gRPC: How can RPC handlers properly detect if `Server` has been `Shutdown()` - c++

Question

Current, I'm using a hackish way – a global variable – to make RPC handlers able to detect that the Server has been (about to be) called Shutdown().
bool g_ServerIsNotDead = true; // Hack!
Status StreamServiceImpl::GetCurrentTemperature(ServerContext *context_,
const UpdateInterval *request_,
ServerWriter<Temperature> *stream_)
{
auto currentTemp = 100.0f;
while(g_ServerIsNotDead) // Hack!!!
{
qDebug() << QThread::currentThreadId() << currentTemp << "farenheit.";
Temperature message;
message.set_temperature(currentTemp);
stream_->Write(message);
QThread::sleep(2);
currentTemp += 1.0f;
}
return Status::OK;
}
void insideSomeFunction() {
// Testing shutdown 5 seconds later
QTimer::singleShot(std::chrono::seconds(5), this, [=]() {
qDebug() << "Shuting down!";
g_ServerIsNotDead = false; // Hack!!
this->server->Shutdown(); // This method actually blocks until all RPC handlers have exited, believe it or not!
emit shutdown();
qDebug() << "All dead.";
});
}
Ref: https://github.com/C0D1UM/grpc-qt-example/blob/master/rpc_server/hellostream_server.cpp
It would be really nice if I could somehow check that Server has been Shutdown() from grpc::ServerContext, but I didn't see any relevant methods to achieve this.
Even better if someone could propose a way to take out the while loop completely (?). I'm using Qt so everything is event-driven.

Answer 1

So, I think it's worth making clear what Shutdown does. There's a detailed comment about this but basically, server Shutdown doesn't fail, cancel, or kill your existing in-progress calls (unless you use the deadline argument and the gRPC C++ async API).
Rather, it stops listening for new connections, stops accepting new calls, fails requested-but-not-yet-accepted calls. If you want to fail or terminate your calls at shutdown, you can do it at application-level code as you've done above.
I would just recommend that instead of using a global variable, you should use a member function of your StreamServiceImpl class so that you can support multiple services running in the same process if you choose.

Answer 2

We can use ServerContext::IsCancelled as a breaking/termination criteria in streaming APIs. I changed GetCurrentTemperature(...) as follows (just replaced g_ServerIsNotDead with !context_->IsCancelled()) and it worked:
Status StreamServiceImpl::GetCurrentTemperature(ServerContext *context_,
const UpdateInterval *request_,
ServerWriter<Temperature> *stream_) {
auto currentTemp = 100.0f;
while(!context_->IsCancelled) {
qDebug() << QThread::currentThreadId() << currentTemp << "farenheit.";
Temperature message;
message.set_temperature(currentTemp);
stream_->Write(message);
QThread::sleep(2);
currentTemp += 1.0f;
}
return Status::OK;
}