com.github.f4b6a3.uuid.clockseq

Class DefaultClockSequenceStrategy

java.lang.Object

com.github.f4b6a3.uuid.sequence.AbstractSequence

com.github.f4b6a3.uuid.clockseq.DefaultClockSequenceStrategy

All Implemented Interfaces:

ClockSequenceStrategy, Sequence

public class DefaultClockSequenceStrategy
extends AbstractSequence
implements ClockSequenceStrategy

This class is an implementation of the 'clock sequence' of the RFC-4122. The maximum value of this sequence is 16,383 or 0x3fff. ### RFC-4122 - 4.1.5. Clock Sequence (1) For UUID version 1, the clock sequence is used to help avoid duplicates that could arise when the clock is set backwards in time or if the node ID changes. (2) If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence has to be changed. If the previous value of the clock sequence is known, it can just be incremented; otherwise it should be set to a random or high-quality pseudo-random value. (3) Similarly, if the node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of clock sequence associated with the changed node ID were known, then the clock sequence could just be incremented, but that is unlikely. (4) 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. This provides maximum protection against node identifiers that may move or switch from system to system rapidly. The initial value MUST NOT be correlated to the node identifier.

Field Summary

Fields

Modifier and Type	Field and Description
protected static int	SEQUENCE_MAX
protected static int	SEQUENCE_MIN

Fields inherited from class com.github.f4b6a3.uuid.sequence.AbstractSequence

MAX_VALUE, MIN_VALUE, value

Constructor Summary

Constructors

Constructor and Description
DefaultClockSequenceStrategy()
DefaultClockSequenceStrategy(long timestamp, long nodeIdentifier, UuidState state) This constructor uses a state stored previously. ### RFC-4122 - 4.1.5.

Method Summary

Modifier and Type	Method and Description
int	getClockSequence(long timestamp, long nodeIdentifier) Get the next value for a timestamp and a node identifier. ### RFC-4122 - 4.1.5.
void	reset()

Methods inherited from class com.github.f4b6a3.uuid.sequence.AbstractSequence

current, max, min, next, set

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

SEQUENCE_MIN

protected static final int SEQUENCE_MIN

See Also:

Constant Field Values

SEQUENCE_MAX

protected static final int SEQUENCE_MAX

See Also:

Constant Field Values

Constructor Detail

DefaultClockSequenceStrategy

public DefaultClockSequenceStrategy()

DefaultClockSequenceStrategy

public DefaultClockSequenceStrategy(long timestamp,
                                    long nodeIdentifier,
                                    UuidState state)

This constructor uses a state stored previously. ### RFC-4122 - 4.1.5. Clock Sequence (1) For UUID version 1, the clock sequence is used to help avoid duplicates that could arise when the clock is set backwards in time or if the node ID changes. (2) If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence has to be changed. If the previous value of the clock sequence is known, it can just be incremented; otherwise it should be set to a random or high-quality pseudo-random value. (3) Similarly, if the node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of clock sequence associated with the changed node ID were known, then the clock sequence could just be incremented, but that is unlikely. (4) 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. This provides maximum protection against node identifiers that may move or switch from system to system rapidly. The initial value MUST NOT be correlated to the node identifier.

Parameters:

timestamp - the current timestamp

nodeIdentifier - the current node identifier

state - the previous state saved

Method Detail

getClockSequence

public int getClockSequence(long timestamp,
                            long nodeIdentifier)

Get the next value for a timestamp and a node identifier. ### RFC-4122 - 4.1.5. Clock Sequence (2a) If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence has to be changed. If the previous value of the clock sequence is known, it can just be incremented; otherwise it should be set to a random or high-quality pseudo-random value. ### RFC-4122 - 4.2.1. Basic Algorithm (6b) If the state was available, but the saved timestamp is later than the current timestamp, increment the clock sequence value. ### RFC-4122 - 4.2.1.2. System Clock Resolution (3c) If a system overruns the generator by requesting too many UUIDs within a single system time interval, the UUID service MUST either return an error, or stall the UUID generator until the system clock catches up.

Specified by:

getClockSequence in interface ClockSequenceStrategy

Parameters:

timestamp - a timestamp

nodeIdentifier - a node identifier

Returns:

a clock sequence

Throws:

OverrunException - an overrun exception

reset

public void reset()

Specified by:

reset in interface Sequence

Overrides:

reset in class AbstractSequence