Twitter的分布式自增ID算法Snowflake

在分布式系统中，需要生成全局UID的场合还是比较多的，twitter的snowflake解决了这种需求，实现也还是很简单的，除去配置信息，核心代码就是毫秒级时间41位 机器ID 10位 毫秒内序列12位。

 

10---0000000000 0000000000 0000000000 0000000000 0 --- 00000 ---00000 ---000000000000

在上面的字符串中，第一位为未使用（实际上也可作为long的符号位），接下来的41位为毫秒级时间，然后5位datacenter标识位，5位机器ID（并不算标识符，实际是为线程标识），然后12位该毫秒内的当前毫秒内的计数，加起来刚好64位，为一个Long型。

这样的好处是，整体上按照时间自增排序，并且整个分布式系统内不会产生ID碰撞（由datacenter和机器ID作区分），并且效率较高，经测试，snowflake每秒能够产生26万ID左右，完全满足需要。

1

2

3package com.twitter.service.snowflake

4 

5import com.twitter.ostrich.stats.Stats

6import com.twitter.service.snowflake.gen._

7import java.util.Random

8import com.twitter.logging.Logger

9 

10

16class IdWorker(val workerId: Long, val datacenterId: Long, private val reporter: Reporter, var sequence: Long = 0L)

17extends Snowflake.Iface {

18  private[this] def genCounter(agent: String) = {

19    Stats.incr("ids_generated")

20    Stats.incr("ids_generated_%s".format(agent))

21  }

22  private[this] val exceptionCounter = Stats.getCounter("exceptions")

23  private[this] val log = Logger.get

24  private[this] val rand = new Random

25 

26  val twepoch = 1288834974657L

27 

28 //机器标识位数

29 

30  private[this] val workerIdBits = 5L

31 

32//数据中心标识位数

33  private[this] val datacenterIdBits = 5L

34 

35//机器ID最大值

36  private[this] val maxWorkerId = -1L ^ (-1L << workerIdBits)

37 

38//数据中心ID最大值

39  private[this] val maxDatacenterId = -1L ^ (-1L << datacenterIdBits)

40 

41//毫秒内自增位

42  private[this] val sequenceBits = 12L

43 

44//机器ID偏左移12位

45 

46  private[this] val workerIdShift = sequenceBits

47 

48//数据中心ID左移17位

49  private[this] val datacenterIdShift = sequenceBits workerIdBits

50 

51//时间毫秒左移22位

52  private[this] val timestampLeftShift = sequenceBits workerIdBits datacenterIdBits

53  private[this] val sequenceMask = -1L ^ (-1L << sequenceBits)

54 

55  private[this] var lastTimestamp = -1L

56 

57  // sanity check for workerId

58  if (workerId > maxWorkerId || workerId < 0) {

59    exceptionCounter.incr(1)

60    throw new IllegalArgumentException("worker Id can't be greater than %d or less than 0".format(maxWorkerId))

61  }

62 

63  if (datacenterId > maxDatacenterId || datacenterId < 0) {

64    exceptionCounter.incr(1)

65    throw new IllegalArgumentException("datacenter Id can't be greater than %d or less than 0".format(maxDatacenterId))

66  }

67 

68  log.info("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",

69    timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId)

70 

71  def get_id(useragent: String): Long = {

72    if (!validUseragent(useragent)) {

73      exceptionCounter.incr(1)

74      throw new InvalidUserAgentError

75    }

76 

77    val id = nextId()

78    genCounter(useragent)

79 

80    reporter.report(new AuditLogEntry(id, useragent, rand.nextLong))

81    id

82  }

83 

84  def get_worker_id(): Long = workerId

85  def get_datacenter_id(): Long = datacenterId

86  def get_timestamp() = System.currentTimeMillis

87 

88  protected[snowflake] def nextId(): Long = synchronized {

89    var timestamp = timeGen()

90 

91 //时间错误

92 

93    if (timestamp < lastTimestamp) {

94      exceptionCounter.incr(1)

95      log.error("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);

96      throw new InvalidSystemClock("Clock moved backwards.  Refusing to generate id for %d milliseconds".format(

97        lastTimestamp - timestamp))

98    }

99 

100    if (lastTimestamp == timestamp) {

101//当前毫秒内，则 1

102      sequence = (sequence  1) & sequenceMask

103      if (sequence == 0) {

104//当前毫秒内计数满了，则等待下一秒

105        timestamp = tilNextMillis(lastTimestamp)

106      }

107    } else {

108      sequence = 0

109    }

110 

111    lastTimestamp = timestamp

112//ID偏移组合生成最终的ID，并返回ID   

113 

114((timestamp - twepoch) << timestampLeftShift) |

115      (datacenterId << datacenterIdShift) |

116      (workerId << workerIdShift) |

117      sequence

118  }

119 

120//等待下一个毫秒的到来 

121 

122protected def tilNextMillis(lastTimestamp: Long): Long = {

123    var timestamp = timeGen()

124    while (timestamp <= lastTimestamp) {

125      timestamp = timeGen()

126    }

127    timestamp

128  }

129 

130  protected def timeGen(): Long = System.currentTimeMillis()

131 

132  val AgentParser = """([a-zA-Z][a-zA-Z\-0-9]*)""".r

133 

134  def validUseragent(useragent: String): Boolean = useragent match {

135    case AgentParser(_) => true

136    case _ => false

137  }

138}

 

 

转载自： http://blog.sina.com.cn/s/blog_6b7c2e660102vbi2.html#userconsent#

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