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<h1 id="背景">背景</h1>
<p>在当前的Pegasus实现中，由于向secondary读取会导致不一致的情况发生，所以目前Pegasus仅仅支持对primary副本的读取。但是在某些情况下（例如：负载均衡、热点写入等）经常会导致primary不稳定。所以我们希望在primary不稳定时能够读取secondary，通过牺牲部分强一致性来降低读请求的长尾并提高系统的可用性。backup request便是用来实现此功能的。</p>
<h1 id="设计实现">设计实现</h1>
<p>backup reqeust的实现原理比较简单：对于读操作（目前写操作不支持backup request），当client向primary发送请求后，如果经过一段时间延时（通常是p999）其response仍然没有返回，则随机选择一台secondary并向其发送backup request。最后获取最快返回来的response进行处理。</p>
<p>这里发送secondary请求的延时我们建议选择p999，因为backup request操作是用来实现消除长尾的，并不是提升集群性能的。如果将该值设置过低，则会由于backup request的请求量过大而导致集群压力增大（假设选择p50作为其延时，这样便会有50%的请求向secondary发送请求，系统负载便会增大50%）。</p>
<h1 id="如何使用">如何使用</h1>
<p>在Pegasus java client v2.0.0中，我们增加了一个接口，通过该接口可以打开某个表的backup reqeust功能。其实现如下：</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="nc">PegasusTableInterface</span> <span class="nf">openTable</span><span class="o">(</span><span class="nc">String</span> <span class="n">tableName</span><span class="o">,</span> <span class="kt">int</span> <span class="n">backupRequestDelayMs</span><span class="o">)</span> <span class="kd">throws</span> <span class="nc">PException</span><span class="o">;</span>
</code></pre></div></div>
<p>相比于老版本的<code class="language-plaintext highlighter-rouge">openTable</code>接口，我们增加了一个<code class="language-plaintext highlighter-rouge">backupRequestDelayMs</code>参数。这个参数便是上文所指的时延，即：向primary发送请求，如果过了<code class="language-plaintext highlighter-rouge">backupRequestDelayMs</code>毫秒response仍没有返回，则向secondary发送backup request。需要注意的是，<code class="language-plaintext highlighter-rouge">backupRequestDelayMs &lt;= 0</code>代表禁用backup reqeust功能。</p>
<p>另外在老版本的<code class="language-plaintext highlighter-rouge">openTable</code>接口中，backup request功能默认是关闭的。</p>
<h1 id="性能测试">性能测试</h1>
<p>下面表格里展示了是否打开backup request的性能对比，这里我们选取了未打开backup request时读请求的p999时间作为backup request的delay时间(138ms)。数据显示，打开backup request之后get请求的p999时延<strong>基本没有变化</strong>，而p9999时延却有了<strong>数倍的降低</strong>。</p>
<p>另外，由于delay时间设置的是p999时间，大约1000个请求里只有1个请求会发送backup request，因此额外请求量（也就是开启backup request的额外开销）比例在0.1%左右。依此类推，若想要降低P999时延，则可以将 <code class="language-plaintext highlighter-rouge">backupRequestDelayMs</code> 设置为P99延迟，由此会增加1%的额外读流量。</p>
<table>
<thead>
<tr>
<th>test case</th>
<th>enable backup request</th>
<th>read p9999</th>
</tr>
</thead>
<tbody>
<tr>
<td>3-clients 15-threads</td>
<td>no</td>
<td>988671</td>
</tr>
<tr>
<td>3-clients 15-threads</td>
<td>yes</td>
<td>153599</td>
</tr>
</tbody>
</table>
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<li><a href="#如何使用">如何使用</a></li>
<li><a href="#性能测试">性能测试</a></li>
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