之前关于brpc的几篇分析：
brpc之mutex源码分析
brpc之ResourcePool源码分析
brpc之bthread_id源码分析
brpc之bthread源码浅淅(一)
brpc之bthread源码浅淅(二)
关于协程的实现分析：
协程库基础知识
Phxrpc协程库实现
Pebble协程库实现
Libco协程库实现
这一篇分析下客户端使用rpc的流程，从简单的echo_c++开始，从client.cpp中main起，可以熟悉下brpc::Channel的使用和设计原理。与protobuf有关联，早之前也用过和分析过[百度文件系统bfs源码分析系列]，但这块目前掌握的不是很深入。
对于demo中的协议：
6 message EchoRequest {
7 required string message = 1;
8 };
10 message EchoResponse {
11 required string message = 1;
12 };
14 service EchoService {
15 rpc Echo(EchoRequest) returns (EchoResponse);
16 };
转换后的结果部分如下：
67 class EchoRequest : public ::google::protobuf::Message /* @@protoc_insertion_point(class_definit ion:example.EchoRequest) */ {
68 public:
69 EchoRequest();
70 virtual ~EchoRequest();
156 // required string message = 1;
157 bool has_message() const;
158 void clear_message();
159 static const int kMessageFieldNumber = 1;
160 const ::std::string& message() const;
161 void set_message(const ::std::string& value);
182 };
185 class EchoResponse : public ::google::protobuf::Message /* @@protoc_insertion_point(class_defini tion:example.EchoResponse) */ {
186 public:
187 EchoResponse();
188 virtual ~EchoResponse();
274 // required string message = 1;
275 bool has_message() const;
276 void clear_message();
277 static const int kMessageFieldNumber = 1;
278 const ::std::string& message() const;
279 void set_message(const ::std::string& value);
300 };
338 class EchoService_Stub : public EchoService {
339 public:
340 EchoService_Stub(::google::protobuf::RpcChannel* channel);
343 ~EchoService_Stub();
348
349 void Echo(::google::protobuf::RpcController* controller,
350 const ::example::EchoRequest* request,
351 ::example::EchoResponse* response,
352 ::google::protobuf::Closure* done);
353 private:
354 ::google::protobuf::RpcChannel* channel_;
357 };
正常运行结果
图一
把server kill掉client这边显示
图二
因为不太方便测试集群的用例就暂时跳过，也可以通过web相应的ip/port查看各种后台数据。
client.cpp中main函数部分代码：
35 int main(int argc, char* argv[]) {
37 GFLAGS_NS::ParseCommandLineFlags(&argc, &argv, true);
38
39 // A Channel represents a communication line to a Server. Notice that
40 // Channel is thread-safe and can be shared by all threads in your program.
41 brpc::Channel channel;
42
44 brpc::ChannelOptions options;
45 //more code...
49 if (channel.Init(FLAGS_server.c_str(), FLAGS_load_balancer.c_str(), &options) != 0) {
51 return -1;
52 }
56 //more code...
91 }
36 class ChannelBase : public google::protobuf::RpcChannel/*non-copyable*/,
37 public Describable {
38 public:
44 virtual int CheckHealth() = 0;
45 };
149 class Channel : public ChannelBase {
150 //more code...
229 butil::intrusive_ptr<SharedLoadBalancer> _lb;
232 };
这里为了分析某些功能和简化另外的一些功能，假设是以集群的方式请求rpc。以上列出的Channel类部分实现，其他的几个是关于打包序列化请求的接口，包括初始化等实现，比较简单。
以集群方式的话那_server_id=INVALID_SOCKET_ID。
320 int Channel::Init(const char* ns_url,
321 const char* lb_name,
322 const ChannelOptions* options) {
323 //more code...
337 if (InitChannelOptions(options) != 0);
338 LoadBalancerWithNaming* lb = new (std::nothrow) LoadBalancerWithNaming;
339 //more code...
350 if (lb->Init(ns_url, lb_name, _options.ns_filter, &ns_opt) != 0) {
352 delete lb;
353 return -1;
354 }
355 _lb.reset(lb);
356 return 0;
357 }
146 int Channel::InitChannelOptions(const ChannelOptions* options) {
147 if (options) { // Override default options if user provided one.
