brpc中同步、异步、半同步和取消操作

直接如题，从brpc开源的文档中看到brpc既支持同步调用，同时也支持异步调用。这里直接给出同步、异步的例子，同时对其进行分析。

1、brpc同步调用

brcp的同步调用是之前的echo的简单例子，所谓同步就是client对远端的server进行调用，同时自己原地等待，等待rpc返回之后，在进行之后的操作。
代码如下：

#include <iostream>
#include <gflags/gflags.h>
#include <butil/logging.h>
#include <butil/time.h>
#include <brpc/channel.h>
#include "echo.pb.h"

using namespace std;

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);
    
    // 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;
    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).
        
        //注意这里是同步调用，因为回调函数done被设置为NULL
        //同步表示stub调用rpc，返回之后才会进行后面的语句
        stub.Echo(&cntl, &request, &response, NULL);
        cout << "after stub_method 0 " << endl;
        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();
        }
        cout << "after stub_method 1 " << endl;
        usleep(FLAGS_interval_ms * 1000L);
    }
	
    LOG(INFO) << "EchoClient is going to quit";
    return 0;
}

注意

1、stub.Echo调用可以看到回调函数被设置为NULL，那么此时调用被设置为同步操作，原地等待。

2、同步访问中的response/controller不会在CallMethod后被框架使用，它们都可以分配在栈上。注意，如果request/response字段特别多字节数特别大的话，还是更适合分配在堆上。

运行结果

同步进行，按照操作流程一步步打印

2、brpc异步调用

brpc的异步调用指的是和同步调用相比，在进行rpc调用之后，此时callmethod就结束了，进行继续执行后续的动作，等到rpc返回之后，会调用事先注册的回调函数，回调函数进行后面rpc返回之后的操作。
代码如下：

#include <iostream>
#include <gflags/gflags.h>
#include <butil/logging.h>
#include <butil/time.h>
#include <brpc/channel.h>
#include "echo.pb.h"

using namespace std;

DEFINE_bool(send_attachment, true, "Carry attachment 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<