Channel (Apache Tomcat 8.5.57 API Documentation)

org.apache.catalina.tribes

## Interface Channel

- All Known Subinterfaces:
    ManagedChannel
  All Known Implementing Classes:
    GroupChannel

      public interface Channel

  Channel interface
  A channel is a representation of a group of nodes all participating in some sort of communication with each other.
  The channel is the main API class for Tribes, this is essentially the only class that an application needs to be aware of. Through the channel the application can:
  1. send messages
  2. receive message (by registering a `ChannelListener`
  3. get all members of the group `getMembers()`
  4. receive notifications of members added and members disappeared by registering a `MembershipListener`
  The channel has 5 major components:
  1. Data receiver, with a built in thread pool to receive messages from other peers
  2. Data sender, an implementation for sending data using NIO or java.io
  3. Membership listener,listens for membership broadcasts
  4. Membership broadcaster, broadcasts membership pings.
  5. Channel interceptors, the ability to manipulate messages as they are sent or arrive
  The channel layout is:

        ChannelListener_1..ChannelListener_N MembershipListener_1..MembershipListener_N [Application Layer]
                  \          \                  /                   /
                   \          \                /                   /
                    \          \              /                   /
                     \          \            /                   /
                      \          \          /                   /
                       \          \        /                   /
                        ---------------------------------------
                                        |
                                        |
                                     Channel
                                        |
                               ChannelInterceptor_1
                                        |                                               [Channel stack]
                               ChannelInterceptor_N
                                        |
                                   Coordinator (implements MessageListener,MembershipListener,ChannelInterceptor)
                                --------------------
                               /        |           \
                              /         |            \
                             /          |             \
                            /           |              \
                           /            |               \
                 MembershipService ChannelSender ChannelReceiver                        [IO layer]

  For example usage @see org.apache.catalina.tribes.group.GroupChannel

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `DEFAULT` Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel DEFAULT - starts or stops all components in the channel
    `static int` | `MBR_RX_SEQ` Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel MBR_RX_SEQ - starts or stops the membership listener.
    `static int` | `MBR_TX_SEQ` Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel MBR_TX_SEQ - starts or stops the membership broadcaster.
    `static int` | `SEND_OPTIONS_ASYNCHRONOUS` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SEND_OPTIONS_BYTE_MESSAGE` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SEND_OPTIONS_DEFAULT` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SEND_OPTIONS_MULTICAST` Send options.
    `static int` | `SEND_OPTIONS_SECURE` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SEND_OPTIONS_SYNCHRONIZED_ACK` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SEND_OPTIONS_UDP` Send options.
    `static int` | `SEND_OPTIONS_USE_ACK` Send options, when a message is sent, it can have an option flag to trigger certain behavior.
    `static int` | `SND_RX_SEQ` Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel SND_RX_SEQ - starts or stops the data receiver.
    `static int` | `SND_TX_SEQ` Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel SND_TX_SEQ - starts or stops the data sender.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addChannelListener(ChannelListener listener)` Add a channel listener, this is a callback object when messages are received If the channel listener implements the Heartbeat interface the `heartbeat()` method will be invoked when the heartbeat runs on the channel
    `void` | `addInterceptor(ChannelInterceptor interceptor)` Adds an interceptor to the channel message chain.
    `void` | `addMembershipListener(MembershipListener listener)` Add a membership listener, will get notified when a new member joins, leaves or crashes If the membership listener implements the Heartbeat interface the `heartbeat()` method will be invoked when the heartbeat runs on the channel
    `Member` | `getLocalMember(boolean incAlive)` Return the member that represents this node.
    `Member` | `getMember(Member mbr)` Returns the member from the membership service with complete and recent data.
    `Member[]` | `getMembers()` Get all current group members
    `String` | `getName()` Return the name of this channel.
    `boolean` | `hasMembers()` Returns true if there are any members in the group, this call is the same as `getMembers().length > 0`
    `void` | `heartbeat()` Sends a heart beat through the interceptor stacks Use this method to alert interceptors and other components to clean up garbage, timed out messages etc.
    `void` | `removeChannelListener(ChannelListener listener)` remove a channel listener, listeners are removed based on Object.hashCode and Object.equals
    `void` | `removeMembershipListener(MembershipListener listener)` remove a membership listener, listeners are removed based on Object.hashCode and Object.equals
    `UniqueId` | `send(Member[] destination, Serializable msg, int options)` Send a message to one or more members in the cluster
    `UniqueId` | `send(Member[] destination, Serializable msg, int options, ErrorHandler handler)` Send a message to one or more members in the cluster
    `void` | `setHeartbeat(boolean enable)` Enables or disables internal heartbeat.
    `void` | `setName(String name)` Set the name of this channel
    `void` | `start(int svc)` Starts up the channel.
    `void` | `stop(int svc)` Shuts down the channel.