148 _options = *options;
149 }
150 const Protocol* protocol = FindProtocol(_options.protocol);
151 //check protocol
155 _serialize_request = protocol->serialize_request;
156 _pack_request = protocol->pack_request;
157 _get_method_name = protocol->get_method_name;
203 return 0;
204 }
在options.proto定义了一些协议以及在policy/directory中定义相应的接口实现，考虑protocol==PROTOCOL_BAIDU_STD。这里会用protocol中的接口设置channel中的，而前者在global中注册：
383 // Protocols
384 Protocol baidu_protocol = { ParseRpcMessage,
385 SerializeRequestDefault, PackRpcRequest,
386 ProcessRpcRequest, ProcessRpcResponse,
387 VerifyRpcRequest, NULL, NULL,
388 CONNECTION_TYPE_ALL, "baidu_std" };
389 if (RegisterProtocol(PROTOCOL_BAIDU_STD, baidu_protocol) != 0) {
390 exit(1);
391 }
30 class LoadBalancerWithNaming : public SharedLoadBalancer,
31 public NamingServiceWatcher {
32 public:
44 //more code...
45 private:
46 butil::intrusive_ptr<NamingServiceThread> _nsthread_ptr;
47 };
113 // A intrusively shareable load balancer created from name.
114 class SharedLoadBalancer : public SharedObject, public NonConstDescribable {
115 public:
116 //more code...
175 LoadBalancer* _lb;
180 };
34 // Inherit this class to observer NamingService changes.
38 class NamingServiceWatcher {
39 public:
40 virtual ~NamingServiceWatcher() {}
41 virtual void OnAddedServers(const std::vector<ServerId>& servers) = 0;
42 virtual void OnRemovedServers(const std::vector<ServerId>& servers) = 0;
43 };
56 // A dedicated thread to map a name to ServerIds
57 class NamingServiceThread : public SharedObject, public Describable {
123 };
有些不是重要的组件暂时不在这里说明，反正知道实现原理和调用过程即可。
31 int LoadBalancerWithNaming::Init(const char* ns_url, const char* lb_name,
32 const NamingServiceFilter* filter,
33 const GetNamingServiceThreadOptions* options) {
34 if (SharedLoadBalancer::Init(lb_name) != 0) {
35 return -1;
36 }
37 if (GetNamingServiceThread(&_nsthread_ptr, ns_url, options) != 0) {
39 return -1;
40 }
41 if (_nsthread_ptr->AddWatcher(this, filter) != 0) {
43 return -1;
44 }
45 return 0;
46 }
上面功能是根据负载均衡名称new一个实例出来；并根据url解析出protocol，比如bns，来创建一个名字服务并Start：
411 int GetNamingServiceThread(
412 butil::intrusive_ptr<NamingServiceThread>* nsthread_out,
413 const char* url,
414 const GetNamingServiceThreadOptions* options) {
412 //more code...
459 if (ptr == NULL) {
460 NamingServiceThread* thr = new (std::nothrow) NamingServiceThread;
461 //more code...
468 new_thread = true;
469 }
470 }
471 if (new_thread) {
472 if (nsthread->Start(source_ns->New(), key.protocol, key.service_name, options) != 0) {
474 std::unique_lock<pthread_mutex_t> mu(g_nsthread_map_mutex);
475 g_nsthread_map->erase(key);
476 return -1;
477 }
478 } else {
479 if (nsthread->WaitForFirstBatchOfServers() != 0) {
480 return -1;
481 }
482 }
483 nsthread_out->swap(nsthread);
484 return 0;
485 }
45 // Mapping a name to ServerNodes.
