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authormuqiuhan <[email protected]>2025-05-08 10:32:53 +0000
committermuqiuhan <[email protected]>2025-05-08 10:32:53 +0000
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-rw-r--r--2025/04/02/N-1-selects-problem-与-Prisma-ORM/index.html2
-rw-r--r--2025/04/13/uuidv7-rdbms/index.html2
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- 多人协作系统中的实现策略 (CRDT,锁等实现方案)
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- Apr 2, 2025
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- <p>最近碰到一块业务:在系统中可以存在多个用户同时对某个项目信息进行编辑,这种多人协作的场景挺有意思的,不过在我们的业务中,并不需要实时协作,只需要保证不会出错就行,话虽如此,但也可以探索一下实时协作的实现方案,防止老年痴呆。</p>
-<p>先来看看第一个方案 —— CRDT(Conflict-free Replicated Data Type,无冲突可复制数据类型)是一类数据结构,它保证了在分布式节点(或多客户端)上进行离线&#x2F;并发更新后,无需中心协调、也无需人工干预,通过“合并策略”就能得到一致的最终状态。 </p>
-<p>核心思想是:所有并发操作都是<strong>幂等</strong>(idempotent)、<strong>可交换</strong>(commutative)的。</p>
-<p>常见类型有:<br>一、G-Counter(只能增计数器)<br>二、PN-Counter(可增可减计数器)<br>三、LWW-Register(最后写入胜出)<br>四、结合 JSON 的树型 CRDT(如 <a target="_blank" rel="noopener" href="https://github.com/automerge/automerge">Automerge</a> &#x2F; <a target="_blank" rel="noopener" href="https://yjs.dev/">Yjs</a>) </p>
-<p>更多原理可参考 Decipad 博客“Collaborative and Offline Editing Using CRDTs”[^1]。</p>
-<blockquote>
-<p>有一个挺有趣的 Rust 项目 <a target="_blank" rel="noopener" href="https://github.com/loro-dev/loro">Loro: Make your JSON data collaborative and version-controlled with CRDTs</a></p>
-</blockquote>
-<p>假设我的项目信息编辑页面允许多人实时&#x2F;离线修改某个研究项目的“名称”、“描述”字段,前端用 SvelteKit + GraphQL 获取和提交变更:</p>
-<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">┌── 用户 A 离线修改了 “description” 的若干段文本 </span><br><span class="line">└── 用户 B 同时在线修改了同一字段的其他段落 </span><br></pre></td></tr></table></figure>
-
-<p>如果后端使用 CRDT(比如把 <code>description</code> 用 JSON-CRDT 存储),两次修改只要在任意顺序合并都能得到完整的内容:</p>
-<p>首先,A 客户端本地 apply 操作并缓存,恢复网络后推给服务器;<br>然后,服务器用 CRDT merge(A.delta, B.delta),得到一致文档<br>最后,服务器广播新文档到所有客户端,A&#x2F;B 均得到相同结果 </p>
-<hr>
-<p>好了说点实际符合业务场景的方案,首先想到的是悲观锁(Pessimistic Locking) ,思路是:用户打开编辑界面时,向后端申请“锁” → 其它用户尝试编辑时被拒绝 → 编辑完成后释放锁&#x2F;超时自动释放。 </p>
-<p>假如有一个这样的锁表:</p>
-<pre><code class="sql">CREATE TABLE project_lock (
- project_id UUID PRIMARY KEY,
- locked_by UUID NOT NULL,
- expires_at TIMESTAMPTZ NOT NULL
-);
-</code></pre>
-<p>可以在事务内申请它:</p>
-<pre><code class="ts">const now = new Date();
-const expires = new Date(now.getTime() + 5*60*1000); // 5 分钟后过期
-await prisma.$transaction(async tx =&gt; &#123;
- const existing = await tx.project_lock.findUnique(&#123; where:&#123; project_id &#125; &#125;);
-
- if (existing &amp;&amp; existing.expires_at &gt; now) &#123;
- throw new Error(&#39;项目正被人编辑&#39;);
- &#125;
-
- await tx.project_lock.upsert(&#123;
- where: &#123; project_id &#125;,
- update: &#123; locked_by: userId, expires_at: expires &#125;,
- create: &#123; project_id, locked_by: userId, expires_at: expires &#125;
