應用場景

有時為了保證可用性，需要同時訪問兩路服務，哪個先返回就取哪個。在brpc中，這有多種做法，根據server是否掛在同一個命名服務內有所區別。

當后端server可以掛在一個命名服務內時

Channel開啟backup request。這個Channel會先向其中一個server發送請求，如果在ChannelOptions.backup_request_ms后還沒回來，再向另一個server發送。之后哪個先回來就取哪個。在設置了合理的backup_request_ms后，大部分時候只會發一個請求，對后端服務只有一倍壓力。

#include <gflags/gflags.h>
#include <butil/logging.h>
#include <butil/time.h>
#include <brpc/channel.h>
#include "echo.pb.h"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(backup_request_ms, 2, "Timeout for sending backup request");int main(int argc, char* argv[]) {// Parse gflags. We recommend you to use gflags as well.GFLAGS_NS::ParseCommandLineFlags(&argc, &argv, true);// A Channel represents a communication line to a Server. Notice that // Channel is thread-safe and can be shared by all threads in your program.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;// 設置backup_request_ms參數，在超時后會自動向另外一個server重發請求，與首次請求一起，哪個先返回用哪個options.backup_request_ms = FLAGS_backup_request_ms;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 counter = 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_index(++counter);// 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[index=" << response.index()<< "] from " << cntl.remote_side()<< " to " << cntl.local_side()<< " latency=" << cntl.latency_us() << "us";} else {LOG(WARNING) << cntl.ErrorText();}sleep(1);}LOG(INFO) << "EchoClient is going to quit";return 0;
}

backup_request_ms推薦值

backup_request_ms=2ms可以大約覆蓋95.5%的請求，選擇backup_request_ms=10ms則可以覆蓋99.99%的請求。

當后端server不能掛在一個命名服務內時

方案一、使用SelectiveChannel：

建立一個開啟backup request的SelectiveChannel，其中包含兩個sub channel。訪問這個SelectiveChannel和上面的情況類似，會先訪問一個sub channel，如果在ChannelOptions.backup_request_ms后沒返回，再訪問另一個sub channel。如果一個sub channel對應一個集群，這個方法就是在兩個集群間做互備。

#include <gflags/gflags.h>
#include <bthread/bthread.h>
#include <butil/logging.h>
#include <brpc/selective_channel.h>
#include <brpc/parallel_channel.h>
#include "echo.pb.h"DEFINE_int32(thread_num, 50, "Number of threads to send requests");
DEFINE_bool(use_bthread, false, "Use bthread to send requests");
DEFINE_int32(attachment_size, 0, "Carry so many byte attachment along with requests");
DEFINE_int32(request_size, 16, "Bytes of each request");
DEFINE_string(connection_type, "", "Connection type. Available values: single, pooled, short");
DEFINE_string(protocol, "baidu_std", "Protocol type. Defined in src/brpc/options.proto");
DEFINE_string(starting_server, "0.0.0.0:8114", "IP Address of the first server, port of i-th server is `first-port + i'");
DEFINE_string(load_balancer, "rr", "Name of load balancer");
DEFINE_int32