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Class JavaGroups.Channel

java.lang.Object
   |
   +----JavaGroups.Channel

public abstract class Channel

extends Object

implements Transport

The FSM for a channel is roughly as follows: a channel is created (unconnected). The channel is connected to a group (connected). Messages can now be sent and received. The channel is disconnected from the group (unconnected). The channel could now be connected to a different group again. The channel is closed (closed).

Only a single sender is allowed to be connected to a channel at a time, but there can be more than one channel in an application.

Messages can be sent to the group members using the Send method and messages can be received using Receive (pull approach).

A channel instance is created using either a ChannelFactory or the public constructor. Each implementation of a channel must provide a subclass of Channel and an implementation of ChannelFactory.

Various degrees of sophistication in message exchange can be achieved using building blocks on top of channels, e.g. light-weight groups, synchronous message invocation, or remote method calls. Channels are on the same abstraction level as sockets, and should really be simple to use. Higher-level abstractions are all built on top of channels.

Author:

Bela Ban

See Also:

DatagramPacket, MulticastSocket

BLOCK

GET_STATE_EVENTS

LOCAL

SUSPECT

VIEW

BlockOk()

Called to acknowledge a Block (callback in MembershipListener or BlockEvent received from call to Receive).

Close()

Destroys the channel and its associated resources (e.g.

Connect(Object)

Connects the channel to a group.

Disconnect()

Disconnects the channel from the current group (if connected), leaving the group.

Down(Event)

Access to event mechanism of channels.

GetAllStates(Vector, long)

Retrieve all states of the group members.

GetGroupAddress()

Returns the group address of the group of which the channel is a member.

GetLocalAddress()

Returns the channel's own address.

GetOpt(int)

Gets an option.

GetState(Object, long)

Retrieve the state of the group.

GetView()

Gets the current view.

Peek(long)

Returns the next message, view, block, suspect or other event without removing it from the queue.

Receive(long)

Receives a message, a view change or a block event.

ReturnState(Object)

Called by the application is response to receiving a GetState object when calling Receive.

Send(Message)

Sends a message to a destination.

SetChannelListener(ChannelListener)

Allows to be notified when a channel event such as connect, disconnect or close occurs.

SetOpt(int, Object)

Sets an option.

SetUpHandler(UpHandler)

When up_handler is set, all events will be passed to it directly.

 public static final int BLOCK

 public static final int VIEW

 public static final int SUSPECT

 public static final int LOCAL

 public static final int GET_STATE_EVENTS

 public abstract void Connect(Object group_address) throws ChannelClosed

Connects the channel to a group. The client is now able to receive group messages, views and block events (depending on the options set) and to send messages to (all or single) group members. This is a null operation if already connected.

All channels with the same group address form a group, that means all messages sent to the group will be received by all channels connected to the same group address.

The group address is opaque, and can only be interpreted by a channel implementation, i.e. the implementation decides what to do with the object.

Throws: ChannelClosed

The channel is closed and therefore cannot be used any longer. A new channel has to be created first.

See Also:

Disconnect

 public abstract void Disconnect()

Disconnects the channel from the current group (if connected), leaving the group. It is a null operation if not connected. It is a null operation if the channel is closed.

See Also:

Connect

 public abstract void Close()

Destroys the channel and its associated resources (e.g. the protocol stack). After a channel has been closed, invoking methods on it throws the ChannelClosed exception (or results in a null operation). It is a null operation if the channel is already closed.

If the channel is connected to a group, Disconnect will be called first.

 public abstract void Send(Message msg) throws ChannelNotConnected, ChannelClosed

Sends a message to a destination. The message contains

1. a destination address (Object). A null address sends the message to all group members.
2. a source address. Can be left empty. Will be filled in by the protocol stack.
3. a byte buffer. The message contents.
4. several additional fields. They can be used by application programs (or patterns). E.g. a message ID, a oneway field which determines whether a response is expected etc.

Parameters:

msg - The message to be sent. Destination and buffer should be set. A null destination means to send to all group members.

Throws: ChannelNotConnected

The channel must be connected to send messages.

Throws: ChannelClosed

The channel is closed and therefore cannot be used any longer. A new channel has to be created first.

 public void Down(Event evt)

Access to event mechanism of channels. Enables to send and receive events, used by building blocks to communicate with (building block) specific protocol layers. Currently useful only with JChannel.

 public abstract Object Receive(long timeout) throws ChannelNotConnected, ChannelClosed, Timeout

Receives a message, a view change or a block event. By using SetOpt, the type of objects to be received can be determined (e.g. not views and blocks, just messages). The possible types returned can be:

1. Message. Normal message
2. Event. All other events (used by JChannel)
3. View. A view change.
4. BlockEvent. A block event indicating an impending view change.
5. SuspectEvent. A notification of a suspected member.
6. GetStateEvent. The current state of the application should be returned using ReturnState.
7. SetStateEvent. The state of a single/all members as requested previously by having called Channel.GetState(s).

The instanceof operator can be used to discriminate between different types returned.

