Class Channel
- All Implemented Interfaces:
Transport
- Direct Known Subclasses:
JChannel
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:
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Field Summary
FieldsModifier and TypeFieldDescriptionstatic final int
static final int
static final int
protected Set
<ChannelListener> static final int
static final int
protected Receiver
static final int
protected UpHandler
static final int
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Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionvoid
addChannelListener
(ChannelListener listener) Allows to be notified when a channel event such as connect, disconnect or close occurs.abstract void
blockOk()
Called to acknowledge a block() (callback inMembershipListener
orBlockEvent
received from call toReceive
).void
abstract void
close()
Destroys the channel and its associated resources (e.g., the protocol stack).abstract void
Connects the channel to a group.abstract void
Connects the channel to a group and fetches the stateabstract void
Disconnects the channel from the current group (if connected), leaving the group.void
Access to event mechanism of channels.Can be used instead of down() when a return value is expected.abstract Map
Returns a map of statistics of the various protocols and of the channel itself.abstract boolean
abstract boolean
getAllStates
(Vector targets, long timeout) Deprecated.Not really needed - we always want to get the state from a single memberabstract String
Deprecated.UsegetClusterName()
insteadabstract String
Returns the cluster name of the group of which the channel is a member.getInfo()
abstract Address
Returns the channel's own address.protected abstract org.apache.commons.logging.Log
getLog()
int
Returns the number of messages that are waiting.abstract Object
getOpt
(int option) Gets an option.abstract boolean
Retrieve the state of the group.abstract boolean
Fetches a partial state identified by state_id.abstract View
getView()
Gets the current view.abstract boolean
Determines whether the channel is connected to a group.abstract boolean
isOpen()
Determines whether the channel is open; i.e., the protocol stack has been created (may not be connected though).protected void
protected void
protected void
protected void
protected void
void
open()
Re-opens a closed channel.static String
option2String
(int option) abstract Object
peek
(long timeout) Deprecated.Use aReceiver
instead, this method will not be available in JGroups 3.0abstract Object
receive
(long timeout) Deprecated.Use aReceiver
insteadvoid
removeChannelListener
(ChannelListener listener) abstract void
returnState
(byte[] state) Called by the application is response to receiving agetState()
object when callingreceive()
.abstract void
returnState
(byte[] state, String state_id) Returns a given substate (state_id of null means return entire state)abstract void
send
(Address dst, Address src, Serializable obj) Helper method.abstract void
Sends a message to a (unicast) destination.void
setChannelListener
(ChannelListener channel_listener) Deprecated.Use addChannelListener() insteadabstract void
abstract void
Sets an option.void
Sets the receiver, which will handle all messages, view changes etcvoid
setUpHandler
(UpHandler up_handler) When up_handler is set, all events will be passed to it directly.abstract void
shutdown()
Shuts down the channel without disconnecting if connected, stops all the threadsabstract boolean
startFlush
(boolean automatic_resume) abstract boolean
startFlush
(long timeout, boolean automatic_resume) abstract boolean
startFlush
(List<Address> flushParticipants, boolean automatic_resume) abstract void
abstract void
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Field Details
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BLOCK
public static final int BLOCK- See Also:
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VIEW
public static final int VIEW- See Also:
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SUSPECT
public static final int SUSPECT- See Also:
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LOCAL
public static final int LOCAL- See Also:
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GET_STATE_EVENTS
public static final int GET_STATE_EVENTS- See Also:
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AUTO_RECONNECT
public static final int AUTO_RECONNECT- See Also:
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AUTO_GETSTATE
public static final int AUTO_GETSTATE- See Also:
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up_handler
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channel_listeners
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receiver
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Constructor Details
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Channel
public Channel()
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Method Details
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getLog
protected abstract org.apache.commons.logging.Log getLog() -
connect
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 name form a group, that means all messages sent to the group will be received by all channels connected to the same channel name.
