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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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