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<h2 class="post-title" itemprop="name headline">[Golang] aggregate in mongo
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<div class="post-description">本篇文章首先跟大家分享一個常見的 Schema 設計,在此情境下,為了讀取一連串的資料,我們有不同種方式可以辦到。其中一種就是本文的主角, Aggregate 的概念。為了解釋 Aggregate 如何運作以及如何實現,本文採用 Golang 作為基本的程式語言,並且使用 mgo 作為與 mongo 進行處理的第三方函式庫。此外也採用了最原始的讀取方式,並且將此方式從撰寫方式以及效能兩方面直接與 Aggregate 進行比較。</div>
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<p>在探討整個主題之前,我們先設計一個簡單的 <code>schema</code> 來符合這次<br>假設今天在資料庫內有主要結構,分別是 <code>User</code> 跟 <code>Pod</code><br><code>User</code> 非常簡單,就是描述一個使用者<br><code>Pod</code> 這邊不用在意他是什麼東西,他是一個資源,然後透過 <code>User</code> 創立的,所以每個 <code>Pod</code> 裡面都會有一個欄位去記錄是哪個 <code>User</code> 創立的。</p>
<p>有了前述的假設之後,接下來想像一個實際需求</p>
<ol>
<li>有一個 <code>UserA</code> 創造了數百個 <code>Pod</code> 資源。</li>
<li>想要 Query 該 <code>UserA</code> 並且列出該<code>User</code>創造的所有<code>Pod</code>資源的細部資料</li>
</ol>
<p><img src="https://i.imgur.com/rFmSBdw.png" alt="Imgur"></p>
<p>在以前 SQL 的時候,我們可能會透過 <code>SELECT</code> 搭配 <code>JOIN</code> 的方式來存取,那今天假如在 <code>mongoDB</code> 的環境下,我們可以怎麼處理?</p>
<p>本文章的所有範例程式碼都可以在<a href="https://github.com/hwchiu/GolangPratice/blob/master/mgoAggregate/main.go" target="_blank" rel="noopener">mgo Aggregate Example</a>找到</p>
<div class="note info">
<p>必須要注意, mongodb 的版本至少要 > 3.0 之後才支援本文描述的 <code>Aggregate</code></p>
</div>
<h2 id="Structure"><a href="#Structure" class="headerlink" title="Structure"></a>Structure</h2><p>這邊就採用 <code>golang</code> 簡單的描述一下我們的結構,這些結構會用在 <code>mongodb</code> 裡面<br><figure class="highlight go"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">type</span> User <span class="keyword">struct</span> {</span><br><span class="line"> ID bson.ObjectId <span class="string">`bson:"_id"`</span></span><br><span class="line"> Name <span class="keyword">string</span> <span class="string">`bson:"name"`</span></span><br><span class="line"> Pods []Pod <span class="string">`bson:"pods"`</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> Pod <span class="keyword">struct</span> {</span><br><span class="line"> ID bson.ObjectId <span class="string">`bson:"_id"`</span></span><br><span class="line"> CreatedBy bson.ObjectId <span class="string">`bson:"createdBy"`</span></span><br><span class="line"> Name <span class="keyword">string</span> <span class="string">`bson:"name"`</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="Find"><a href="#Find" class="headerlink" title="Find"></a>Find</h2><p>第一種方法其實非常直覺</p>
<ol>
<li>就是先取得該 <code>User</code> 的實例</li>
<li>去 <code>Pod</code> 的 <code>Collection</code> 內進行搜尋,找出所有的 <code>Pod</code> 其記錄的<code>User</code>是<code>UserA</code>.</li>
</ol>
<p>假設我們已經有該 <code>User</code> 的物件了,接下來很簡單透過 <code>find</code> 的方式找出所有由該 <code>User</code> 所創造的 <code>Pod</code>.<br>並且把找到的結果都放到 <code>User</code> 物件裡面的 <code>Pods</code> 變數<br><figure class="highlight go"><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">c := db.C(<span class="string">"pods"</span>)</span><br><span class="line">c.Find(bson.M{<span class="string">"createdBy"</span>: user.ID}).All(&user.Pods)</span><br></pre></td></tr></table></figure></p>
<h2 id="Aggregate"><a href="#Aggregate" class="headerlink" title="Aggregate"></a>Aggregate</h2><p>在 <code>mgo</code>裡面,如果想要做到 <code>Aaggregate</code> 類似這種 <code>Join</code> 的方式,其實可以透過一個叫做 <code>pipeline</code> 的函式來完成</p>
<p>為了使用 <code>pipeline</code> 的,我們必須要先描述我們想要如何去尋找這些資料,這些資料是由<code>bson.M{}</code> 陣列組成的。<br>該陣列內,至少要提供兩種不同的查詢方式</p>
<ol>
<li>$match: 這邊用來描述我們要如何查詢 <code>User</code> 物件,在此範例中我們使用特定的<code>User.ID</code>來描述該 <code>User</code></li>
