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<?xml version="1.0" encoding="utf-8"?>
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<title>GRPC golang版源码分析之客户端(二)</title>
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<h1 class="title">GRPC golang版源码分析之客户端(二)</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#org5288d76">1. 前言</a></li>
<li><a href="#orge36d607">2. 负载均衡</a></li>
<li><a href="#org3270768">3. 相关链接</a></li>
</ul>
</div>
</div>
<div id="outline-container-org5288d76" class="outline-2">
<h2 id="org5288d76"><span class="section-number-2">1</span> 前言</h2>
<div class="outline-text-2" id="text-1">
<p>
前面一篇文章分析了一个grpc call的大致调用流程，顺着源码走了一遍，但是grpc中有一些特性并没有进行分析，这篇blog主要就是分析负载均衡这个特性。负载均衡可以让你在使用grpc调用方法是连接不同的服务器。在启动的时候，grpc client会建立对所有服务器的连接，然后通过轮询算法拿不同的服务器。
</p>
</div>
</div>
<div id="outline-container-orge36d607" class="outline-2">
<h2 id="orge36d607"><span class="section-number-2">2</span> 负载均衡</h2>
<div class="outline-text-2" id="text-2">
<p>
负载均衡是在客户端实现的，通过在初始化一个grpc客户端传入一个负载均衡器，默认是通过轮询算法每次调用时切换调用的服务器，由于是在客户端做的，所以只能让服务器平摊压力，并不能实时的根据服务器的状态来进行负载均衡。比如某一刻所有的grpc客户端都轮到了同一个服务器（这里只是举例）。
</p>
<div class="org-src-container">
<pre class="src src-go"><span style="color: #676E95;">//</span><span style="color: #676E95;">在clientconn.go的DialContext函数中</span>
<span style="color: #89DDFF;">go</span> <span style="color: #89DDFF;">func</span>() {
<span style="color: #89DDFF;">var</span> <span style="color: #ffcb6b;">addrs</span> []<span style="color: #c792ea;">Address</span>
<span style="color: #89DDFF;">if</span> cc.dopts.balancer == <span style="color: #f78c6c;">nil</span> {
<span style="color: #676E95;">// </span><span style="color: #676E95;">Connect to target directly if balancer is nil.</span>
<span style="color: #676E95;">//</span><span style="color: #676E95;">如果没有设置负载均衡器，则直接连接</span>
addrs = <span style="color: #82aaff;">append</span>(addrs, <span style="color: #c792ea;">Address</span>{<span style="color: #f78c6c;">Addr</span>: target})
} <span style="color: #89DDFF;">else</span> {
<span style="color: #89DDFF;">var</span> <span style="color: #ffcb6b;">credsClone</span> <span style="color: #c792ea;">credentials.TransportCredentials</span>
<span style="color: #89DDFF;">if</span> creds != <span style="color: #f78c6c;">nil</span> {
credsClone = creds.<span style="color: #82aaff;">Clone</span>()
}
<span style="color: #ffcb6b;">config</span> := <span style="color: #c792ea;">BalancerConfig</span>{
<span style="color: #f78c6c;">DialCreds</span>: credsClone,
}
<span style="color: #676E95;">//</span><span style="color: #676E95;">启动一个负载均衡器,start函数会启动一个watch监听地址的变化，而Notify返回一个通道，在每次服务器地址变化后的最新地址信息.</span>
<span style="color: #89DDFF;">if</span> <span style="color: #ffcb6b;">err</span> := cc.dopts.balancer.<span style="color: #82aaff;">Start</span>(target, config); err != <span style="color: #f78c6c;">nil</span> {
waitC <- err
<span style="color: #89DDFF;">return</span>
}
<span style="color: #ffcb6b;">ch</span> := cc.dopts.balancer.<span style="color: #82aaff;">Notify</span>()
<span style="color: #89DDFF;">if</span> ch == <span style="color: #f78c6c;">nil</span> {
<span style="color: #676E95;">// </span><span style="color: #676E95;">There is no name resolver installed.</span>
addrs = <span style="color: #82aaff;">append</span>(addrs, <span style="color: #c792ea;">Address</span>{<span style="color: #f78c6c;">Addr</span>: target})
} <span style="color: #89DDFF;">else</span> {
addrs, ok = <-ch
<span style="color: #89DDFF;">if</span> <span style="color: #89DDFF; font-weight: bold;">!</span>ok || <span style="color: #82aaff;">len</span>(addrs) == 0 {
waitC <- errNoAddr
<span style="color: #89DDFF;">return</span>
}
}
}
<span style="color: #676E95;">//</span><span style="color: #676E95;">对每一个地址进行连接, 因为这是客户端启动时，所以需要对所有地址操作.</span>
<span style="color: #89DDFF;">for</span> <span style="color: #ffcb6b;">_</span>, <span style="color: #ffcb6b;">a</span> := <span style="color: #89DDFF;">range</span> addrs {
<span style="color: #89DDFF;">if</span> <span style="color: #ffcb6b;">err</span> := cc.<span style="color: #82aaff;">resetAddrConn</span>(a, <span style="color: #f78c6c;">false</span>, <span style="color: #f78c6c;">nil</span>); err != <span style="color: #f78c6c;">nil</span> {
waitC <- err
<span style="color: #89DDFF;">return</span>
}
}
<span style="color: #82aaff;">close</span>(waitC)
}()
..............