- - ### Field Detail

    - #### DEFAULT

          static final int DEFAULT

      Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel
      DEFAULT - starts or stops all components in the channel
      See Also:
        `start(int)`, `stop(int)`, Constant Field Values

    - #### SND_RX_SEQ

          static final int SND_RX_SEQ

      Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel
      SND_RX_SEQ - starts or stops the data receiver. Start means opening a server socket in case of a TCP implementation
      See Also:
        `start(int)`, `stop(int)`, Constant Field Values

    - #### SND_TX_SEQ

          static final int SND_TX_SEQ

      Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel
      SND_TX_SEQ - starts or stops the data sender. This should not open any sockets, as sockets are opened on demand when a message is being sent
      See Also:
        `start(int)`, `stop(int)`, Constant Field Values

    - #### MBR_RX_SEQ

          static final int MBR_RX_SEQ

      Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel
      MBR_RX_SEQ - starts or stops the membership listener. In a multicast implementation this will open a datagram socket and join a group and listen for membership messages members joining
      See Also:
        `start(int)`, `stop(int)`, Constant Field Values

    - #### MBR_TX_SEQ

          static final int MBR_TX_SEQ

      Start and stop sequences can be controlled by these constants This allows you to start separate components of the channel
      MBR_TX_SEQ - starts or stops the membership broadcaster. In a multicast implementation this will open a datagram socket and join a group and broadcast the local member information
      See Also:
        `start(int)`, `stop(int)`, Constant Field Values

    - #### SEND_OPTIONS_BYTE_MESSAGE

          static final int SEND_OPTIONS_BYTE_MESSAGE

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_BYTE_MESSAGE - The message is a pure byte message and no marshaling or unmarshaling will be performed.
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_USE_ACK

          static final int SEND_OPTIONS_USE_ACK

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_USE_ACK - Message is sent and an ACK is received when the message has been received by the recipient
      If no ack is received, the message is not considered successful
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_SYNCHRONIZED_ACK

          static final int SEND_OPTIONS_SYNCHRONIZED_ACK

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_SYNCHRONIZED_ACK - Message is sent and an ACK is received when the message has been received and processed by the recipient
      If no ack is received, the message is not considered successful
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_ASYNCHRONOUS

          static final int SEND_OPTIONS_ASYNCHRONOUS

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_ASYNCHRONOUS - Message is sent and an ACK is received when the message has been received and processed by the recipient
      If no ack is received, the message is not considered successful
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_SECURE

          static final int SEND_OPTIONS_SECURE

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_SECURE - Message is sent over an encrypted channel
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_UDP

          static final int SEND_OPTIONS_UDP

      Send options. When a message is sent with this flag on the system sends the message using UDP instead of TCP
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_MULTICAST

          static final int SEND_OPTIONS_MULTICAST

      Send options. When a message is sent with this flag on the system sends a UDP message on the Multicast address instead of UDP or TCP to individual addresses
      See Also:
        `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

    - #### SEND_OPTIONS_DEFAULT

          static final int SEND_OPTIONS_DEFAULT

      Send options, when a message is sent, it can have an option flag to trigger certain behavior. Most flags are used to trigger channel interceptors as the message passes through the channel stack.
      However, there are five default flags that every channel implementation must implement
      SEND_OPTIONS_DEFAULT - the default sending options, just a helper variable.
      The default is `int SEND_OPTIONS_DEFAULT = SEND_OPTIONS_USE_ACK;`
      See Also:
        `SEND_OPTIONS_USE_ACK`, `send(Member[], Serializable , int)`, `send(Member[], Serializable, int, ErrorHandler)`, Constant Field Values

  - ### Method Detail

    - #### addInterceptor

          void addInterceptor(ChannelInterceptor interceptor)

      Adds an interceptor to the channel message chain.
      Parameters:
        `interceptor` - ChannelInterceptor

    - #### start

          void start(int svc)
                     throws ChannelException

      Starts up the channel. This can be called multiple times for individual services to start The svc parameter can be the logical or value of any constants
      Parameters:
        `svc` - int value of
        DEFAULT - will start all services
        MBR_RX_SEQ - starts the membership receiver
        MBR_TX_SEQ - starts the membership broadcaster
        SND_TX_SEQ - starts the replication transmitter
        SND_RX_SEQ - starts the replication receiver
        Note: In order for the membership broadcaster to transmit the correct information, it has to be started after the replication receiver.
      Throws:
        `ChannelException` - if a startup error occurs or the service is already started or an error occurs.

