Details of Qnet data communication

As mentioned before, Qnet relies on the message passing paradigm of Neutrino. Before any message pass, however, the application (e.g. the client) must establish a connection to the server using the low-level ConnectAttach() function call:

ConnectAttach(nd, pid, chid, index, flags);

In the above call, nd is the node descriptor that identifies each node uniquely. The node descriptor is the only visible means to determine whether the Neutrino is running as a network or as a standalone operating system. If nd is zero, you're specifying a local server process, and you'll get local message passing from the client to the server, carried out by the local kernel as shown below:

When you specify a nonzero value for nd, the application transparently passes message to a server on another machine, and connects to a server on another machine. This way, Qnet not only builds a network of trusted machines, it lets all these machines share their resources with little overhead.

The advantage of this approach lies in using the same API. The key design features are:

These features maximize performance for large payloads and minimize turnaround time for small packets.