- Parameters:
cluster_name
- The name of the chanel to connect to.- Throws:
ChannelException
- The protocol stack cannot be startedChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.- See Also:
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connect
public abstract void connect(String cluster_name, Address target, String state_id, long timeout) throws ChannelException Connects the channel to a group and fetches the state- Parameters:
cluster_name
- The name of the cluster to connect to.target
- The address of the member from which the state is to be retrieved. If it is null, the state is retrieved from coordinator is contacted.state_id
- The ID of a substate. If the full state is to be fetched, set this parameter to nulltimeout
- Milliseconds to wait for the state response (0 = wait indefinitely).- Throws:
ChannelException
- thrown if connecting to cluster was not successfulStateTransferException
- thrown if state transfer was not successful
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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:
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close
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 theChannelClosed
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,
disconnec()t
will be called first. -
shutdown
public abstract void shutdown()Shuts down the channel without disconnecting if connected, stops all the threads -
open
Re-opens a closed channel. Throws an exception if the channel is already open. After this method returns, connect() may be called to join a group. The address of this member will be different from the previous incarnation.- Throws:
ChannelException
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isOpen
public abstract boolean isOpen()Determines whether the channel is open; i.e., the protocol stack has been created (may not be connected though). -
isConnected
public abstract boolean isConnected()Determines whether the channel is connected to a group. This implies it is open. If true is returned, then the channel can be used to send and receive messages. -
getNumMessages
public int getNumMessages()Returns the number of messages that are waiting. Those messages can be removed byreceive(long)
. Note that this number could change after calling this method and before calling receive() (e.g. the latter method might be called by a different thread).- Returns:
- The number of messages on the queue, or -1 if the queue/channel is closed/disconnected.
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dumpQueue
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dumpStats
Returns a map of statistics of the various protocols and of the channel itself.- Returns:
- Mapinvalid input: '<'String,Map>. A map where the keys are the protocols ("channel" pseudo key is used for the channel itself") and the values are property maps.
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send
Sends a message to a (unicast) destination. The message contains- a destination address (Address). A
null
address sends the message to all group members. - a source address. Can be left empty. Will be filled in by the protocol stack.
- a byte buffer. The message contents.
- 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.
- Specified by:
send
in interfaceTransport
- Parameters:
msg
- The message to be sent. Destination and buffer should be set. A null destination means to send to all group members.- Throws:
ChannelNotConnectedException
- The channel must be connected to send messages.ChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.
- a destination address (Address). A
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send
public abstract void send(Address dst, Address src, Serializable obj) throws ChannelNotConnectedException, ChannelClosedException Helper method. Will create a Message(dst, src, obj) and use send(Message).- Parameters:
dst
- Destination address for message. If null, message will be sent to all current group memberssrc
- Source (sender's) address. If null, it will be set by the protocol's transport layer before being put on the wire. Can usually be set to null.obj
- Serializable object. Will be serialized into the byte buffer of the Message. If it is not serializable, the byte buffer will be null.- Throws:
ChannelNotConnectedException
ChannelClosedException
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down
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. -
downcall
Can be used instead of down() when a return value is expected. This will be removed in 3.0 when we change the signature of down() to return Object rather than void- Parameters:
evt
-- Returns:
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receive
public abstract Object receive(long timeout) throws ChannelNotConnectedException, ChannelClosedException, TimeoutException Deprecated.Use aReceiver
insteadReceives a message, a view change or a block event. By usingsetOpt
, the type of objects to be received can be determined (e.g., not views and blocks, just messages). The possible types returned can be:Message
. Normal messageEvent
. All other events (used by JChannel)View
. A view change.BlockEvent
. A block event indicating that a flush protocol has been started, and we should not send any more messages. This event should be ack'ed by callingblockOk()
. Any messages sent after blockOk() returns might get blocked until the flush protocol has completed.UnblockEvent
. An unblock event indicating that the flush protocol has completed and we can resume sending messagesSuspectEvent
. A notification of a suspected member.GetStateEvent
. The current state of the application should be returned usingReturnState
.SetStateEvent
. The state of a single/all members as requested previously by having calledChannel.getState(s).
ExitEvent
. Signals that this member was forced to leave the group (e.g., caused by the member being suspected.) The member can rejoin the group by calling open(). If the AUTO_RECONNECT is set (see setOpt()), the reconnect will be done automatically.
instanceof
operator can be used to discriminate between different types returned.- Specified by:
receive
in interfaceTransport
- Parameters:
timeout
- Value in milliseconds. Value invalid input: '<'= 0 means wait forever- Returns:
- A Message, View, BlockEvent, SuspectEvent, GetStateEvent, SetStateEvent or ExitEvent, depending on what is on top of the internal queue.
- Throws:
ChannelNotConnectedException
- The channel must be connected to receive messages.ChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.TimeoutException
- Thrown when a timeout has occurred.
