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itemtype=http://schema.org/Organization><meta content="Nicksxs's Blog" itemprop=name><meta content="Nicksxs 的技术博客,深入 Java 运行时与生产诊断、低成本大模型推理和 Agent 工程实践。" itemprop=description></span><span hidden itemprop=post itemscope itemtype=http://schema.org/CreativeWork><meta content="Redis_分布式锁 | Nicksxs's Blog" itemprop=name><meta content="今天看了一下 redis 分布式锁 redlock 的实现,简单记录下,。原先我对 redis 锁的概念就是加锁使用 setnx,解锁使用 lua 脚本,但是 setnx 具体是啥,lua 脚本是啥不是很清楚 首先简单思考下这个问题,首先为啥不是先 get 一下 key 存不存…" itemprop=description></span><header class=post-header><h1 itemprop="name headline" class=post-title>Redis_分布式锁</h1><div class=post-meta-container><div class=post-meta><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar"></i> </span><span class=post-meta-item-text>发表于</span> <time itemprop="dateCreated datePublished" title="创建时间:2019-12-10 23:26:26" datetime=2019-12-10T23:26:26+08:00>2019-12-10</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-folder"></i> </span><span class=post-meta-item-text>分类于</span> <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Redis/ itemprop=url rel=index><span itemprop=name>Redis</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Redis/Distributed-Lock/ itemprop=url rel=index><span itemprop=name>Distributed Lock</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/C/ itemprop=url rel=index><span itemprop=name>C</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/C/Redis/ itemprop=url rel=index><span itemprop=name>Redis</span></a> </span></span><span class="post-meta-item cloudflare-page-counter" data-increment=true data-path=/2019/12/10/Redis-Part-1/ data-title=Redis_分布式锁 title=阅读次数><span class=post-meta-item-icon><i class="far fa-eye"></i> </span><span class=post-meta-item-text>阅读次数:</span> <span class=cloudflare-page-counter-count></span></span></div><div class=post-description>今天看了一下 redis 分布式锁 redlock 的实现,简单记录下,。原先我对 redis 锁的概念就是加锁使用 setnx,解锁使用 lua 脚本,但是 setnx 具体是啥,lua 脚本是啥不是很清楚 首先简单思考下这个问题,首先为啥不是先 get 一下 key 存不存…</div></div></header><div class=post-body itemprop=articleBody><p>今天看了一下 <code>redis</code> 分布式锁 <code>redlock</code> 的实现,简单记录下,<h2 id=加锁><a class=headerlink href=#加锁 title=加锁></a>加锁</h2><p>原先我对 <code>redis</code> 锁的概念就是加锁使用 <code>setnx</code>,解锁使用 <code>lua</code> 脚本,但是 <code>setnx</code> 具体是啥,<code>lua</code> 脚本是啥不是很清楚<br>首先简单思考下这个问题,首先为啥不是先 <code>get</code> 一下 <code>key</code> 存不存在,然后再 <code>set</code> 一个 <code>key value</code>,因为加锁这个操作我们是要保证两点,一个是不能中途被打断,也就是说要原子性,如果是先 <code>get</code> 一下 <code>key</code>,如果不存在再 <code>set</code> 值的话,那就不是原子操作了;第二个是可不可以直接 <code>set</code> 值呢,显然不行,锁要保证唯一性,有且只能有一个线程或者其他应用单位获得该锁,正好 <code>setnx</code> 给了我们这种原子命令<p>然后是 <code>setnx</code> 的键和值分别是啥,键比较容易想到是要锁住的资源,比如 <code>user_id</code>, 这里有个我自己之前比较容易陷进去的误区,但是这个误区后<br>面再说,这里其实是把<code>user_id</code> 作为要锁住的资源,在我获得锁的时候别的线程不允许操作,以此保证业务的正确性,不会被多个线程同时修改,确定了键,再来看看值是啥,其实原先我认为值是啥都没关系,我只要锁住了,光键就够我用了,但是考虑下多个线程的问题,如果我这个线程加了锁,然后我因为 <code>gc</code> 停顿等原因卡死了,这个时候<code>redis</code> 的锁或者说就是 <code>redis</code> 的缓存已经过期了,这时候另一个线程获得锁成功,然后我这个线程又活过来了,然后我就仍然认为我拿着锁,我去对数据进行修改或者释放锁,是不是就出现问题了,所以是不是我们还需要一个东西来区分这个锁是哪个线程加的,所以我们可以将值设置成为一个线程独有识别的值,至少在相对长的一段时间内不会重复。