More about channels

In the server example above, we saw that the server created just one channel. It could certainly have created more, but generally, servers don't do that. (The most obvious example of a server with two channels is the Transparent Distributed Processing (TDP, also known as Qnet) native network manager—definitely an odd piece of software!)

As it turns out, there really isn't much need to create multiple channels in the real world. The main purpose of a channel is to give the server a well-defined place to “listen” for messages, and to give the clients a well-defined place to send their messages (via a connection). About the only time that you'd have multiple channels in a server is if the server wanted to provide either different services, or different classes of services, depending on which channel the message arrived on. The second channel could be used, for example, as a place to drop wake up pulses—this ensures that they're treated as a different “class” of service than messages arriving on the first channel.

In a previous paragraph, I had said that you could have a pool of threads running in a server, ready to accept messages from clients, and that it didn't really matter which thread got the request. This is another aspect of the channel abstraction. Under previous versions of the QNX family of operating systems (notably QNX 4), a client would target messages at a server identified by a node ID and process ID. Since QNX 4 is single-threaded, this means that there cannot be confusion about “to whom” the message is being sent. However, once you introduce threads into the picture, the design decision had to be made as to how you would address the threads (really, the “service providers”). Since threads are ephemeral, it really didn't make sense to have the client connect to a particular node ID, process ID, and thread ID. Also, what if that particular thread was busy? We'd have to provide some method to allow a client to select a “non-busy thread within a defined pool of service-providing threads.”

Well, that's exactly what a channel is. It's the “address” of a “pool of service-providing threads.” The implication here is that a bunch of threads can issue a MsgReceive() function call on a particular channel, and block, with only one thread getting a message at a time.