默认情况下，grpc服务端和客户端的收发消息大小都是4M，可以通过自己修改该大小。由于设置的函数参数是int的，所以最大的能使用的消息大小也是2G。如果超过2G的数据可以使用stream的方式。
grpc默认接受数据大小为4MB，如果发送的数据超过该值，需要进行修改。未经设置的默认值如下设置：

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/** Default send/receive message size limits in bytes. -1 for unlimited. */
/** TODO(roth) Make this match the default receive limit after next release */
#define GRPC_DEFAULT_MAX_SEND_MESSAGE_LENGTH (-1)
#define GRPC_DEFAULT_MAX_RECV_MESSAGE_LENGTH (4 * 1024 * 1024)

服务端调整消息大小

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int kMaxMessageSize = INT_MAX;

grpc::ServerBuilder builder;
builder.SetMaxReceiveMessageSize(kMaxMessageSize);
builder.SetMaxSendMessageSize(kMaxMessageSize);
builder.AddListeningPort("0.0.0.0:8081", grpc::InsecureServerCredentials());
builder.RegisterService(&mMyService);

std::unique_ptr<grpc::Server> server(builder.BuildAndStart());
server->Wait();

客户端调整消息大小

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grpc::ChannelArguments cArgs;
cArgs.SetInt(GRPC_ARG_MAX_RECEIVE_MESSAGE_LENGTH, INT_MAX);
std::shared_ptr<grpc::Channel> channel =
    grpc::CreateCustomChannel("localhost:8081", grpc::InsecureChannelCredentials(), cArgs);
std::unique_ptr<arc::MyService::Stub> stub(arc::MyService::NewStub(channel));

grpc stream传输大文件

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request.clear_data();
auto image = request.mutable_image();
int64_t size = 1024 * 1024 * 10;
char* buf = new char[size];
std::ifstream file(path, std::ios::in | std::ios::binary);
if (!file.is_open()) {
    return;
}

while (!file.eof()) {
    std::streamsize len = file.read(buf, size).gcount();
    image->set_image_content(buf, len);
    writer->Write(request);
}

writer->Write(request);
delete buf;

writer->WritesDone();
grpc::Status status = writer->Finish();

message与json字符串的转换

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google::protobuf::util::JsonOptions opt;
opt.add_whitespace = true;
opt.always_print_primitive_fields = true;
opt.preserve_proto_field_names = true;
std::string str;
google::protobuf::util::MessageToJsonString(msg, &str, opt);

// 另外有反向转换函数
google::protobuf::util::JsonStringToMessage(StringPiece input, Message *message);