<span style="color: #676E95;">//</span><span style="color: #676E95;">省略一些代码</span>
<span style="color: #89DDFF;">if</span> ok {
<span style="color: #676E95;">//</span><span style="color: #676E95;">这里开启一个监听goroutine，主要是监听服务器地址变化并对新的地址建立连接，对老的地址关闭连接</span>
<span style="color: #89DDFF;">go</span> cc.<span style="color: #82aaff;">lbWatcher</span>()
}
</pre>
</div>
<p>
上面的代码大致是说明了一个负载均衡器是怎么使用的，下面看看负载均衡具体的结构:
</p>
<div class="org-src-container">
<pre class="src src-go"><span style="color: #89DDFF;">type</span> <span style="color: #c792ea;">Balancer</span> <span style="color: #89DDFF;">interface</span> {
<span style="color: #676E95;">//</span><span style="color: #676E95;">启动一个负载均衡，内部会启动一个名称服务器的watcher，不断监听地址的变化</span>
<span style="color: #82aaff;">Start</span>(<span style="color: #ffcb6b;">target</span> <span style="color: #c792ea;">string</span>, <span style="color: #ffcb6b;">config</span> <span style="color: #c792ea;">BalancerConfig</span>) <span style="color: #c792ea;">error</span>
<span style="color: #82aaff;">Up</span>(<span style="color: #ffcb6b;">addr</span> <span style="color: #c792ea;">Address</span>) (<span style="color: #ffcb6b;">down</span> <span style="color: #89DDFF;">func</span>(<span style="color: #c792ea;">error</span>))
<span style="color: #676E95;">//</span><span style="color: #676E95;">得到下一次连接的地址</span>
<span style="color: #82aaff;">Get</span>(<span style="color: #ffcb6b;">ctx</span> <span style="color: #c792ea;">context.Context</span>, <span style="color: #ffcb6b;">opts</span> <span style="color: #c792ea;">BalancerGetOptions</span>) (<span style="color: #ffcb6b;">addr</span> <span style="color: #c792ea;">Address</span>, <span style="color: #ffcb6b;">put</span> <span style="color: #89DDFF;">func</span>(), <span style="color: #ffcb6b;">err</span> <span style="color: #c792ea;">error</span>)
<span style="color: #676E95;">//</span><span style="color: #676E95;">从服务器得到更新的地址,这个地址是更新后的所有地址</span>
<span style="color: #82aaff;">Notify</span>() <-<span style="color: #89DDFF;">chan</span> []<span style="color: #c792ea;">Address</span>
<span style="color: #676E95;">//</span><span style="color: #676E95;">关闭负载均衡器</span>
<span style="color: #82aaff;">Close</span>() <span style="color: #c792ea;">error</span>
}
</pre>
</div>
<p>
负载均衡器有个默认实现在balancer.go中，使用的轮询算法。这个实现中包含了一个名字服务器的服务，我们可以通过实现一个名字服务器的接口，然后封装到这个负载均衡器中，这样就不需要自己实现整个负载均衡器。名字服务器的接口如下:
</p>
<div class="org-src-container">
<pre class="src src-go"><span style="color: #89DDFF;">type</span> <span style="color: #c792ea;">Update</span> <span style="color: #89DDFF;">struct</span> {
<span style="color: #676E95;">// </span><span style="color: #676E95;">Op indicates the operation of the update.</span>
Op <span style="color: #c792ea;">Operation</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">Addr is the updated address. It is empty string if there is no address update.</span>
Addr <span style="color: #c792ea;">string</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">Metadata is the updated metadata. It is nil if there is no metadata update.</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">Metadata is not required for a custom naming implementation.</span>
Metadata <span style="color: #89DDFF;">interface</span>{}
}
<span style="color: #676E95;">// </span><span style="color: #676E95;">Resolver creates a Watcher for a target to track its resolution changes.</span>
<span style="color: #89DDFF;">type</span> <span style="color: #c792ea;">Resolver</span> <span style="color: #89DDFF;">interface</span> {
<span style="color: #676E95;">// </span><span style="color: #676E95;">Resolve creates a Watcher for target.</span>
<span style="color: #676E95;">//</span><span style="color: #676E95;">通过一个名字得到一个watcher，监听服务器地址变化。</span>
<span style="color: #82aaff;">Resolve</span>(<span style="color: #ffcb6b;">target</span> <span style="color: #c792ea;">string</span>) (<span style="color: #c792ea;">Watcher</span>, <span style="color: #c792ea;">error</span>)
}
<span style="color: #676E95;">// </span><span style="color: #676E95;">Watcher watches for the updates on the specified target.</span>
<span style="color: #89DDFF;">type</span> <span style="color: #c792ea;">Watcher</span> <span style="color: #89DDFF;">interface</span> {
<span style="color: #676E95;">// </span><span style="color: #676E95;">Next blocks until an update or error happens. It may return one or more</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">updates. The first call should get the full set of the results. It should</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">return an error if and only if Watcher cannot recover.</span>
<span style="color: #676E95;">// </span><span style="color: #676E95;">得到下次更新的地址</span>
<span style="color: #82aaff;">Next</span>() ([]*<span style="color: #c792ea;">Update</span>, <span style="color: #c792ea;">error</span>)
<span style="color: #676E95;">// </span><span style="color: #676E95;">Close closes the Watcher.</span>
<span style="color: #82aaff;">Close</span>()
}
</pre>
</div>
<p>
负载均衡器的全貌大概就这些了，其中还有些细节我没有看到，目前来说我也不希望过早深入这些细节。所以负载均衡就到这里啦。
</p>
</div>
</div>
<div id="outline-container-org3270768" class="outline-2">
<h2 id="org3270768"><span class="section-number-2">3</span> 相关链接</h2>
<div class="outline-text-2" id="text-3">
<ol class="org-ol">
<li><a href="http://www.grpc.io/docs/guides/auth.html#overview">http://www.grpc.io/docs/guides/auth.html#overview</a> 验证文档</li>
</ol>
</div>
</div>
</div>
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