46 class NamingService : public Describable, public Destroyable {
54 virtual int RunNamingService(const char* service_name,
55 NamingServiceActions* actions) = 0;
62 virtual bool RunNamingServiceReturnsQuickly() { return false; }
66 virtual NamingService* New() const = 0;
68 protected:
69 virtual ~NamingService() {}
70 };
28 class PeriodicNamingService : public NamingService {
29 protected:
30 virtual int GetServers(const char *service_name,
31 std::vector<ServerNode>* servers) = 0;
32
33 int RunNamingService(const char* service_name,
34 NamingServiceActions* actions);
35 };
28 // Acquire server list from Baidu-Naming-Service, aka BNS
29 class BaiduNamingService : public PeriodicNamingService {
30 public:
31 // You can specify port by appending port selector:
32 // e.g.: bns://DPOP-inner-API-inner-API.jpaas.hosts:main
33 // ^^^^^
34 int GetServers(const char *service_name,
35 std::vector<ServerNode>* servers);
36
37 void Describe(std::ostream& os, const DescribeOptions&) const;
39 NamingService* New() const;
41 void Destroy();
42 };
275 int NamingServiceThread::Start(NamingService* naming_service,
276 const std::string& protocol,
277 const std::string& service_name,
278 const GetNamingServiceThreadOptions* opt_in) {
279 if (naming_service == NULL) {
281 return -1;
282 }
283 _ns = naming_service;
284 _protocol = protocol;
285 _service_name = service_name;
286 if (opt_in) {
287 _options = *opt_in;
288 }
289 _last_sockets.clear();
290 if (_ns->RunNamingServiceReturnsQuickly()) {
291 RunThis(this);
292 } else {
293 int rc = bthread_start_urgent(&_tid, NULL, RunThis, this);
294 if (rc) {
296 return -1;
297 }
298 }
299 return WaitForFirstBatchOfServers();
300 }
对于nameservice的启动，这里不再贴过多的实现代码，大致流程为NamingServiceThread::Start->NamingServiceThread::RunThis->NamingServiceThread::Run->PeriodicNamingService::RunNamingService，其中在PeriodicNamingService::RunNamingService中又根据具体的不同名字如bns，会调用virtual GetServers比如bns中：
28 int BaiduNamingService::GetServers(const char *service_name,
29 std::vector<ServerNode>* servers) {
30 servers->clear();
31 BnsInput input;
32 input.set_service_name(service_name);
33 BnsOutput output;
34 const int rc = webfoot::get_instance_by_service(input, &output);
35 if (rc != webfoot::WEBFOOT_RET_SUCCESS) {
36 //more code...
44 }
45 const int instance_number = output.instance_size();
46 if (instance_number == 0) {
48 return 0;
49 }
50 for (int i = 0; i < instance_number; i++) {
51 const BnsInstance& instance = output.instance(i);
52 if (instance.status() == 0) {
53 butil::ip_t ip;
54 if (butil::str2ip(instance.host_ip().c_str(), &ip) != 0) {
56 continue;
57 }
58 servers->push_back(ServerNode(ip, instance.port(), instance.tag()));
59 }
60 }
61 return 0;
62 }
上面的协议没有找到，从实现中来看，估计根据某个域名拉取一些可用的ip列表类似的功能。比如和BaiduNamingService一样的结构的FileNamingService，会从本地文件中加载可用的ip列表，并监视该文件是否有变动，有变动重新load什么的。
借用命名服务上的一张图
图四
Channel初始化完毕后，接着下面的demo代码：
54 // Normally, you should not call a Channel directly, but instead construct
55 // a stub Service wrapping it. stub can be shared by all threads as well.
56 example::EchoService_Stub stub(&channel);
58 // Send a request and wait for the response every 1 second.
59 int log_id = 0;
60 while (!brpc::IsAskedToQuit()) {
61 // We will receive response synchronously, safe to put variables
62 // on stack.
63 example::EchoRequest request;
64 example::EchoResponse response;
65 brpc::Controller cntl;
66
67 request.set_message("hello world");
68
69 cntl.set_log_id(log_id ++); // set by user
70 // Set attachment which is wired to network directly instead of
71 // being serialized into protobuf messages.
72 cntl.request_attachment().append(FLAGS_attachment);
73
74 // Because `done'(last parameter) is NULL, this function waits until
75 // the response comes back or error occurs(including timedout).
76 stub.Echo(&cntl, &request, &response, NULL);
87 }
90 return 0;
其中Controller作用如下面的注释：
106 // A Controller mediates a single method call. The primary purpose of
107 // the controller is to provide a way to manipulate settings per RPC-call
108 // and to find out about RPC-level errors.
