Package org.jgroups

Class Channel

java.lang.Object
org.jgroups.Channel
All Implemented Interfaces:
Transport
Direct Known Subclasses:
JChannel

public abstract class Channel extends Object implements Transport
A channel represents a group communication endpoint (like BSD datagram sockets). A client joins a group by connecting the channel to a group address and leaves it by disconnecting. Messages sent over the channel are received by all group members that are connected to the same group (that is, all members that have the same group address).

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:
  • Field Details

  • Constructor Details

    • Channel

      public Channel()
  • Method Details

    • getLog

      protected abstract org.apache.commons.logging.Log getLog()
    • connect

      public abstract void connect(String cluster_name) throws ChannelException
      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 started
      ChannelClosedException - The channel is closed and therefore cannot be used any longer. A new channel has to be created first.
      See Also:
    • 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 null
      timeout - Milliseconds to wait for the state response (0 = wait indefinitely).
      Throws:
      ChannelException - thrown if connecting to cluster was not successful
      StateTransferException - thrown if state transfer was not successful
    • 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:
    • 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 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, disconnec()t will be called first.

    • shutdown

      public abstract void shutdown()
      Shuts down the channel without disconnecting if connected, stops all the threads
    • open

      public void open() throws ChannelException
      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
    • 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 by receive(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.
    • dumpQueue

      public String dumpQueue()
    • dumpStats

      public abstract Map 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.
    • send

      public abstract void send(Message msg) throws ChannelNotConnectedException, ChannelClosedException
      Sends a message to a (unicast) destination. The message contains
      1. a destination address (Address). 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.
      Specified by:
      send in interface Transport
      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.
    • 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 members
      src - 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
    • down

      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.
    • downcall

      public Object downcall(Event evt)
      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:
    • receive

      public abstract Object receive(long timeout) throws ChannelNotConnectedException, ChannelClosedException, TimeoutException
      Deprecated.
      Use a Receiver instead
      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 that a flush protocol has been started, and we should not send any more messages. This event should be ack'ed by calling blockOk() . Any messages sent after blockOk() returns might get blocked until the flush protocol has completed.
      5. UnblockEvent. An unblock event indicating that the flush protocol has completed and we can resume sending messages
      6. SuspectEvent. A notification of a suspected member.
      7. GetStateEvent. The current state of the application should be returned using ReturnState.
      8. SetStateEvent. The state of a single/all members as requested previously by having called Channel.getState(s).
      9. 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.
      The instanceof operator can be used to discriminate between different types returned.
      Specified by:
      receive in interface Transport
      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.
    • peek

      public abstract Object peek(long timeout) throws ChannelNotConnectedException, ChannelClosedException, TimeoutException
      Deprecated.
      Use a Receiver instead, this method will not be available in JGroups 3.0
      Returns 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:
    • getView

      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.
    • getLocalAddress

      public abstract Address 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.
    • getChannelName

      public abstract String getChannelName()
      Deprecated.
      Use getClusterName() instead
      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
    • getClusterName

      public abstract String getClusterName()
      Returns the cluster name 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 cluster name
    • setUpHandler

      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.
    • setChannelListener

      public void setChannelListener(ChannelListener channel_listener)
      Deprecated.
      Use addChannelListener() instead
      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.
    • addChannelListener

      public void addChannelListener(ChannelListener 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.
    • removeChannelListener

      public void removeChannelListener(ChannelListener listener)
    • clearChannelListeners

      public void clearChannelListeners()
    • setReceiver

      public void setReceiver(Receiver r)
      Sets the receiver, which will handle all messages, view changes etc
    • getReceiver

      public Receiver getReceiver()
    • setOpt

      public abstract void setOpt(int option, Object value)
      Sets an option. The following options are currently recognized:
      1. BLOCK. Turn the reception of BLOCK events on/off (value is Boolean). Default is off
      2. LOCAL. Receive its own broadcast messages to the group (value is Boolean). Default is on.
      3. AUTO_RECONNECT. Turn auto-reconnection on/off. If on, when a member if forced out of a group (EXIT event), then we will reconnect.
      4. AUTO_GETSTATE. Turn automatic fetching of state after an auto-reconnect on/off. This also sets AUTO_RECONNECT to true (if not yet set).
      This method can be called on an unconnected channel. Calling this method on a closed channel has no effect.
    • getOpt

      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.
    • flushSupported

      public abstract boolean flushSupported()
    • startFlush

      public abstract boolean startFlush(List<Address> flushParticipants, boolean automatic_resume)
    • startFlush

      public abstract boolean startFlush(boolean automatic_resume)
    • startFlush

      public abstract boolean startFlush(long timeout, boolean automatic_resume)
    • stopFlush

      public abstract void stopFlush()
    • stopFlush

      public abstract void stopFlush(List<Address> flushParticipants)
    • blockOk

      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.
    • 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, 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 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.
    • 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
    • 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 member
      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.
      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.
    • returnState

      public abstract void returnState(byte[] state)
      Called by the application is response to receiving a getState() object when calling receive().
      Parameters:
      state - The state of the application as a byte buffer (to send over the network).
    • returnState

      public abstract void returnState(byte[] state, String state_id)
      Returns a given substate (state_id of null means return entire state)
    • getInfo

      public abstract Map<String,Object> getInfo()
    • setInfo

      public abstract void setInfo(String key, Object value)
    • option2String

      public static String option2String(int option)
    • notifyChannelConnected

      protected void notifyChannelConnected(Channel c)
    • notifyChannelDisconnected

      protected void notifyChannelDisconnected(Channel c)
    • notifyChannelClosed

      protected void notifyChannelClosed(Channel c)
    • notifyChannelShunned

      protected void notifyChannelShunned()
    • notifyChannelReconnected

      protected void notifyChannelReconnected(Address addr)