[zeromq-dev] ZeroMQ correctly forking parent / child worker and cleanly exiting the child

[abhay.pradhan at ubs.com]

Hello,

Here is what I want to do (C++)

I have a parent process that creates a context and a socket that listens on a port for client requests. A client comes in and asks for an endpoint (port) for a worker node.

At that point, my parent process forks and the forked process creates a worker node that creates another context and a socket that is listening on the worker port. The parent process returns the worker port to the client and now the client is then free to communicate with the worker, giving it tasks.

All of this works.

However, I want the client to be able to send an EXIT message to the worker. At this point, my client should cleanly terminate (closing its worker socket and context). Here is where my problems start


1)      The worker process calling close() on the context throws an assert (line 310 in zmq.hpp)

2)      Once my worker process exits, I cannot get new clients to connect to my parent process. The parent context hasn't been destroyed and the socket is still open (netstat shows LISTENING).

I was wondering if there were tips on how to handle forked processes exiting cleanly. The guide states that I should create contexts and sockets after I fork() however, I do want my parent process to also bind to a port and serve worker ports as endpoints.

cheers
Abhay


// Pseudo code


class Parent {
                void doStuff() {
                                create parent context;
                                create parent socket
                                bool looping = true;
                                bool child = false;

                                while (looping) {
                                                zmq::message_t request;
                                                socket.recv(&request);  // get request from client
            string reqStr = string(static_cast<char *>(request.data()), request.size());

            // should parent exit?
            if (reqStr == "Exit")
                looping = false;

            // configure child and return child's port
            if (reqStr == "Give me worker port") {
                int port = doFork(reqStr);
                            if (port > 0) {
                                string result = NumberToString(port);
                                zmq::message_t reply(result.length());
                                memcpy((void *) reply.data(), result.c_str(), result.length());
                                socket.send(reply);   // send reply to worker
                            }
                            else {
                                // child exiting
                                looping = false;
                                child = true;
                            }
            }
                                }
                                if (!child) {
                                                // parent is getting out
                                    socket.close();
                                    context.close();
                    }

                    // at this point, program essentially terminates .. either the child process or the parent
                }


                int doFork(reqStr) {
                                int port = CreateAPortForChild();
                    switch (pid = fork()) {
                        case -1:
                            cout << "Error in fork";
                            return -1;

                        case 0:
                            LOG(INFO) << "Created child process with pid: " << getpid() << endl;
                            {
                                WorkerPtr workerPtr(new Worker(NumberToString(port)));
                                workerPtr->spawn();
                            }
                            cout << "Child Process exiting on port: " << port << endl;
                            return 0;

                        default:
                            cout << "Parent process. My process id is " << getpid() << endl;
                    }
                    return port;
                }
}

class Worker {
                void spwan() {
                                create child context
                                create child socket

                                bool looping = true;
                                while (looping) {
                        zmq::message_t request;
                        socket.recv(&request);  // Wait
                        string reqStr = string(static_cast<char *>(request.data()), request.size());
                        if (reqStr == "Do Work") {
                                doWork();
                        }
                        else if (reqStr == "EXIT") {
                                looping = false;
                        }
                                }
                                close child socket
                                should i close child context?
                                // child forked process returns to line after workerPtr->spawn()
                                // how do i exit cleanly?
                }
}

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