    - #### stop

          void stop(int svc)
                    throws ChannelException

      Shuts down the channel. This can be called multiple times for individual services to shutdown The svc parameter can be the logical or value of any constants
      Parameters:
        `svc` - int value of
        DEFAULT - will shutdown all services
        MBR_RX_SEQ - stops the membership receiver
        MBR_TX_SEQ - stops the membership broadcaster
        SND_TX_SEQ - stops the replication transmitter
        SND_RX_SEQ - stops the replication receiver
      Throws:
        `ChannelException` - if a startup error occurs or the service is already stopped or an error occurs.

    - #### send

          UniqueId send(Member[] destination,
                      Serializable msg,
                      int options)
                        throws ChannelException

      Send a message to one or more members in the cluster
      Parameters:
        `destination` - Member[] - the destinations, cannot be null or zero length, the reason for that is that a membership change can occur and at that time the application is uncertain what group the message actually got sent to.
        `msg` - Serializable - the message to send, has to be serializable, or a `ByteMessage` to send a pure byte array
        `options` - int - sender options, see class documentation for each interceptor that is configured in order to trigger interceptors
      Returns:
        a unique Id that identifies the message that is sent
      Throws:
        `ChannelException` - if a serialization error happens.
      See Also:
        `ByteMessage`, `SEND_OPTIONS_USE_ACK`, `SEND_OPTIONS_ASYNCHRONOUS`, `SEND_OPTIONS_SYNCHRONIZED_ACK`

    - #### send

          UniqueId send(Member[] destination,
                      Serializable msg,
                      int options,
                      ErrorHandler handler)
                        throws ChannelException

      Send a message to one or more members in the cluster
      Parameters:
        `destination` - Member[] - the destinations, null or zero length means all
        `msg` - ClusterMessage - the message to send
        `options` - int - sender options, see class documentation
        `handler` - ErrorHandler - handle errors through a callback, rather than throw it
      Returns:
        a unique Id that identifies the message that is sent
      Throws:
        `ChannelException` - - if a serialization error happens.

    - #### heartbeat

          void heartbeat()

      Sends a heart beat through the interceptor stacks Use this method to alert interceptors and other components to clean up garbage, timed out messages etc.
      If you application has a background thread, then you can save one thread, by configuring your channel to not use an internal heartbeat thread and invoking this method.
      See Also:
        `setHeartbeat(boolean)`

    - #### setHeartbeat

          void setHeartbeat(boolean enable)

      Enables or disables internal heartbeat.
      Parameters:
        `enable` - boolean - default value is implementation specific
      See Also:
        `heartbeat()`

    - #### addMembershipListener

          void addMembershipListener(MembershipListener listener)

      Add a membership listener, will get notified when a new member joins, leaves or crashes
      If the membership listener implements the Heartbeat interface the `heartbeat()` method will be invoked when the heartbeat runs on the channel
      Parameters:
        `listener` - MembershipListener
      See Also:
        `MembershipListener`

    - #### addChannelListener

          void addChannelListener(ChannelListener listener)

      Add a channel listener, this is a callback object when messages are received
      If the channel listener implements the Heartbeat interface the `heartbeat()` method will be invoked when the heartbeat runs on the channel
      Parameters:
        `listener` - ChannelListener
      See Also:
        `ChannelListener`, `Heartbeat`

    - #### removeMembershipListener

          void removeMembershipListener(MembershipListener listener)

      remove a membership listener, listeners are removed based on Object.hashCode and Object.equals
      Parameters:
        `listener` - MembershipListener
      See Also:
        `MembershipListener`

    - #### removeChannelListener

          void removeChannelListener(ChannelListener listener)

      remove a channel listener, listeners are removed based on Object.hashCode and Object.equals
      Parameters:
        `listener` - ChannelListener
      See Also:
        `ChannelListener`

    - #### hasMembers

          boolean hasMembers()

      Returns true if there are any members in the group, this call is the same as `getMembers().length > 0`
      Returns:
        boolean - true if there are any members automatically discovered

    - #### getMembers

          Member[] getMembers()

      Get all current group members
      Returns:
        all members or empty array, never null

    - #### getLocalMember

          Member getLocalMember(boolean incAlive)

      Return the member that represents this node. This is also the data that gets broadcasted through the membership broadcaster component
      Parameters:
        `incAlive` - - optimization, true if you want it to calculate alive time since the membership service started.
      Returns:
        Member

    - #### getMember

          Member getMember(Member mbr)

      Returns the member from the membership service with complete and recent data. Some implementations might serialize and send membership information along with a message, and instead of sending complete membership details, only send the primary identifier for the member but not the payload or other information. When such message is received the application can retrieve the cached member through this call.
      In most cases, this is not necessary.
      Parameters:
        `mbr` - Member
      Returns:
        Member

    - #### getName

          String getName()

      Return the name of this channel.
      Returns:
        channel name

    - #### setName

          void setName(String name)

      Set the name of this channel
      Parameters:
        `name` - The new channel name

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