<li>$lookup: 用來描述要如何 <code>Aggregate</code>, 其中有四個欄位要填寫<ul>
<li>from: 用來進行 <code>Aggregate</code> 的 <code>collection</code> 名稱,此範例中我們設定為 <code>pods</code> 這個 <code>collection</code>.</li>
<li>localField: 在該 <code>pipeline</code> 要使用的 <code>collection</code> 內要使用的欄位,這邊我們會使用 <code>User</code> 裡面的 <code>_id</code>.</li>
<li>foreignField: 在 <code>from</code> 這個 <code>collection</code> 裡面要使用的欄位,此範例就是 <code>Pods</code> 裡面的 <code>createdBy</code>.</li>
<li>as: 這個是用來存放查詢後的資料,首先其本身必須是個<code>Array</code>,同時該欄位必須定義在 <code>User</code> 結構裡面。</li>
</ul>
</li>
</ol>
<p>程式碼如下,該程式碼的意思很簡單,我們對 <code>users</code> 這個 <code>collection</code> 裡面透過 <code>$match</code> 去尋找特定的 <code>User</code>,同時透過 <code>$lookup</code> 的方式到 <code>pods</code> 該 <code>collection</code> 裡面去把 <code>User.ID == Pods.CreatedBy</code> 相同的 <code>pods</code> 都找出來,最後放到 <code>user.pods</code> 裡面<br><figure class="highlight go"><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></pre></td><td class="code"><pre><span class="line">c := db.C(<span class="string">"users"</span>)</span><br><span class="line">pipeline := []bson.M{</span><br><span class="line"> {<span class="string">"$lookup"</span>: bson.M{<span class="string">"from"</span>: <span class="string">"pods"</span>, <span class="string">"localField"</span>: <span class="string">"_id"</span>, <span class="string">"foreignField"</span>: <span class="string">"createdBy"</span>, <span class="string">"as"</span>: <span class="string">"pods"</span>}},</span><br><span class="line"> {<span class="string">"$match"</span>: bson.M{<span class="string">"_id"</span>: user.ID}},</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> resp User</span><br><span class="line">err := c.Pipe(pipeline).One(&resp)</span><br><span class="line"><span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> fmt.Println(err)</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h2 id="Performance"><a href="#Performance" class="headerlink" title="Performance"></a>Performance</h2><p>因為這次的範例非常簡單,所以並不能用來代表在所有的使用情境下的結果,只能用來表示如本文的情境下的效率問題。</p>
<p>在上述的範例專案內,一開始會先隨機產生一個使用者,並且隨機產生<code>30000</code> 筆 <code>Pod</code> 資料,並且嘗試比較不同數量的情況下,透過 <code>Find</code> 以及 <code>Aggregate</code> 實際上的效能問題。</p>
<p>下列的時間都是 <strong>(ms)</strong></p>
<table>
<thead>
<tr>
<th>Methond\Numbers</th>
<th>1000</th>
<th>5000</th>
<th>10000</th>
<th>30000</th>
<th>50000</th>
</tr>
</thead>
<tbody>
<tr>
<td>Find</td>
<td>5.7</td>
<td>18</td>
<td>35</td>
<td>111</td>
<td>250</td>
</tr>
<tr>
<td>Aggregate</td>
<td>7.4</td>
<td>22</td>
<td>77</td>
<td>180</td>
<td>NaN</td>
</tr>
</tbody>
</table>
<div class="note danger">
<p>這邊要特別注意的是,當 <code>Pods</code> 的數量過高的時候,使用 <code>pipeline</code> 來處理就會得到下列的錯誤訊息</p><p><code>Total size of documents in pods matching { $match: { $and: [ { createdBy: { $eq: ObjectId('5b86dac74807c532d70bea52') } }, {} ] } } exceeds maximum document size</code></p><p>根據官網描述,預設的情況下,只能存放 <code>16MB</code> 的資料。<br>這意味者資料過多的情況下,使用者要注意這個現象,避免資料存取失敗。</p>
</div>
<h2 id="Summary"><a href="#Summary" class="headerlink" title="Summary"></a>Summary</h2><p>本文假設的情境非常簡單,基本上兩種方法都可以完成,但是在資料多寡的情況下,花費的時間就有所區別。<br>若是有更複雜的需求,在 <code>pipeline</code> 裡面還可以設定除了 <code>$lookup</code> 以及 <code>$match</code> 以外的用法,透過一次的呼叫就把資料給查詢完畢。<br>至少如果只是本文這種很簡單的情境,其實自己額外查詢即可,不論在簡潔性跟效率上我認為都更高。</p>
<p>所以還是一樣,不同情境下,每個功能都會有不同的使用方法跟考量點。<br>這邊就讓各位自己去評估囉</p>
<h2 id="Reference"><a href="#Reference" class="headerlink" title="Reference"></a>Reference</h2><ul>
<li><a href="https://docs.mongodb.com/manual/aggregation/" target="_blank" rel="noopener">https://docs.mongodb.com/manual/aggregation/</a></li>
</ul>
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