gRPC异步处理应答
(金庆的专栏)
gRPC的示例 greeter_async_client.cc 不算是异步客户端，
它使用了异步请求，但是阻塞式等待应答，结果成为一个同步调用。
std::string SayHello(const std::string& user) {
...
std::unique_ptr<ClientAsyncResponseReader<HelloReply> > rpc(
stub_->AsyncSayHello(&context, request, &cq));
rpc->Finish(&reply, &status, (void*)1);
void* got_tag;
bool ok = false;
// Block until the next result is available in the completion queue "cq".
cq.Next(&got_tag, &ok);
...
return reply.message();
}
为了实现真正的异步RPC请求，发出请求后立即返回，然后在一个线程中处理所有应答。
以下代码经测试表明可以使用。
// Grpc异步应答处理，线程中运行.
void HandleGrpcResponses()
{
...
grpc::CompletionQueue & rCq = rMgr.GetCq();
for (;;)
{
void * pTag;
bool ok = false;
// Block until the next result is available in the completion queue "cq".
rCq.Next(&pTag, &ok);
// Act upon the status of the actual RPC.
std::unique_ptr<IGrpcCb> pCb(static_cast<IGrpcCb*>(pTag));
const grpc::Status & rStatus = pCb->GetStatus();
if (rStatus.ok())
(*pCb)(); // run callback
}
}
IGrpcCb是回调类，定义如下：
class IGrpcCb
{
public:
explicit IGrpcCb(...) {};
virtual ~IGrpcCb(void) {};
grpc::ClientContext & GetContext() { return m_context; }
grpc::Status & GetStatus() { return m_status; }
public:
virtual void operator()() {};
protected:
grpc::ClientContext m_context;
grpc::Status m_status;
...
};
// R is response class like rpc::CreateRoomResponse.
template <class R>
class GrpcCb final : public IGrpcCb
{
public:
explicit GrpcCb(...)
: IGrpcCb(...)
{};
virtual ~GrpcCb(void) override {};
public:
typedef std::unique_ptr<grpc::ClientAsyncResponseReader<R> > RpcPtr;
public:
R & GetResp() { return m_resp; }
void SetRpcPtrAndFinish(RpcPtr pRpc)
{
m_pRpc.swap(pRpc);
m_pRpc->Finish(&m_resp, &m_status, (void*)this);
}
public:
virtual void operator()() override
{
// Deal m_resp...
}
private:
RpcPtr m_pRpc;
R m_resp;
};
异步请求代码示例如下：
grpc::CompletionQueue & cq = GetCq();
rpc::CreateRoomRequest req;
// pGcb will be deleted in HandleGrpcResponses().
auto pGcb = new GrpcCb<rpc::CreateRoomResponse>(...);
pGcb->SetRpcPtrAndFinish(
m_pStub->AsyncCreateRoom(&pGcb->GetContext(), req, &cq));