关于分布式唯一ID，snowflake的一些思考及改进(完美解决时钟回拨问题)

1.写唯一ID生成器的原由

在阅读工程源码的时候，发现有一个工具职责生成一个消息ID，方便进行全链路的查询，实现方式特别简单，核心源码不过两行，根据时间戳以及随机数生成一个ID，这种算法ID在分布式系统中重复的风险就很明显了。本来以为只是日志打印功能，根据于此在不同系统调用间关联业务日志而已，不过后来发现此ID需要入库，看到这里就觉得有些风险了，于是就想着怎么改造它。

String timeString = String.valueOf(System.currentTimeMillis());
return Long.parseLong(timeString.substring(timeString.length() - 8, timeString.length()))
                * RandomUtils.nextInt(1, 9);

2. Twitter snowflake

既然是分布式唯一ID，自然而然想到了Twitter的snowflake算法，在以前的做的部分业务中也用到过它来生成数据库主键ID，不过当时仅限于使用，以及将64 bit的long数字拆分成几个部分，以保证唯一，对具体实现没有深入研究。正好借着机会深入下。

snowflake拆分long的示意图

该ID生成方式，用41位时间戳保存当前时间的毫秒数（69年后一个轮回），十位机器码，最多可以供一项业务1024台实例，每个毫秒数，12位序列自增，每秒理论上单机环境可生产409.6万个ID，分享一个官方github的scala实现，twitter-archive/snowflake。

3.关于snowflake的一些思考

1.监视器锁

此锁的目的是为了保证在多线程的情况下，只有一个线程进入方法体生成ID，保证并发情况下生成ID的唯一性，如果在竞争激烈情况下，自选锁+ CAS原子变量的方式或许是更为合理的选择，可以达到优化部分性能的目的。

源码中的监视器锁

2.时间回退问题

时间校准，以及其他因素，可能导致服务器时间回退（时间向前快进不会有问题），如果恰巧回退前生成过一些ID，而时间回退后，生成的ID就有可能重复。官方对于此并没有给出解决方案，而是简单的抛错处理，这样会造成在时间被追回之前的这段时间服务不可用，显然我无法接受这一点。

官方的抛错处理

而对于此的思考是，既然snowflake理论情况下单机可实现每秒409.6万个ID的生成上限，实际上能想得到的业务都不太可能产生如此高的并发，那么就会存在在过去的一段时间内，有大量的时间戳“被浪费”，达不到该上限，可能在某一毫秒内只生成几个ID,如果发生了时间回退，这些“被浪费”的资源是不是就能利用起来，而不是抛错。

被浪费的时间戳

如果在内存中建立一个数组，这个数组设定固定长度，比如说200，这些数组中存储上一次该位置对应的毫秒数的messageId，那么就能在时间回退到追回时间这段时间内，再至多提供819200（(2^12) *200）个messageId，如果发生时间回退，就只用在上一次messageId进行+1操作，直到系统时间被追回（此段结合后续源码进行解释）。

4.改进版的snowflake

1.机器码生成器 MachineIdService设计及其实现：

public interface MachineIdService {
    /**
     * 生成MachineId的方法
     *
     * @return machineId 机器码
     * @throws  MessageIdException 获取机器码可能因为外部因素失败
     */
    Long getMachineId() throws MessageIdException;
}

实现该接口确保一个集群中，每台实例生成不同的machineID，并且MachineID 不能超过(2^10) 1023,具体实现方式，可使用MySQL数据库，文件描述映射，Redis自增等方式，这里我使用了Redis自增的方式（所以在需要用到该ID生成器的地方需要依赖Redis），具体实现方式如下：

public class RedisMachineIdServiceImpl implements MachineIdService {
<span class="token keyword">private</span> <span class="token keyword">static</span> final String <span class="token constant">MAX_ID</span> <span class="token operator">=</span> <span class="token string">"MAX_ID"</span><span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> final String <span class="token constant">IP_MACHINE_ID_MAPPING</span> <span class="token operator">=</span> <span class="token string">"IP_MACHINE_ID_MAPPING"</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> RedisTemplate<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> String<span class="token operator">&gt;</span> redisTemplate<span class="token punctuation">;</span>


<span class="token keyword">private</span> String redisKeyPrefix<span class="token punctuation">;</span>

