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<title>GRPC Core: Service Config in gRPC</title>
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<div class="title">Service Config in gRPC </div>  </div>
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<div class="textblock"><h1>Objective</h1>
<p>The service config is a mechanism that allows service owners to publish parameters to be automatically used by all clients of their service.</p>
<h1>Format</h1>
<p>The service config is a JSON string of the following form:</p>
<div class="fragment"><div class="line">{</div><div class="line">  // Load balancing policy name (case insensitive).</div><div class="line">  // Currently, the only selectable client-side policy provided with gRPC</div><div class="line">  // is &#39;round_robin&#39;, but third parties may add their own policies.</div><div class="line">  // This field is optional; if unset, the default behavior is to pick</div><div class="line">  // the first available backend.</div><div class="line">  // If the policy name is set via the client API, that value overrides</div><div class="line">  // the value specified here.</div><div class="line">  //</div><div class="line">  // Note that if the resolver returns at least one balancer address (as</div><div class="line">  // opposed to backend addresses), gRPC will use grpclb (see</div><div class="line">  // https://github.com/grpc/grpc/blob/master/doc/load-balancing.md),</div><div class="line">  // regardless of what LB policy is requested either here or via the</div><div class="line">  // client API.</div><div class="line">  &#39;loadBalancingPolicy&#39;: string,</div><div class="line"></div><div class="line">  // Per-method configuration.  Optional.</div><div class="line">  &#39;methodConfig&#39;: [</div><div class="line">    {</div><div class="line">      // The names of the methods to which this method config applies. There</div><div class="line">      // must be at least one name. Each name entry must be unique across the</div><div class="line">      // entire service config. If the &#39;method&#39; field is empty, then this</div><div class="line">      // method config specifies the defaults for all methods for the specified</div><div class="line">      // service.</div><div class="line">      //</div><div class="line">      // For example, let&#39;s say that the service config contains the following</div><div class="line">      // method config entries:</div><div class="line">      //</div><div class="line">      // &#39;methodConfig&#39;: [</div><div class="line">      //   { &#39;name&#39;: [ { &#39;service&#39;: &#39;MyService&#39; } ] ... },</div><div class="line">      //   { &#39;name&#39;: [ { &#39;service&#39;: &#39;MyService&#39;, &#39;method&#39;: &#39;Foo&#39; } ] ... }</div><div class="line">      // ]</div><div class="line">      //</div><div class="line">      // For a request for MyService/Foo, we will use the second entry, because</div><div class="line">      // it exactly matches the service and method name.</div><div class="line">      // For a request for MyService/Bar, we will use the first entry, because</div><div class="line">      // it provides the default for all methods of MyService.</div><div class="line">      &#39;name&#39;: [</div><div class="line">        {</div><div class="line">          // RPC service name.  Required.</div><div class="line">          // If using gRPC with protobuf as the IDL, then this will be of</div><div class="line">          // the form &quot;pkg.service_name&quot;, where &quot;pkg&quot; is the package name</div><div class="line">          // defined in the proto file.</div><div class="line">          &#39;service&#39;: string,</div><div class="line"></div><div class="line">          // RPC method name.  Optional (see above).</div><div class="line">          &#39;method&#39;: string,</div><div class="line">        }</div><div class="line">      ],</div><div class="line"></div><div class="line">      // Whether RPCs sent to this method should wait until the connection is</div><div class="line">      // ready by default. If false, the RPC will abort immediately if there</div><div class="line">      // is a transient failure connecting to the server. Otherwise, gRPC will</div><div class="line">      // attempt to connect until the deadline is exceeded.</div><div class="line">      //</div><div class="line">      // The value specified via the gRPC client API will override the value</div><div class="line">      // set here. However, note that setting the value in the client API will</div><div class="line">      // also affect transient errors encountered during name resolution,</div><div class="line">      // which cannot be caught by the value here, since the service config</div><div class="line">      // is obtained by the gRPC client via name resolution.</div><div class="line">      &#39;waitForReady&#39;: bool,</div><div class="line"></div><div class="line">      // The default timeout in seconds for RPCs sent to this method. This can</div><div class="line">      // be overridden in code. If no reply is received in the specified amount</div><div class="line">      // of time, the request is aborted and a deadline-exceeded error status</div><div class="line">      // is returned to the caller.</div><div class="line">      //</div><div class="line">      // The actual deadline used will be the minimum of the value specified</div><div class="line">      // here and the value set by the application via the gRPC client API.</div><div class="line">      // If either one is not set, then the other will be used.</div><div class="line">      // If neither is set, then the request has no deadline.</div><div class="line">      //</div><div class="line">      // The format of the value is that of the &#39;Duration&#39; type defined here:</div><div class="line">      // https://developers.google.com/protocol-buffers/docs/proto3#json</div><div class="line">      &#39;timeout&#39;: string,</div><div class="line"></div><div class="line">      // The maximum allowed payload size for an individual request or object</div><div class="line">      // in a stream (client-&gt;server) in bytes. The size which is measured is</div><div class="line">      // the serialized, uncompressed payload in bytes. This applies both</div><div class="line">      // to streaming and non-streaming requests.</div><div class="line">      //</div><div class="line">      // The actual value used is the minimum of the value specified here and</div><div class="line">      // the value set by the application via the gRPC client API.</div><div class="line">      // If either one is not set, then the other will be used.</div><div class="line">      // If neither is set, then the built-in default is used.</div><div class="line">      //</div><div class="line">      // If a client attempts to send an object larger than this value, it</div><div class="line">      // will not be sent and the client will see an error.</div><div class="line">      // Note that 0 is a valid value, meaning that the request message must</div><div class="line">      // be empty.</div><div class="line">      &#39;maxRequestMessageBytes&#39;: number,</div><div class="line"></div><div class="line">      // The maximum allowed payload size for an individual response or object</div><div class="line">      // in a stream (server-&gt;client) in bytes. The size which is measured is</div><div class="line">      // the serialized, uncompressed payload in bytes. This applies both</div><div class="line">      // to streaming and non-streaming requests.</div><div class="line">      //</div><div class="line">      // The actual value used is the minimum of the value specified here and</div><div class="line">      // the value set by the application via the gRPC client API.</div><div class="line">      // If either one is not set, then the other will be used.</div><div class="line">      // If neither is set, then the built-in default is used.</div><div class="line">      //</div><div class="line">      // If a server attempts to send an object larger than this value, it</div><div class="line">      // will not be sent, and the client will see an error.</div><div class="line">      // Note that 0 is a valid value, meaning that the response message must</div><div class="line">      // be empty.</div><div class="line">      &#39;maxResponseMessageBytes&#39;: number</div><div class="line">    }</div><div class="line">  ]</div><div class="line">}</div></div><!-- fragment --><p>Note that new per-method parameters may be added in the future as new functionality is introduced.</p>
<h1>Architecture</h1>
<p>A service config is associated with a server name. The <a class="el" href="naming_8md.html">name resolver</a> plugin, when asked to resolve a particular server name, will return both the resolved addresses and the service config.</p>
<p>TODO(roth): Design how the service config will be encoded in DNS.</p>
<h1>APIs</h1>
<p>The service config is used in the following APIs:</p>
<ul>
<li>In the resolver API, used by resolver plugins to return the service config to the gRPC client.</li>
<li>In the gRPC client API, where users can query the channel to obtain the service config associated with the channel (for debugging purposes).</li>
<li>In the gRPC client API, where users can set the service config explicitly. This is intended for use in unit tests. </li>
</ul>
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