【brpc学习实践六】backup request场景案例

应用场景

有时为了保证可用性，需要同时访问两路服务，哪个先返回就取哪个。在brpc中，这有多种做法，根据server是否挂在同一个命名服务内有所区别。

当后端server可以挂在一个命名服务内时

Channel开启backup request。这个Channel会先向其中一个server发送请求，如果在ChannelOptions.backup_request_ms后还没回来，再向另一个server发送。之后哪个先回来就取哪个。在设置了合理的backup_request_ms后，大部分时候只会发一个请求，对后端服务只有一倍压力。

#include 
#include 
#include 
#include 
#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;
    // 设置backup_request_ms参数，在超时后会自动向另外一个server重发请求，与首次请求一起，哪个先返回用哪个
    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推荐值

backup_request_ms=2ms可以大约覆盖95.5%的请求，选择backup_request_ms=10ms则可以覆盖99.99%的请求。

当后端server不能挂在一个命名服务内时

方案一、使用SelectiveChannel：

建立一个开启backup request的SelectiveChannel，其中包含两个sub channel。访问这个SelectiveChannel和上面的情况类似，会先访问一个sub channel，如果在ChannelOptions.backup_request_ms后没返回，再访问另一个sub channel。如果一个sub channel对应一个集群，这个方法就是在两个集群间做互备。

#include 
#include 
#include 
#include 
#include 
#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