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Meet the Channel Model: ICommunicationObject

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A Common a State Machine

Objects that deal with communication, e.g. sockets, usually present a state machine whose state transitions relate to allocating network resources, making or accepting connections, closing connections and aborting communication. The channel state machine provides a uniform model of the states of a communication object that abstracts the underlying implementation of that object. The interface provides a set of states, state transition methods, and state transition events. All channels, channel factories and channel listeners implement the channel state machine.

ICommunicationObject

ICommunicationObject is a CLR interface that describes the channel state machine contract:

 public interface ICommunicationObject : IDisposable
{
    CommunicationState State { get; }

    event EventHandler Closed;
    event EventHandler Closing;
    event EventHandler Faulted;
    event EventHandler Opened;
    event EventHandler Opening;

    void Abort();

    void Close();
    void Close(TimeSpan timeout);
    IAsyncResult BeginClose(AsyncCallback callback, object state);
    IAsyncResult BeginClose(TimeSpan timeout, 
                            AsyncCallback callback, object state);
    void EndClose(IAsyncResult result);

    void Open();
    void Open(TimeSpan timeout);
    IAsyncResult BeginOpen(AsyncCallback callback, object state);
    IAsyncResult BeginOpen(TimeSpan timeout, 
                           AsyncCallback callback, object state);
    void EndOpen(IAsyncResult result);
}

The events Closed, Closing, Faulted, Opened and Opening signal an external observer after a state transition occurs.
The methods Abort, Close, and Open (and their async equivalents) cause state transitions.
The state property returns the current state as defined by CommunicationState:

 public enum CommunicationState
{
    Created,
    Opening,
    Opened,
    Closing,
    Closed,
    Faulted
}

An ICommunicationObject starts out in the Created state where it’s various properties can be configured. Once in the Opened state, the object is usable for sending and/or receiving messages but its properties are considered immutable. If an unrecoverable error occurs, the object transitions to the Faulted state where it can be inspected for information about the error and ultimately closed. When in the Closed state the object has essentially reached the end of the state machine. In general, once an object transitions from one state to the next, it does not go back to a previous state.

States and Transition
The figure below shows the ICommunicationObject states and state transitions. State transitions can be caused by calling one of the three methods: Abort, Open, or Close. They could also be caused by calling other implementation-specific methods. Transitioning to the Faulted state could happen as a result of errors while opening or after having opened the communication object. Every ICommunicationObject starts out in the Created state. In this state, an application can configure the object by setting its properties (e.g. setting the address of a channel listener by calling its SetUri method). Once an object is in a state other than Created, it is considered immutable. For example, calling SetUri on a channel listener in the Opened state should throw an InvalidOperationException.

WCF provides an abstract base class named CommunicationObject which implements ICommunicationObject and acts as the base of all channel objects. That will be the subject of my next posting on the channel model.

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