client基本概念及学习指南
https://github.com/luozesong/brpc/blob/master/docs/cn/client.md
一、编写proto
这里与服务一致，实际开发中需要双端共同确定proto内容；
二、初始化channel
rpc channel可以视为socket编程中的client对象
定义一个channel,Channel可以被所有线程共用，你不需要为每个线程创建独立的Channel，也不需要用锁互斥。不过Channel的创建和Init并不是线程安全的，请确保在Init成功后再被多线程访问，在没有线程访问后再析构
三、初始化channel options
填充连接相关的的参数如超时、协议、最大重试次数、负载均衡算法等
四、channel init with options
初始化channel
五、初始化自定义service_stub
protobuf会为我们定义的rpc 函数生成一个xxx_stub用来和服务交互。我们需要用自定义service_stub初始化我们的客户端对象。如果proto没定义rpc 服务，可以用Channel.CallMethod。
六、发起访问
一般来说，我们不直接调用Channel.CallMethod，而是通过protobuf生成的桩XXX_Stub，过程更像是“调用函数”。stub内没什么成员变量，建议在栈上创建和使用，而不必new，当然你也可以把stub存下来复用。Channel::CallMethod和stub访问都是线程安全的，可以被所有线程同时访问。
XXX_Stub stub(&channel);
stub.some_method(controller, request, response, done);
这里的some_method为在proto中定义的rpc服务经过编译后得到的抽象接口，像service_stub.ParallelSearchPlans。
如果我们没定义rpc服务，用brpc内部已定义好的服务时，需要用到callmethod()发起请求。而我们可以通过addchannel组装多个请求，一次调用，可以用在提升服务的性能。下面是一个实际示例
while (request_num--) {pchan.AddChannel(channel, baidu::rpc::DOESNT_OWN_CHANNEL, rpc_call_mapper, rpc_response_merger);}uint64_t logid = UII_GET_LOGID();cntl->set_log_id(logid);AccessLogGuard access_log_guard(service_name.c_str(), "nshead", req, res, cntl);pchan.CallMethod(NULL, cntl, req, res, baidu::rpc::DoNothing());if (cntl->Failed()) {UII_LOG_WARNING("Fail to access service:%s Error:%s", service_name.c_str(), cntl->ErrorText().c_str());return -1;}
上面代码使用rpc::DoNothing()实现了一个半同步的请求响应，阻塞到所有的请求返回。
七、返回处理
我们发起访问后，可以根据controler调用cntl->Failed()判断请求是否成功。若成功，就可以对res响应进行处理。
八、基础客户端案例及释义
// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.// A client sending requests to server every 1 second.#include <gflags/gflags.h>
#include <butil/logging.h>
#include <butil/time.h>
#include <brpc/channel.h>
#include "echo.pb.h"DEFINE_string(attachment, "", "Carry this along with requests");
DEFINE_string(protocol, "baidu_std", "Protocol type. Defined in src/brpc/options.proto");
DEFINE_string(connection_type, "", "Connection type. Available values: single, pooled, short");
DEFINE_string(server, "0.0.0.0:8000", "IP Address of server");
DEFINE_string(load_balancer, "", "The algorithm for load balancing");
DEFINE_int32(timeout_ms, 100, "RPC timeout in milliseconds");
DEFINE_int32(max_retry, 3, "Max retries(not including the first RPC)");
DEFINE_int32(interval_ms, 1000, "Milliseconds between consecutive requests");int main(int argc, char* argv[]) {// Parse gflags. We recommend you to use gflags as well.GFLAGS_NS::ParseCommandLineFlags(&argc, &argv, true);brpc::Channel channel;// Initialize the channel, NULL means using default options.brpc::ChannelOptions options;options.protocol = FLAGS_protocol;options.connection_type = FLAGS_connection_type;options.timeout_ms = FLAGS_timeout_ms/*milliseconds*/;options.max_retry = FLAGS_max_retry;if (channel.Init(FLAGS_server.c_str(), FLAGS_load_balancer.c_str(), &options) != 0) {LOG(ERROR) << "Fail to initialize channel";return -1;}// Normally, you should not call a Channel directly, but instead construct// a stub Service wrapping it. stub can be shared by all threads as well.example::EchoService_Stub stub(&channel);// Send a request and wait for the response every 1 second.int log_id = 0;while (!brpc::IsAskedToQuit()) {// We will receive response synchronously, safe to put variables// on stack.example::EchoRequest request;example::EchoResponse response;brpc::Controller cntl;request.set_message("hello world");cntl.set_log_id(log_id ++); // set by user// Set attachment which is wired to network directly instead of // being serialized into protobuf messages.cntl.request_attachment().append(FLAGS_attachment);// Because `done'(last parameter) is NULL, this function waits until// the response comes back or error occurs(including timedout).stub.Echo(&cntl, &request, &response, NULL);if (!cntl.Failed()) {LOG(INFO) << "Received response from " << cntl.remote_side()<< " to " << cntl.local_side()<< ": " << response.message() << " (attached="<< cntl.response_attachment() << ")"<< " latency=" << cntl.latency_us() << "us";} else {LOG(WARNING) << cntl.ErrorText();}usleep(FLAGS_interval_ms * 1000L);}LOG(INFO) << "EchoClient is going to quit";return 0;
}