Parameters:

timeout - Value in milliseconds. Value <= 0 means wait forever

Returns:

A Message, View, BlockEvent,SuspectEvent, GetStateEvent object or SetStateEvent, depending on what is on top of the internal queue.

Throws: ChannelNotConnected

The channel must be connected to receive messages.

Throws: ChannelClosed

The channel is closed and therefore cannot be used any longer. A new channel has to be created first.

Throws: Timeout

Thrown when a timeout has occurred.

 public abstract Object Peek(long timeout) throws ChannelNotConnected, ChannelClosed, Timeout

Returns the next message, view, block, suspect or other event without removing it from the queue.

Parameters:

timeout - Value in milliseconds. Value <= 0 means wait forever

Returns:

A Message, View, BlockEvent, SuspectEvent, GetStateEvent or SetStateEvent object, depending on what is on top of the internal queue.

Throws: ChannelNotConnected

The channel must be connected to receive messages.

Throws: ChannelClosed

The channel is closed and therefore cannot be used any longer. A new channel has to be created first.

Throws: Timeout

Thrown when a timeout has occurred.

See Also:

Receive

 public abstract View GetView()

Gets the current view. This does not retrieve a new view, use Receive to do so. The view may only be available after a successful Connect. The result of calling this method on an unconnected channel is implementation defined (may return null). Calling it on a channel that is not enabled to receive view events (via SetOpt) returns null. Calling this method on a closed channel returns a null view.

Returns:

The current view.

 public abstract Object GetLocalAddress()

Returns the channel's own address. The result of calling this method on an unconnected channel is implementation defined (may return null). Calling this method on a closed channel returns null.

Returns:

The channel's address. Generated by the underlying transport, and opaque. Addresses can be used as destination in the Send operation.

 public abstract Object GetGroupAddress()

Returns the group address of the group of which the channel is a member. This is the object that was the argument to Connect. Calling this method on a closed channel returns null.

Returns:

The group address

 public void SetUpHandler(UpHandler up_handler)

When up_handler is set, all events will be passed to it directly. These will not be received by the channel (except connect/disconnect, state retrieval and the like). This can be used by building blocks on top of a channel; thus the channel is used as a pass-through medium, and the building blocks take over some of the channel's tasks. However, tasks such as connection management and state transfer is still handled by the channel.

 public void SetChannelListener(ChannelListener channel_listener)

Allows to be notified when a channel event such as connect, disconnect or close occurs. E.g. a PullPushAdapter may choose to stop when the channel is closed, or to start when it is opened.

 public abstract void SetOpt(int option,
                             Object value)

Sets an option. The following options are currently recognized:

1. BLOCK (0). Turn the reception of BLOCK events on/off (value is Boolean). Default is off. If set to on, receiving VIEW events will be set to on, too.
2. VIEW (1). Turn the reception of VIEW events on/off (value is Boolean). Default is on.
3. SUSPECT (2). Turn the reception of SUSPECT events on/off (value is Boolean). Default is on.
4. LOCAL (3). Receive its own broadcast messages to the group (value is Boolean). Default is on.
5. GET_STATE_EVENTS (4). Turn the reception of GetState events on/off (value is Boolean). Default is off, which means that no other members can ask this member for its state (null will be returned).

This method can be called on an unconnected channel. Calling this method on a closed channel has no effect.

 public abstract Object GetOpt(int option)

Gets an option. This method can be called on an unconnected channel. Calling this method on a closed channel returns null.

Parameters:

option - The option to be returned.

Returns:

The object associated with an option.

 public abstract void BlockOk()

Called to acknowledge a Block (callback in MembershipListener or BlockEvent received from call to Receive). After sending BlockOk, no messages should be sent until a new view has been received. Calling this method on a closed channel has no effect.

 public abstract boolean GetState(Object target,
                                  long timeout)

Retrieve the state of the group. Will usually contact the oldest group member to get the state. When the method returns true, a SetStateEvent will have been added to the channel's queue, causing Receive to return the state in one of the next invocations. If false, no state will be retrieved by Receive.

Parameters:

target - The address of the member from which the state is to be retrieved. If it is null, the oldest member is contacted.

timeout - Milliseconds to wait for the response (0 = wait indefinitely).

Returns:

boolean True if the state was retrieved successfully, otherwise false.

 public abstract boolean GetAllStates(Vector targets,
                                      long timeout)

Retrieve all states of the group members. Will contact all group members to get the states. When the method returns true, a SetStateEvent will have been added to the channel's queue, causing Receive to return the states in one of the next invocations. If false, no states will be retrieved by Receive.

Parameters:

targets - A list of members which are contacted for states. If the list is null, all the current members of the group will be contacted.

timeout - Milliseconds to wait for the response (0 = wait indefinitely).

Returns:

boolean True if the state was retrieved successfully, otherwise false.

 public void ReturnState(Object state)

Called by the application is response to receiving a GetState object when calling Receive.

Parameters:

state - The state of the application as a Serializable or Externaizable (to send over the network).

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