- &#125;);
-&#125;);
-</code></pre>
-<p>释放锁就直接从锁表里删掉对应的数据即可:</p>
-<pre><code class="ts">await prisma.project_lock.delete(&#123; where:&#123; project_id &#125; &#125;);
-</code></pre>
-<p>前端的话,大概就是:</p>
-<p>在进入编辑前请求一下 <code>/api/project/:id/lock</code> 之类的 API,失败则提示“被占用”;<br>在 <code>onbeforeunload</code> 时执行 <code>/unlock</code>;<br>超时后后端自动允许新锁。 </p>
-<hr>
-<p>第二个方案是乐观并发控制(Optimistic Concurrency) :记录资源的<strong>版本号</strong>或<strong>时间戳</strong>;客户端提交更新时带上自己的版本号,后端检查版本是否一致,不一致则认为冲突,返回 409,由客户端告知用户“数据已过期,请刷新后合并”。 </p>
-<p>具体实现中,可以尝试在 <code>project</code> 表加上 <code>version INT NOT NULL DEFAULT 1, updated_at TIMESTAMPTZ</code> ,然后更新项目时:</p>
-<pre><code class="ts">async updateProject(parent, &#123; id, version, input &#125;, ctx) &#123;
- const result = await prisma.$executeRaw`
- UPDATE project
- SET name = $&#123;input.name&#125;,
- description = $&#123;input.description&#125;,
- version = version + 1,
- updated_at = now()
- WHERE id = $&#123;id&#125; AND version = $&#123;version&#125;
- `;
- if (result === 0) &#123;
- throw new ConflictException(&#39;版本冲突,请刷新后重试&#39;);
- &#125;
- return prisma.project.findUnique(&#123; where:&#123; id &#125; &#125;);
-&#125;
-</code></pre>
-<p>前端捕获到冲突错误可以用一个弹窗提示“另有用户已更新此项目,是否合并&#x2F;重新加载?” 之类的玩意儿。</p>
-<hr>
-<p>第三个方案是:操作转化(Operational Transformation,OT) </p>
-<p>也就是记录用户每次的“操作”(insert&#x2F;delete at position),服务器根据历史操作序列对并发操作做<strong>转化</strong>(transform),确保先到达的操作调整后再应用后到达的。 </p>
-<p>有一些实现案例:<br>一、ShareDB(Node.js)<br>二、Google Docs 中的同步算法 </p>
-<p>具体实现的话,可能要现在前端逐字符&#x2F;块地包装成操作并 WebSocket 推送,服务器再维护一个“操作历史队列”,每来一个 op 就 transform 并 broadcast,而客户端收到广播后,按顺序 replay 保证视图一致。</p>
-<hr>
-<p>最后可能还可以用事件溯源(Event Sourcing)+ 场景命令模式来实现:</p>
-<p>不直接存状态,而是存所有“命令 &#x2F; 事件”(Event),回放事件得到当前状态。冲突通过合并策略或补偿事件(Compensating Events)解决。 </p>
-<p>例如:<br>在每次更新时推送 <code>ProjectUpdated &#123; projectId, fieldsChanged, userId, timestamp &#125;</code> ,<br>然后写入事件存储(如 Kafka &#x2F; EventStoreDB),<br>读端 Consumer 按顺序重建最新状态或按领域聚合 ,<br>最后在并发时如果两个事件都修改了同一字段,可在写端做校验&#x2F;补偿,或在读端做最后写入胜出等策略 。</p>
-<hr>
-<p>总结来说,</p>
-<ul>
-<li>CRDT 最擅长 <strong>去中心化</strong>、<strong>离线编辑</strong>、<strong>自动合并</strong>; </li>
-<li>若不引入 CRDT,可根据业务侧重点选用: <ol>
-<li>悲观锁 → 强制串行编辑,简单粗暴; </li>
-<li>乐观并发 → 适合大多数业务场景,成本低; </li>
-<li>OT → 适合富文本或实时协同场景,复杂度中等; </li>
-<li>事件溯源 → 适合需要全历史审计、可回放的场景。</li>
-</ol>
-</li>
-</ul>
-<hr>
-<p>[^1]: Decipad 博客 “Collaborative and Offline Editing Using CRDTs”<br> <a target="_blank" rel="noopener" href="https://www.decipad.com/blog/decipads-innovative-method-collaborative-and-offline-editing-using-crdts">https://www.decipad.com/blog/decipads-innovative-method-collaborative-and-offline-editing-using-crdts</a></p>
-<p>[^2]: Hacker News 讨论(CRDT 相关线程)<br> <a target="_blank" rel="noopener" href="https://news.ycombinator.com/item?id=38289327">https://news.ycombinator.com/item?id=38289327</a></p>
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