109 class Controller : public google::protobuf::RpcController/*non-copyable*/ {
761 };
由于该类实现过于复杂，这里不再贴相关代码。这里是同步call所以logid没啥用，在ParallelChannel中会有用，接着stub.Echo(&cntl, &request, &response, NULL)：
759 void EchoService_Stub::Echo(::google::protobuf::RpcController* controller,
760 const ::example::EchoRequest* request,
761 ::example::EchoResponse* response,
762 ::google::protobuf::Closure* done) {
763 channel_->CallMethod(descriptor()->method(0),
764 controller, request, response, done);
765 }
369 void Channel::CallMethod(const google::protobuf::MethodDescriptor* method, .../*同上四个参数*/) {
374 const int64_t start_send_real_us = butil::gettimeofday_us();
375 Controller* cntl = static_cast<Controller*>(controller_base);
376 cntl->OnRPCBegin(start_send_real_us);
377 //more code...
397 const CallId correlation_id = cntl->call_id();
398 const int rc = bthread_id_lock_and_reset_range(
399 correlation_id, NULL, 2 + cntl->max_retry());
400 //more code...
456 cntl->_response = response;
457 cntl->_done = done;
458 cntl->_pack_request = _pack_request;
459 cntl->_method = method;
460 cntl->_auth = _options.auth;
468 cntl->_lb = _lb;
482 _serialize_request(&cntl->_request_buf, cntl, request);
493 if (cntl->backup_request_ms() >= 0 &&
494 (cntl->backup_request_ms() < cntl->timeout_ms() ||
495 cntl->timeout_ms() < 0)) {
496 // Setup timer for backup request. When it occurs, we'll setup a
497 // timer of timeout_ms before sending backup request.
498
499 // Send another request if RPC does not finish after so many milliseconds
500 // The request will be sent to a different server by best effort.
501 //more code...
515 } else if (cntl->timeout_ms() >= 0) {
516 // Setup timer for RPC timetout
517
518 // _deadline_us is for truncating _connect_timeout_ms
519 cntl->_deadline_us = cntl->timeout_ms() * 1000L + start_send_real_us;
520 const int rc = bthread_timer_add(
521 &cntl->_timeout_id,
522 butil::microseconds_to_timespec(cntl->_deadline_us),
523 HandleTimeout, (void*)correlation_id.value);
524 if (BAIDU_UNLIKELY(rc != 0)) {
526 return cntl->HandleSendFailed();
527 }
528 } else {
529 cntl->_deadline_us = -1;
530 }
531
532 cntl->IssueRPC(start_send_real_us);
533 if (done == NULL) {
534 // MUST wait for response when sending synchronous RPC. It will
535 // be woken up by callback when RPC finishes (succeeds or still
536 // fails after retry)
537 Join(correlation_id);
538 if (cntl->_span) {
539 cntl->SubmitSpan();
540 }
541 cntl->OnRPCEnd(butil::gettimeofday_us());
542 }
543 }
上面列出了一些关键实现的CallMethod代码，CallMethod作为Channel继承RpcChannel是需要重新定义该接口，主要用于一些控制类的功能。
上面会设置rpc的开始时间，设置选项参数(代码跳过)，创建一个callid，这个实现后面再分析bthread_id.md，这其中会设置channel:: HandleTimeout[对应Channel上所有RPC的总超时，brpc中的超时是deadline，超过就意味着RPC结束，超时后没有重试。]：
359 static void HandleTimeout(void* arg) {
360 bthread_id_t correlation_id = { (uint64_t)arg };
362 bthread_id_error(correlation_id, ERPCTIMEDOUT);
363 }
其中对于一些错误判断会进行HandleSocketFailed：
1179 int Controller::HandleSocketFailed(bthread_id_t id, void* data, int error_code,
1180 const std::string& error_text) {
1181 Controller* cntl = static_cast<Controller*>(data);
1182 if (!cntl->is_used_by_rpc()) {
1183 // Cannot destroy the call_id before RPC otherwise an async RPC
1184 // using the controller cannot be joined and related resources may be
1185 // destroyed before done->Run() running in another bthread.
1186 // The error set will be detected in Channel::CallMethod and fail
1187 // the RPC.
