How to tell if connecting to the server fails? (C++)

[thej...@gmail.com]

How do I detect that I can't connect to the specified server in C++? As far as I can tell, the attempt to connect currently just hangs forever. Is there any way to return programmatically on failure to connect to the server?

[Florian Schott]

Hi,
I got the same problem, my current solution ist to set a deadline to my rpc calls and if this exceeds, I assume the connection is down.
This can be achieved relatively easy: the context object has a method set_deadline which can take a  timespec.
If the RPC Call times out, you'll get the grpc status DEADLINE_EXCEEDED.

Example with five seconds timeout:

    ClientContext context;

    gpr_timespec timeOut;

    timeOut.tv_sec = 5;
    timeOut.tv_nsec = 0;
    timeOut.clock_type = GPR_TIMESPAN;
    context.set_deadline(timeOut);

    grpc::Status retStat = serviceStub->myRPCCall(&context, request, &response);

    if (!retStat.ok() && retStat.error_code() == DEADLINE_EXCEEDED)
    {
        // call timed out

[Yang Gao]

You may want to check out the channel connectivity API in channel.h. The state definition is at https://github.com/grpc/grpc/blob/master/doc/connectivity-semantics-and-api.md.

A WaitUntilReady() may look like this:

bool WaitUntilReady(channel, deadline) {

  auto state = channel->GetState(true);

  while (state != GRPC_CHANNEL_READY) {

    if (!WaitForStateChange(state, deadline)) { return false; }

    state = channel->GetState(true);

  }

  return true;

}

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[Florian Schott]

Hi,

yes this would be a nice solution, i also tried this, but in my case I didn't get this to work. I only get the state GRPC_CHANNEL_IDLE, regardless if it is conencted or not (in fact even when there is no server present at all).

[Yang Gao]

Did you call GetState(true) ? Passing in true will make the channel start connecting.

To view this discussion on the web visit https://groups.google.com/d/msgid/grpc-io/c9f67f99-5fb2-42da-8c79-db9bf402b4c2%40googlegroups.com.

[Florian Schott]

yes I did..

just tried it again, because something like this would be really nice to have for me. I do channel->GetSteate(true) in  a loop with a Sleep and while waching it right now the state does not change. Even doesn't matter if I start/shut down the server in the background...
BTW I'm using 0.11.0 right now.

However, I worked around with the deadline-check I mentioned above. But if this solution (ChannelState) would work, it qould be very nice to set up a background-monitoring-thread......

[Yang Gao]

It should not stay in IDLE no matter there is a server or not. If you have a (non)working example code snippet, I can help look at it.

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[Florian Schott]

Hi,

it is as simple as this:


    std::shared_ptr<grpc::Channel> channel = grpc::CreateChannel(os.str(), grpc::InsecureCredentials());
    _serviceStub = dataService::NewStub(channel);

    auto state = channel->GetState(true);

os is a ostringstream wcontaining "localhost:21000"
State is GRPC_CHANNEL_IDLE, no matter if server is up or down. Also later in the program, if i check the status, it's always GRPC_CHANNEL_IDLE..