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peek
public abstract Object peek(long timeout) throws ChannelNotConnectedException, ChannelClosedException, TimeoutException Deprecated.Use aReceiver
instead, this method will not be available in JGroups 3.0Returns the next message, view, block, suspect or other event without removing it from the queue.- Parameters:
timeout
- Value in milliseconds. Value invalid input: '<'= 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:
ChannelNotConnectedException
- The channel must be connected to receive messages.ChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.TimeoutException
- Thrown when a timeout has occurred.- See Also:
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getView
Gets the current view. This does not retrieve a new view, usereceive()
to do so. The view may only be available after a successfulconnect()
. 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 (viasetOpt
) returnsnull
. Calling this method on a closed channel returns a null view.- Returns:
- The current view.
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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.
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getChannelName
Deprecated.UsegetClusterName()
insteadReturns the group address of the group of which the channel is a member. This is the object that was the argument toconnect()
. Calling this method on a closed channel returnsnull
.- Returns:
- The group address
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getClusterName
Returns the cluster name of the group of which the channel is a member. This is the object that was the argument toconnect()
. Calling this method on a closed channel returnsnull
.- Returns:
- The cluster name
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setUpHandler
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. -
setChannelListener
Deprecated.Use addChannelListener() insteadAllows 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. -
addChannelListener
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. -
removeChannelListener
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clearChannelListeners
public void clearChannelListeners() -
setReceiver
Sets the receiver, which will handle all messages, view changes etc -
getReceiver
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setOpt
Sets an option. The following options are currently recognized:BLOCK
. Turn the reception of BLOCK events on/off (value is Boolean). Default is offLOCAL
. Receive its own broadcast messages to the group (value is Boolean). Default is on.AUTO_RECONNECT
. Turn auto-reconnection on/off. If on, when a member if forced out of a group (EXIT event), then we will reconnect.AUTO_GETSTATE
. Turn automatic fetching of state after an auto-reconnect on/off. This also sets AUTO_RECONNECT to true (if not yet set).
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getOpt
Gets an option. This method can be called on an unconnected channel. Calling this method on a closed channel returnsnull
.- Parameters:
option
- The option to be returned.- Returns:
- The object associated with an option.
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flushSupported
public abstract boolean flushSupported() -
startFlush
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startFlush
public abstract boolean startFlush(boolean automatic_resume) -
startFlush
public abstract boolean startFlush(long timeout, boolean automatic_resume) -
stopFlush
public abstract void stopFlush() -
stopFlush
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blockOk
public abstract void blockOk()Called to acknowledge a block() (callback inMembershipListener
orBlockEvent
received from call toReceive
). 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. -
getState
public abstract boolean getState(Address target, long timeout) throws ChannelNotConnectedException, ChannelClosedException Retrieve the state of the group. Will usually contact the oldest group member to get the state. When the method returns true, aSetStateEvent
will have been added to the channel's queue, causingreceive()
to return the state in one of the next invocations. If false, no state will be retrieved byreceive()
.- Parameters:
target
- The address of the member from which the state is to be retrieved. If it is null, the coordinator is contacted.timeout
- Milliseconds to wait for the response (0 = wait indefinitely).- Returns:
- boolean True if the state was retrieved successfully, otherwise false.
- Throws:
ChannelNotConnectedException
- The channel must be connected to receive messages.ChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.
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getState
public abstract boolean getState(Address target, String state_id, long timeout) throws ChannelNotConnectedException, ChannelClosedException Fetches a partial state identified by state_id.- Parameters:
target
-state_id
-timeout
-- Returns:
- Throws:
ChannelNotConnectedException
ChannelClosedException
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getAllStates
public abstract boolean getAllStates(Vector targets, long timeout) throws ChannelNotConnectedException, ChannelClosedException Deprecated.Not really needed - we always want to get the state from a single memberRetrieve all states of the group members. Will contact all group members to get the states. When the method returns true, aSetStateEvent
will have been added to the channel's queue, causingReceive
to return the states in one of the next invocations. If false, no states will be retrieved byReceive
.- 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.
- Throws:
ChannelNotConnectedException
- The channel must be connected to receive messages.ChannelClosedException
- The channel is closed and therefore cannot be used any longer. A new channel has to be created first.
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returnState
public abstract void returnState(byte[] state) Called by the application is response to receiving agetState()
object when callingreceive()
.- Parameters:
state
- The state of the application as a byte buffer (to send over the network).
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returnState
Returns a given substate (state_id of null means return entire state) -
getInfo
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setInfo
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option2String
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notifyChannelConnected
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notifyChannelDisconnected
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notifyChannelClosed
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notifyChannelShunned
protected void notifyChannelShunned() -
notifyChannelReconnected
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