<span class="token comment">//设置RedisTemplate实例</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setRedisTemplate</span><span class="token punctuation">(</span><span class="token parameter">RedisTemplate<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> String<span class="token operator">&gt;</span> redisTemplate</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>redisTemplate <span class="token operator">=</span> redisTemplate<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 设置redisKey前缀，如果多个业务使用同一个Redis集群，使用不同的Redis前缀进行区分</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setRedisKeyPrefix</span><span class="token punctuation">(</span><span class="token parameter">String redisKeyPrefix</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>redisKeyPrefix <span class="token operator">=</span> redisKeyPrefix<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

@Override
<span class="token keyword">public</span> Long <span class="token function">getMachineId</span><span class="token punctuation">(</span><span class="token punctuation">)</span> throws MessageIdException <span class="token punctuation">{</span>
    String host<span class="token punctuation">;</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
        <span class="token comment">//获取本机IP地址</span>
        host <span class="token operator">=</span> InetAddress<span class="token punctuation">.</span><span class="token function">getLocalHost</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getHostAddress</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>UnknownHostException e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">MessageIdException</span><span class="token punctuation">(</span><span class="token string">"Can not get the host!"</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>redisTemplate <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">MessageIdException</span><span class="token punctuation">(</span><span class="token string">"Can not get the redisTemplate instance!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>redisKeyPrefix <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">MessageIdException</span><span class="token punctuation">(</span><span class="token string">"The redis key prefix is null,please set redis key prefix first!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    HashOperations<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> String<span class="token punctuation">,</span> Long<span class="token operator">&gt;</span> hashOperations <span class="token operator">=</span> redisTemplate<span class="token punctuation">.</span><span class="token function">opsForHash</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//通过IP地址在Redis中的映射，找到本机的MachineId</span>
    Long result <span class="token operator">=</span> hashOperations<span class="token punctuation">.</span><span class="token keyword">get</span><span class="token punctuation">(</span>redisKeyPrefix <span class="token operator">+</span> <span class="token constant">IP_MACHINE_ID_MAPPING</span><span class="token punctuation">,</span> host<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>result <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> result<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token comment">//如果没有找到，说明需要对该实例进行新增MachineId，使用Redis的自增函数，生成一个新的MachineId</span>
    Long incrementResult <span class="token operator">=</span> redisTemplate<span class="token punctuation">.</span><span class="token function">opsForValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">increment</span><span class="token punctuation">(</span>redisKeyPrefix <span class="token operator">+</span> <span class="token constant">MAX_ID</span><span class="token punctuation">,</span> <span class="token number">1</span>L<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>incrementResult <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">MessageIdException</span><span class="token punctuation">(</span><span class="token string">"Get the machine id failed,please check the redis environment!"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token comment">//将生成的MachineId放入Redis中，方便下次查找映射</span>
    hashOperations<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>redisKeyPrefix <span class="token operator">+</span> <span class="token constant">IP_MACHINE_ID_MAPPING</span><span class="token punctuation">,</span> host<span class="token punctuation">,</span> incrementResult<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> incrementResult<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

2.MessageIdService设计以及实现

public interface MessageIdService {
<span class="token comment"><span class="hljs-comment">/**
 * 生成一个保证全局唯一的MessageId
 *
 * <span class="hljs-doctag">@return</span> messageId
 */</span></span>
<span class="token keyword"><span class="hljs-function"><span class="hljs-keyword">long</span></span></span><span class="hljs-function"> </span><span class="token function"><span class="hljs-function"><span class="hljs-title">genMessageId</span></span></span><span class="token punctuation"><span class="hljs-function"><span class="hljs-params">(</span></span></span><span class="token punctuation"><span class="hljs-function"><span class="hljs-params">)</span></span></span><span class="token punctuation">;</span>

<span class="token comment"><span class="hljs-comment">/**
 * 初始化方法
 *
 * <span class="hljs-doctag">@throws</span> MessageIdException
 */</span></span>
<span class="token keyword"><span class="hljs-function"><span class="hljs-keyword">void</span></span></span><span class="hljs-function"> </span><span class="token function"><span class="hljs-function"><span class="hljs-title">init</span></span></span><span class="token punctuation"><span class="hljs-function"><span class="hljs-params">(</span></span></span><span class="token punctuation"><span class="hljs-function"><span class="hljs-params">)</span></span></span><span class="hljs-function"> </span><span class="token keyword"><span class="hljs-function"><span class="hljs-keyword">throws</span></span></span><span class="hljs-function"> </span><span class="token class-name"><span class="hljs-function">MessageIdException</span></span><span class="token punctuation">;</span>