1190 return bthread_id_unlock(id);
1191 }
1192 const int saved_error = cntl->ErrorCode();
1193 if (error_code == ERPCTIMEDOUT) {
1197 } else if (error_code == EBACKUPREQUEST) {
1201 } else if (!error_text.empty()) {
1203 } else {
1204 //more code...
1206 }
1207 CompletionInfo info = { id, false };
1208 cntl->OnVersionedRPCReturned(info, true, saved_error);
1209 return 0;
1210 }
OnVersionedRPCReturned后面再分析。接着行456〜460设置Controller* cntl一些数据成员，并序列化请求至_request_buf，并设置定时器，这里只考虑timeout_ms，超时后回调(是立即执行还是等待后再执行)
359 static void HandleTimeout(void* arg) {
360 bthread_id_t correlation_id = { (uint64_t)arg };
361 bthread_id_error(correlation_id, ERPCTIMEDOUT);
362 }
里面的回调最终回到上面的HandleSocketFailed，即error_code为ERPCTIMEDOUT。之后开始发起rpc请求IssueRPC，如果是同步即done == NULL需要等待(先跳过具体的分析)：
529 void Join(CallId id) {
530 bthread_id_join(id);
531 }
967 void Controller::IssueRPC(int64_t start_realtime_us) {
968 _current_call.begin_time_us = start_realtime_us;
969 // Clear last error, Don't clear _error_text because we append to it.
970 _error_code = 0;
971
972 // Make versioned correlation_id.
973 // call_id : unversioned, mainly for ECANCELED and ERPCTIMEDOUT
974 // call_id + 1 : first try.
975 // call_id + 2 : retry 1
976 // ...
977 // call_id + N + 1 : retry N
978 // All ids except call_id are versioned. Say if we've sent retry 1 and
979 // a failed response of first try comes back, it will be ignored.
980 const CallId cid = current_id();
992 // Pick a target server for sending RPC
993 _current_call.need_feedback = false;
994 _current_call.enable_circuit_breaker = has_enabled_circuit_breaker();
996 if (SingleServer()) {
997 //more code...
1007 } else {
1008 LoadBalancer::SelectIn sel_in =
1009 { start_realtime_us, true,
1010 has_request_code(), _request_code, _accessed };
1011 LoadBalancer::SelectOut sel_out(&tmp_sock);
1012 const int rc = _lb->SelectServer(sel_in, &sel_out);
1013 if (rc != 0) {
1014 //more code...
1020 }
1021 _current_call.need_feedback = sel_out.need_feedback;
1022 _current_call.peer_id = tmp_sock->id();
1027 _remote_side = tmp_sock->remote_side();
1028 }
1113 // Make request
1114 butil::IOBuf packet;
1115 SocketMessage* user_packet = NULL;
1116 _pack_request(&packet, &user_packet, cid.value, _method, this,
1117 _request_buf, using_auth);//baidu_protocol:PackRpcRequest
1142 Socket::WriteOptions wopt;
1143 wopt.id_wait = cid;
1144 wopt.abstime = pabstime;
1145 wopt.pipelined_count = _pipelined_count;
1146 wopt.with_auth = has_flag(FLAGS_REQUEST_WITH_AUTH);
1147 wopt.ignore_eovercrowded = has_flag(FLAGS_IGNORE_EOVERCROWDED);
1148 int rc;
1149 size_t packet_size = 0;
1150 if (user_packet_guard) {
1151 //
1155 } else {
1156 packet_size = packet.size();
1157 rc = _current_call.sending_sock->Write(&packet, &wopt);
1158 }
1175 }
上面的逻辑为正式发请求，省略了一些判断，在发之前，会处理一些逻辑，比如如果是集群则选择一个server作为目的server，根据连接类型选择某个socket，接着_pack_request，并Write数据。
当收到响应时，由于这里使用的是PROTOCOL_BAIDU_STD：
544 void ProcessRpcResponse(InputMessageBase* msg_base) {
545 const int64_t start_parse_us = butil::cpuwide_time_us();
546 DestroyingPtr<MostCommonMessage> msg(static_cast<MostCommonMessage*>(msg_base));
547 RpcMeta meta;
548 if (!ParsePbFromIOBuf(&meta, msg->meta)) {
550 return;
551 }
552
553 const bthread_id_t cid = { static_cast<uint64_t>(meta.correlation_id()) };
555 Controller* cntl = NULL;
556 const int rc = bthread_id_lock(cid, (void**)&cntl);
557 if (rc != 0) {
558 //more code...