<p>上面其实还有两个问题,一个是当 <code>gc</code> 超时时,我这个线程如何知道我手里的锁已经过期了,一种方法是我在加好锁之后就维护一个超时时间,这里其实还有个问题,不过跟第二个问题相关,就一起说了,就是设置超时时间,有些对于不是锁的 <code>redis</code> 缓存操作可以是先设置好值,然后在设置过期时间,那么这就又有上面说到的不是原子性的问题,那么就需要在同一条指令里把超时时间也设置了,幸好 <code>redis</code> 提供了这种支持<figure class="highlight plaintext"><table><tr><td class=gutter><pre><span class=line>1</span><br></pre><td class=code><pre><span class=line>SET resource_name my_random_value NX PX 30000</span><br></pre></table></figure><p>这里借鉴一下解释下,<code>resource_name</code>就是 key,代表要锁住的东西,<code>my_random_value</code>就是识别我这个线程的,<code>NX</code>代表只有在不存在的时候才设置,然后<code>PX 30000</code>表示超时时间是 30秒自动过期<p>PS:记录下我原先有的一个误区,是不是要用 key 来区分加锁的线程,这样只有一个用处,就是自身线程可以识别是否是自己加的锁,但是最大的问题是别的线程不知道,其实这个用户的出发点是我在担心前面提过的一个问题,就是当 gc 停顿后,我要去判断当前的这个锁是否是我加的,还有就是当释放锁的时候,如果保证不会错误释放了其他线程加的锁,但是这样附带很多其他问题,最大的就是其他线程怎么知道能不能加这个锁。<h2 id=解锁><a class=headerlink href=#解锁 title=解锁></a>解锁</h2><p>当线程在锁过期之前就处理完了业务逻辑,那就可以提前释放这个锁,那么提前释放要怎么操作,直接<code>del key</code>显然是不行的,因为这样就是我前面想用线程随机值加资源名作为锁的初衷,我不能去释放别的线程加的锁,那么我要怎么办呢,先 <code>get</code> 一下看是不是我的?那又变成非原子的操作了,幸好redis 也考虑到了这个问题,给了<code>lua</code> 脚本来操作这种<figure class="highlight lua"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br></pre><td class=code><pre><span class=line><span class=keyword>if</span> redis.call(<span class=string>"get"</span>,KEYS[<span class=number>1</span>]) == ARGV[<span class=number>1</span>] <span class=keyword>then</span></span><br><span class=line> <span class=keyword>return</span> redis.call(<span class=string>"del"</span>,KEYS[<span class=number>1</span>])</span><br><span class=line><span class=keyword>else</span></span><br><span class=line> <span class=keyword>return</span> <span class=number>0</span></span><br><span class=line><span class=keyword>end</span></span><br></pre></table></figure><p>这里的<code>KEYS[1]</code>就是前面加锁的<code>resource_name</code>,<code>ARGV[1]</code>就是线程的随机值<code>my_random_value</code><h2 id=多节点><a class=headerlink href=#多节点 title=多节点></a>多节点</h2><p>前面说的其实是单节点 <code>redis</code> 作为分布式锁的情况,那么当我们的 <code>redis</code> 有多节点的情况呢,如果多节点下处于加锁或者解锁或者锁有效情况下<br><code>redis</code> 的某个节点宕掉了怎么办,这里就有一些需要思考的地方,是否单独搞一个单节点的 <code>redis</code>作为分布式锁专用的,但是如果这个单节点的挂了呢,还有就是成本问题,所以我们需要一个多节点的分布式锁方案<br>这里就引出了开头说到的<code>redlock</code>,这个可是 <code>redis</code>的作者写的, 他的加锁过程是分以下几步去做这个事情<ul><li>获取当前时间(毫秒数)。<li>按顺序依次向N个Redis节点执行获取锁的操作。这个获取操作跟前面基于单Redis节点的获取锁的过程相同,包含随机字符串my_random_value,也包含过期时间(比如PX 30000,即锁的有效时间)。为了保证在某个Redis节点不可用的时候算法能够继续运行,这个获取锁的操作还有一个超时时间(time out),它要远小于锁的有效时间(几十毫秒量级)。客户端在向某个Redis节点获取锁失败以后,应该立即尝试下一个Redis节点。这里的失败,应该包含任何类型的失败,比如该Redis节点不可用,或者该Redis节点上的锁已经被其它客户端持有(注:Redlock原文中这里只提到了Redis节点不可用的情况,但也应该包含其它的失败情况)。<li>计算整个获取锁的过程总共消耗了多长时间,计算方法是用当前时间减去第1步记录的时间。如果客户端从大多数Redis节点(>= N/2+1)成功获取到了锁,并且获取锁总共消耗的时间没有超过锁的有效时间(lock validity time),那么这时客户端才认为最终获取锁成功;否则,认为最终获取锁失败。<li>如果最终获取锁成功了,那么这个锁的有效时间应该重新计算,它等于最初的锁的有效时间减去第3步计算出来的获取锁消耗的时间。<li>如果最终获取锁失败了(可能由于获取到锁的Redis节点个数少于N/2+1,或者整个获取锁的过程消耗的时间超过了锁的最初有效时间),那么客户端应该立即向所有Redis节点发起释放锁的操作(即前面介绍的Redis Lua脚本)。<br>释放锁的过程比较简单:客户端向所有Redis节点发起释放锁的操作,不管这些节点当时在获取锁的时候成功与否。这里为什么要向所有的节点发送释放锁的操作呢,这里是因为有部分的节点的失败原因可能是加锁时阻塞,加锁成功的结果没有及时返回,所以为了防止这种情况还是需要向所有发起这个释放锁的操作。<br>初步记录就先到这。</ul></div><footer class=post-footer><div class="affiliate-creative affiliate-creative--post-end" data-affiliate-id=tencent-cloud-general><a rel="sponsored noopener" aria-label=腾讯云产品优惠 href=https://cloud.tencent.com/act/cps/redirect?redirect=2446&cps_key=2e33a1eedb380997d03618b4a5ba6ca7&from=console target=_blank><img alt=腾讯云服务器、数据库和云产品优惠 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