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</pre>
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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