563 return;
564 }
620 accessor.OnResponse(cid, saved_error);
621 }
46 void OnResponse(CallId id, int saved_error) {
47 const Controller::CompletionInfo info = { id, true };
48 _cntl->OnVersionedRPCReturned(info, false, saved_error);
49 }
以上会尝试解析消息并回调OnVersionedRPCReturned，中间的一些错误处理情况跳过。
542 void Controller::OnVersionedRPCReturned(const CompletionInfo& info,
543 bool new_bthread, int saved_error) {
546 // TODO(gejun): Simplify call-ending code.
547 // Intercept previous calls
548 while (info.id != _correlation_id && info.id != current_id()) {
549 //more code...
564 return;
565 }
566
567 if ((!_error_code && _retry_policy == NULL) ||
568 _current_call.nretry >= _max_retry) {
570 goto END_OF_RPC;
571 }
572 if (_error_code == EBACKUPREQUEST) {
607 } else if (_retry_policy ? _retry_policy->DoRetry(this)
608 : DefaultRetryPolicy()->DoRetry(this)) {
610 // The error must come from _current_call because:
611 // * we intercepted error from _unfinished_call in OnVersionedRPCReturned
612 // * ERPCTIMEDOUT/ECANCELED are not retrying error by default.
613 CHECK_EQ(current_id(), info.id) << "error_code=" << _error_code;
614 if (!SingleServer()) {
615 if (_accessed == NULL) {
616 _accessed = ExcludedServers::Create(
617 std::min(_max_retry, RETRY_AVOIDANCE));
618 if (NULL == _accessed) {
619 SetFailed(ENOMEM, "Fail to create ExcludedServers");
620 goto END_OF_RPC;
621 }
622 }
623 _accessed->Add(_current_call.peer_id);
624 }
625 _current_call.OnComplete(this, _error_code, info.responded, false);
626 ++_current_call.nretry;
627 // Clear http responses before retrying, otherwise the response may
628 // be mixed with older (and undefined) stuff. This is actually not
629 // done before r32008.
630 if (_http_response) {
631 _http_response->Clear();
632 }
633 response_attachment().clear();
634 return IssueRPC(butil::gettimeofday_us());
635 }
637 END_OF_RPC:
638 if (new_bthread) {
664 if ((FLAGS_usercode_in_pthread || _done != NULL/*Note[_done]*/) &&
665 !has_flag(FLAGS_DESTROY_CID_IN_DONE)/*Note[cid]*/) {
666 bthread_id_about_to_destroy(info.id);
667 }
668 // No need to join this bthread since RPC caller won't wake up
669 // (or user's done won't be called) until this bthread finishes
670 bthread_t bt;
671 bthread_attr_t attr = (FLAGS_usercode_in_pthread ?
672 BTHREAD_ATTR_PTHREAD : BTHREAD_ATTR_NORMAL);
673 _tmp_completion_info = info;
674 if (bthread_start_background(&bt, &attr, RunEndRPC, this) != 0) {
676 EndRPC(info);
677 }
678 } else {
679 if (_done != NULL/*Note[_done]*/ &&
680 !has_flag(FLAGS_DESTROY_CID_IN_DONE)/*Note[cid]*/) {
681 bthread_id_about_to_destroy(info.id);
682 }
683 EndRPC(info);
684 }
685 }
以上当某个retry请求回来时，可能要中断之前的请求，这部分代码没贴上来；判断是否还有retry次数和是否retryDoRetry；对请求失败的情况下，若是集群情况，下一次rpc请求会避免上一次的选择，完成本次请求OnComplete，并增加retry次数，并再进行发起IssueRPC，这里列一下常见的retry的情况brpc的错误码：
ETIMEDOUT：连接超时
EAGAIN：同时发送的请求过多。软限，很少出现。
EFAILEDSOCKET：RPC进行过程中TCP连接出现问题
ELIMIT：同时处理的请求数超过ServerOptions.max_concurrency了
709 void Controller::Call::OnComplete(
710 Controller* c, int error_code/*note*/, bool responded, bool end_of_rpc) {
711 //more code...
