Java SocketChannel write blocked while reading

Question

I am trying to use SocketChannel.write and SocketChannel.read at the same time in two different threads (Android API Level 25).

I configured the SocketChannel as blocking mode .

For reading, I created an endless loop to read everything from server:

// Make socketChannel.read() return after 500ms at most
socketChannel.socket().setSoTimeout(500);
while(!SHUTDOWN) {
    int read = socketChannel.read(buffer);
    if(read > 0 ){
      // Do something nice
      break;
    }
}

And for writing, I write data each 10 seconds.

The problem is, I found that sometimes the writing operations were blocked while reading.

But if I make the reading thread sleep for a short period in each loop, eg 100ms, this problem won't appear anymore.

looks like reading thread is blocking writing thread

AFAIK, TCP connections can offer bi-direction operations at the same time. Can anyone help to explain this?

Answer 1

As explained in TCP Wikipedia - Flow Control :

TCP uses an end-to-end flow control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. Having a mechanism for flow control is essential in an environment where machines of diverse network speeds communicate. For example, if a PC sends data to a smartphone that is slowly processing received data, the smartphone must regulate the data flow so as not to be overwhelmed.