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领域驱动设计中的“聚合根” |
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领域驱动设计中的“聚合根”
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Mar 13, 2025
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<p>在领域驱动设计（Domain-Driven Design，简称DDD）中，聚合根（Aggregate Root）是聚合（Aggregate）中的核心实体，是一个聚合的入口点和控制者，负责维护聚合内部的一致性和不变性条件。聚合是一组紧密相关的领域对象的集合，这些对象通过一定的业务规则绑定在一起，并被视为一个单元。</p>
<p>主要的作用如下：</p>
<ul>
<li>维护不变性：聚合根确保聚合内所有对象的一致性和不变性条件不被破坏。它负责封装与聚合相关的业务逻辑，保证聚合内的对象符合业务规则。</li>
<li>管理生命周期：聚合根负责管理其内部对象的创建、修改和删除。它控制着聚合内部成员的生命周期，包括它们的创建、更新和删除。</li>
<li>处理业务逻辑：聚合根负责处理与聚合相关的业务逻辑和操作，外部系统通过调用聚合根的方法来执行这些操作。它不仅是数据的容器，还负责封装与聚合相关的业务逻辑。</li>
</ul>
<p>其具有以下特性：</p>
<ul>
<li>唯一入口：聚合根是聚合内部对象的唯一入口，外部系统只能与聚合根交互，而无法直接访问聚合内部的其他对象。这样可以避免外部系统直接修改聚合内的实体，确保聚合的一致性和业务逻辑的完整性。</li>
<li>标识唯一性：每个聚合根都有一个全局唯一的标识符（ID），用以区分不同的聚合实例。</li>
<li>事务边界：聚合根常常作为事务的边界，确保事务内的所有操作要么全部成功，要么全部失败，以此来维护数据的完整性。</li>
</ul>
<p>用 F# 来描述，以订单管理为例，大概写一下：</p>
<figure class="highlight fsharp"><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><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">type</span> <span class="title class_">OrderStatus</span> <span class="operator">=</span> </span><br><span class="line"> <span class="operator">|</span> New</span><br><span class="line"> <span class="operator">|</span> Shipped</span><br><span class="line"> <span class="operator">|</span> Delivered</span><br><span class="line"> <span class="operator">|</span> Cancelled</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> <span class="title class_">OrderItem</span> (productName<span class="operator">:</span> <span class="type">string</span>, price<span class="operator">:</span> <span class="type">float</span>, quantity<span class="operator">:</span> <span class="type">int</span>) <span class="operator">=</span> </span><br><span class="line"> <span class="keyword">do</span></span><br><span class="line"> <span class="keyword">if</span> quantity <span class="operator"><=</span> <span class="number">0</span> <span class="keyword">then</span></span><br><span class="line"> <span class="built_in">failwith</span> <span class="string">"Quantity must be positive"</span></span><br><span class="line"> </span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.ProductName <span class="operator">=</span> productName</span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.Price <span class="operator">=</span> price</span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.Quantity <span class="operator">=</span> quantity</span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.TotalPrice () <span class="operator">=</span> price <span class="operator">*</span> quantity</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> <span class="title class_">Order</span> (<span class="built_in">id</span><span class="operator">:</span> <span class="type">int</span>, customerName<span class="operator">:</span> <span class="type">string</span>) <span class="operator">=</span> </span><br><span class="line"> <span class="keyword">let</span> <span class="keyword">mutable</span> status <span class="operator">=</span> OrderStatus.New</span><br><span class="line"> <span class="keyword">let</span> <span class="keyword">mutable</span> orderItems <span class="operator">=</span> []</span><br><span class="line"></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.Id <span class="operator">=</span> <span class="built_in">id</span></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.CustomerName <span class="operator">=</span> customerName</span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.Status <span class="operator">=</span> status</span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.OrderItems <span class="operator">=</span> orderItems</span><br><span class="line"></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.AddItem (item<span class="operator">:</span> OrderItem, price<span class="operator">:</span> <span class="type">float</span>, quantity<span class="operator">:</span> <span class="type">int</span>) <span class="operator">=</span></span><br><span class="line"> <span class="keyword">if</span> quantity <span class="operator"><=</span> <span class="number">0</span> <span class="keyword">then</span></span><br><span class="line"> <span class="built_in">failwith</span> <span class="string">"Quantity must be positive"</span></span><br><span class="line"> </span><br><span class="line"> orderItems <span class="operator"><-</span> orderItems <span class="operator">@</span> [OrderItem(item.ProductName, price, quantity)]</span><br><span class="line"></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.ChangeStatus (status<span class="operator">:</span> OrderStatus) <span class="operator">=</span></span><br><span class="line"> this.Status <span class="operator"><-</span> status</span><br><span class="line"></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.TotalPrice () <span class="operator">=</span></span><br><span class="line"> orderItems <span class="operator">|></span> List.sumBy (<span class="keyword">fun</span> item <span class="operator">-></span> item.TotalPrice())</span><br><span class="line"></span><br><span class="line"> <span class="keyword">member</span> <span class="keyword">public</span> this.GetTotalPrice () <span class="operator">=</span></span><br><span class="line"> orderItems <span class="operator">|></span> List.sumBy (<span class="keyword">fun</span> item <span class="operator">-></span> item.TotalPrice())</span><br></pre></td></tr></table></figure>
<p>在这个例子中，<code>Order</code> 是聚合根，它通过 <code>AddItem</code> 方法来添加订单项，保证每个订单项符合业务规则。同时，聚合根 <code>Order</code> 还负责订单状态的管理，例如通过 <code>ChangeStatus</code> 方法来更新订单状态。<code>OrderItem</code> 是聚合内的一个实体，表示订单项，它通过 <code>GetTotalPrice</code> 方法来计算每个订单项的总价。外部系统只能通过 <code>Order</code> 聚合根来访问和操作订单项，而不能直接访问或修改 <code>OrderItem</code></p>
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