}

public class MessageIdServiceImpl implements MessageIdService {

    private static final Logger LOGGER = LoggerFactory.getLogger(MessageIdServiceImpl.class);
    //最大的MachineId，1024个
    private static final long MAX_MACHINE_ID = 1023L;
    //AtomicLongArray 环的大小，可保存200毫秒内，每个毫秒数上一次的MessageId，时间回退的时候依赖与此
    private static final int CAPACITY = 200;
    // 时间戳在messageId中左移的位数
    private static final int TIMESTAMP_SHIFT_COUNT = 22;
    // 机器码在messageId中左移的位数
    private static final int MACHINE_ID_SHIFT_COUNT = 12;
    // 序列号的掩码 2^12 4096
    private static final long SEQUENCE_MASK = 4095L;

    //messageId ，开始的时间戳，start the world，世界初始之日
    private static long START_THE_WORLD_MILLIS;
    //机器码变量
    private long machineId;
    // messageId环，解决时间回退的关键，亦可在多线程情况下减少毫秒数切换的竞争
    private AtomicLongArray messageIdCycle = new AtomicLongArray(CAPACITY);
    //生成MachineIds的实例
    private MachineIdService machineIdService;

    static {
        try {
            //使用一个固定的时间作为start the world的初始值
            START_THE_WORLD_MILLIS = SimpleDateFormat.getDateTimeInstance().parse("2018-09-13 00:00:00").getTime();
        } catch (ParseException e) {
            throw new RuntimeException("init start the world millis failed", e);
        }
    }

    public void setMachineIdService(MachineIdService machineIdService) {
        this.machineIdService = machineIdService;
    }

    /**
     * init方法中通过machineIdService 获取本机的machineId
     * @throws MessageIdException
     */
    @Override
    public void init() throws MessageIdException {
        if (machineId == 0L) {
            machineId = machineIdService.getMachineId();
        }
        //获取的machineId 不能超过最大值
        if (machineId <= 0L || machineId > MAX_MACHINE_ID) {
            throw new MessageIdException("the machine id is out of range,it must between 1 and 1023");
        }
    }
    /**
     * 核心实现的代码
     */
    @Override
    public long genMessageId() {
        do {
            // 获取当前时间戳，此时间戳是当前时间减去start the world的毫秒数
            long timestamp = System.currentTimeMillis() - START_THE_WORLD_MILLIS;
            // 获取当前时间在messageIdCycle 中的下标，用于获取环中上一个MessageId
            int index = (int)(timestamp % CAPACITY);
            long messageIdInCycle = messageIdCycle.get(index);
            //通过在messageIdCycle 获取到的messageIdInCycle，计算上一个MessageId的时间戳
            long timestampInCycle = messageIdInCycle >> TIMESTAMP_SHIFT_COUNT;
            // 如果timestampInCycle 并没有设置时间戳，或时间戳小于当前时间，认为需要设置新的时间戳
            if (messageIdInCycle == 0 || timestampInCycle < timestamp) {
                long messageId = timestamp << TIMESTAMP_SHIFT_COUNT | machineId << MACHINE_ID_SHIFT_COUNT;
                // 使用CAS的方式保证在该条件下，messageId 不被重复
                if (messageIdCycle.compareAndSet(index, messageIdInCycle, messageId)) {
                    return messageId;
                }
                LOGGER.debug("messageId cycle CAS1 failed");
            }
            // 如果当前时间戳与messageIdCycle的时间戳相等，使用环中的序列号+1的方式，生成新的序列号
            // 如果发生了时间回退的情况，（即timestampInCycle > timestamp的情况）那么不能也更新messageIdCycle 的时间戳，使用Cycle中MessageId+1
            if (timestampInCycle >= timestamp) {
                long sequence = messageIdInCycle & SEQUENCE_MASK;
                if (sequence >= SEQUENCE_MASK) {
                    LOGGER.debug("over sequence mask :{}", sequence);
                    continue;
                }
                long messageId = messageIdInCycle + 1L;
                // 使用CAS的方式保证在该条件下，messageId 不被重复
                if (messageIdCycle.compareAndSet(index, messageIdInCycle, messageId)) {
                    return messageId;
                }
                LOGGER.debug("messageId cycle CAS2 failed");
            }
            // 整个生成过程中，采用的spinLock
        } while (true);
    }

}

原文地址：https://www.jianshu.com/p/b1124283fc43