How to modify the hello-world-client-cpp20 example to unblock on timeout or response?

[tradias]

How to modify the hello-world-client-cpp20 example to unblock on timeout or response?

[Tradias]

gRPC provides two ways of handling cancellation of RPCs:

1. Set a deadline for the entire RPC. In that case, if the deadline is reached during the call to agrpc::finish the grpc::Status will be set to DEADLINE_EXCEEDED. The returned bool will still be true because the call actually made it to the server successfully, see also here.

2. grpc::ClientContext::TryCancel(). If the cancellation was successful then apgrc::finish's grpc::Status will be set to CANCELLED. You could e.g. have a shorter deadline for an individual RPC step like so:

grpc::Alarm alarm;
agrpc::wait(alarm, std::chrono::system_clock::now() + std::chrono::seconds(1),
            boost::asio::bind_executor(grpc_context,
                                        [&](bool alarm_expired)
                                        {
                                            if (alarm_expired)
                                            {
                                                client_context.TryCancel();
                                            }
                                        }));
co_await agrpc::finish(*reader, response, status);
alarm.Cancel();

and I might be able to get this to work with the new awaitable operators at some point as well. Note that for those to work the grpc::ClientContext would need to be passed to agrpc::finish so that it can be cancelled which is not yet possible.

[rbresalier]

Thanks @Tradias for your answer.

[rbresalier]

@Tradias : Since you are going to update some of the examples based on issue #7 , perhaps you could give an example of setting a deadline timer for one of the rpc calls.

[Tradias]

I have added an example on how to cancel a RPC step. The example is based on a unary RPC which should rather be deadlined with grpc::ClientContext::set_deadline, but it can be applied to streaming RPC steps as well. I leave it as an exercise to you to do that :).

    grpc::ClientContext client_context;
    client_context.set_deadline(std::chrono::system_clock::now() + std::chrono::seconds(5));