already initialized. Uninitizlied UUID generator has no
    # last known clock.
    next_sequence unless @@last_clock
    clock = (Time.new.to_f * CLOCK_MULTIPLIER).to_i & 0xFFFFFFFFFFFFFFF0
    if clock > @@last_clock
      @@drift = 0
      @@last_clock = clock
    elsif clock = @@last_clock
      drift = @@drift += 1
      if drift < 10000
        @@last_clock += 1
      else
        Thread.pass
        nil
      end
    else
      next_sequence
      @@last_clock = clock
    end
  end while not clock
  sprintf template, clock & 0xFFFFFFFF, (clock >> 32)& 0xFFFF, ((clock >> 48) & 0xFFFF | VERSION_CLOCK),
          @@sequence & 0xFFFF, @@mac_hex & 0xFFFFFFFFFFFF
end

.setup ⇒ `Object`

Create a uuid.state file by finding the IEEE 802 NIC MAC address for this machine. Works for UNIX (ifconfig) and Windows (ipconfig). Creates the uuid.state file in the installation directory (typically the GEM’s lib).

# File 'lib/uuid.rb', line 255

def self.setup
  file = File.expand_path(File.dirname(__FILE__))
  file = File.basename(file) == 'lib' ? file = File.join(file, '..', STATE_FILE) : file = File.join(file, STATE_FILE)
  file = File.expand_path(file)
  if File.exist? file
    puts "#{PACKAGE}: Found an existing UUID state file: #{file}"
  else
    puts "#{PACKAGE}: No UUID state file found, attempting to create one for you:"
    # Run ifconfig for UNIX, or ipconfig for Windows.
    config = ""
    Kernel.open "|ifconfig" do |input|
      input.each_line { |line| config << line }
    end rescue nil
    Kernel.open "|ipconfig /all" do |input|
      input.each_line { |line| config << line }
    end rescue nil

    addresses = config.scan(IFCONFIG_PATTERN).collect { |addr| addr[1..-2] }
    if addresses.empty?
      puts "Could not find any IEEE 802 NIC MAC addresses for this machine."
      puts "You need to create the uuid.state file manually."
    else
      puts "Found the following IEEE 802 NIC MAC addresses on your computer:"
      addresses.each { |addr| puts "  #{addr}" }
      puts "Selecting the first address #{addresses[0]} for use in your UUID state file."
      File.open file, "w" do |output|
        output.puts "mac_addr: #{addresses[0]}"
        output.puts format("sequence: \"0x%04x\"", rand(0x10000))
      end
      puts "Created a new UUID state file: #{file}"
    end
  end
  file
end

.uuid ⇒ `Object`

Generates and returns a new UUID string.

The argument format specifies which formatting template to use:

:default – Produces 36 characters, including hyphens separating the UUID value parts
:compact – Produces a 32 digits (hexadecimal) value with no hyphens
:urn – Aadds the prefix urn:uuid: to the :default format

For example:

print UUID.new :default

or just

print UUID.new

:call-seq:

UUID.new([format])  -> string

# File 'lib/uuid.rb', line 220

def new format = nil
  # Determine which format we're using for the UUID string.
  template = FORMATS[format || :default] or
      raise RuntimeError, "I don't know the format '#{format}'"

  # 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
    # Initialize UUID generator if not already initialized. Uninitizlied UUID generator has no
    # last known clock.
    next_sequence unless @@last_clock
    clock = (Time.new.to_f * CLOCK_MULTIPLIER).to_i & 0xFFFFFFFFFFFFFFF0
    if clock > @@last_clock
      @@drift = 0
      @@last_clock = clock
    elsif clock = @@last_clock
      drift = @@drift += 1
      if drift < 10000
        @@last_clock += 1
      else
        Thread.pass
        nil
      end
    else
      next_sequence
      @@last_clock = clock
    end
  end while not clock
  sprintf template, clock & 0xFFFFFFFF, (clock >> 32)& 0xFFFF, ((clock >> 48) & 0xFFFF | VERSION_CLOCK),
          @@sequence & 0xFFFF, @@mac_hex & 0xFFFFFFFFFFFF
end

Module: UUID

Overview

Generating UUIDs

UUIDs in Brief

Configuring the UUID generator

Time-based UUID

UUID state file

Constant Summary collapse

Class Method Summary collapse

Class Method Details

.config(options) ⇒ Object

.new(format = nil) ⇒ Object

.setup ⇒ Object

.uuid ⇒ Object

.config(options) ⇒ `Object`

.new(format = nil) ⇒ `Object`

.setup ⇒ `Object`

.uuid ⇒ `Object`