793 if (need_feedback) {
794 const LoadBalancer::CallInfo info =
795 { begin_time_us, peer_id, error_code, c };
796 c->_lb->Feedback(info);
797 }
798
799 // Release the `Socket' we used to send/receive data
800 sending_sock.reset(NULL);
801 }
OnComplete中有大部分代码跟sending_sock操作有关，这里暂时不分析，上面部分代码是对一次rpc过程的结束，最后决定是否要需要lb重新计算权值。
这里测试时，对一些关键路径上加了打印，对于正常情况：
E0921 14:57:40 775 /Users//opensource/brpc/src/brpc/channel.cpp:399] Bthread Rpc Call Init 4294967297
E0921 14:57:40 9475 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967298 0=== 4294967297 xxx 4294967298
E0921 14:57:40 9475 /Users//opensource/brpc/src/brpc/controller.cpp:569] OnVersionedRPCReturned step3 4294967298
I0921 14:57:40 775 /Users//opensource/brpc/example/echo_c++/client.cpp:78] Received response from 0.0.0.0:8000 to 127.0.0.1:52693: hello world (attached=) latency=2489us
在server收到请求时不回应直接sleep(xx)秒，然后打印出：
E0921 15:00:46 775 /Users//opensource/brpc/src/brpc/channel.cpp:399] Bthread Rpc Call Init 4294967297
E0921 15:00:46 10499 /Users//opensource/brpc/src/brpc/channel.cpp:361] Bthread Rpc Call Timeout 4294967297
E0921 15:00:46 10499 /Users//opensource/brpc/src/brpc/controller.cpp:1216] HandleSocketFailed 4294967297 0---1008 ~~~~
E0921 15:00:46 10499 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967297 0=== 4294967297 xxx 4294967298
E0921 15:00:46 10499 /Users//opensource/brpc/src/brpc/controller.cpp:663] OnVersionedRPCReturned step6 4294967297
W0921 15:00:46 775 /Users//opensource/brpc/example/echo_c++/client.cpp:84] [E1008]Reached timeout=100ms @0.0.0.0:8000
不启动server直接启动client：
E0921 15:04:09 775 /Users//opensource/brpc/src/brpc/channel.cpp:399] Bthread Rpc Call Init 4294967297
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:1216] HandleSocketFailed 4294967298 0---61 ~~~~ Fail to connect Socket{id=0 addr=0.0.0.0:8000} (0x0x1121c8000): Connection refused
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967298 0=== 4294967297 xxx 4294967298
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:609] OnVersionedRPCReturned step5 4294967298
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:963] HandleSendFailed 4294967299 64
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967299 64=== 4294967297 xxx 4294967299
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:609] OnVersionedRPCReturned step5 4294967299
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:963] HandleSendFailed 4294967300 64
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967300 64=== 4294967297 xxx 4294967300
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:609] OnVersionedRPCReturned step5 4294967300
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:963] HandleSendFailed 4294967301 64
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:544] OnVersionedRPCReturned step1 4294967301 64=== 4294967297 xxx 4294967301
E0921 15:04:09 5891 /Users//opensource/brpc/src/brpc/controller.cpp:569] OnVersionedRPCReturned step3 4294967301
W0921 15:04:09 775 /Users//opensource/brpc/example/echo_c++/client.cpp:84] [E61]Fail to connect Socket{id=0 addr=0.0.0.0:8000} (0x0x1121c8000): Connection refused [R1][E64]Not connected to 0.0.0.0:8000 yet, server_id=0 [R2][E64]Not connected to 0.0.0.0:8000 yet, server_id=0 [R3][E64]Not connected to 0.0.0.0:8000 yet, server_id=0
I0921 15:04:10 9475 /Users//opensource/brpc/src/brpc/socket.cpp:2204] Checking Socket{id=0 addr=0.0.0.0:8000} (0x1121c8000)
上面没有说到并发请求的情况，这些可以参考multi_threaded_echo_c++中实现，“Channel::CallMethod和stub访问都是线程安全的，可以被所有线程同时访问。”
后面分析下server端实现。
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