Class ReplicationMessage

java.lang.Object
org.apache.derby.impl.store.replication.net.ReplicationMessage
All Implemented Interfaces:
Externalizable, Serializable

public class ReplicationMessage extends Object implements Externalizable
This message is used for the communication between the master and the slave during Replication. The message is composed of a type flag and corresponding object. Each type flag indicating the type of the content bundled inside the message.
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  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    private Object
    This is the content of the replication message.
    static final long
    The version number is determined based on the database version of the current database.
    private int
    The valid values for the type of the content of the message are listed below.
    static final int
    This flag is used to send an acknowledgment of successful completion of a requested operation.
    static final int
    Indicates that the requested operation was not able to be completed successfully.
    static final int
    Used to signal the slave that it must failover.
    static final int
    Used during the intial handshake between the master and the slave.
    static final int
    used during the intial handshake between the master and the slave.
    static final int
    This flag will be used for all messages that carry log records.
    static final int
    A message used to check if the replication network is working.
    static final int
    A message sent as a response to a TYPE_PING message.
    static final int
    Used to send a stop replication signal to the slave.
  • Constructor Summary

    Constructors
    Constructor
    Description
    public No args constructor required with Externalizable.
    ReplicationMessage(int type, Object message)
    Constructor used to set the type and message.
  • Method Summary

    Modifier and Type
    Method
    Description
    Used to get the actual message that is wrapped inside the ReplicationMessage object.
    int
    Used to get the type of this ReplicationMessage.
    void
    Used to restore the contents of this object.
    void
    Used to save the contents of this Object.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
  • Field Details

    • serialVersionUID

      public static final long serialVersionUID
      The version number is determined based on the database version of the current database. This would help to establish if the master and the slave in replication are at compatible database versions.
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    • message

      private Object message
      This is the content of the replication message.
    • type

      private int type
      The valid values for the type of the content of the message are listed below.
    • TYPE_INITIATE_VERSION

      public static final int TYPE_INITIATE_VERSION
      used during the intial handshake between the master and the slave. The initial handshake helps to negotiate the message UIDs and send a appropriate error or acknowledgment. The object this message contains will be a Long. IMPORTANT: This constant must not be changed in future versions since we need it to decide slave/master version mismatch
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    • TYPE_INITIATE_INSTANT

      public static final int TYPE_INITIATE_INSTANT
      Used during the intial handshake between the master and the slave. Messages of this type are used to ensure that master and slave have identical log files by checking that they will insert the next log record on the same byte position in the log. The object this message contains will be a Long.
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    • TYPE_LOG

      public static final int TYPE_LOG
      This flag will be used for all messages that carry log records. The Object this message type contains will be a byte array. The content of the byte array is the log records in the binary form.
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    • TYPE_ACK

      public static final int TYPE_ACK
      This flag is used to send an acknowledgment of successful completion of a requested operation. It will however not be used to signify reception for every message transmission. The object this message type contains will be a String. The SQLState of the error message can be used here.
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    • TYPE_ERROR

      public static final int TYPE_ERROR
      Indicates that the requested operation was not able to be completed successfully. The object this message type contains will be a String[] where the first length-1 fields are used as arguments to create the exception and the last field contains the SQLState. The SQLState is the last element in the Array, because this means that the whole Array can be used as input to the StandardException creator. The SQLState will be ignored by the exception creator because there is one argument too many. This way we don't have to make a copy of the received Array, containing all elements except the SQLState element.
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    • TYPE_PING

      public static final int TYPE_PING
      A message used to check if the replication network is working. This message type has no object
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    • TYPE_PONG

      public static final int TYPE_PONG
      A message sent as a response to a TYPE_PING message. This message type has no object
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    • TYPE_STOP

      public static final int TYPE_STOP
      Used to send a stop replication signal to the slave. Since this is a control signal the object this message contains will be null.
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    • TYPE_FAILOVER

      public static final int TYPE_FAILOVER
      Used to signal the slave that it must failover. The object associated with this message will be null.
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  • Constructor Details

    • ReplicationMessage

      public ReplicationMessage()
      public No args constructor required with Externalizable.
    • ReplicationMessage

      public ReplicationMessage(int type, Object message)
      Constructor used to set the type and message. This is used while creating messages for sending while readExternal is used on the receiving end.
      Parameters:
      type - The type of this message. Must be one of the message type constants of this class (TYPE_LOG, TYPE_ACK, TYPE_ERROR, TYPE_INITIATE).
      message - The message to be transmitted.
  • Method Details

    • getMessage

      public Object getMessage()
      Used to get the actual message that is wrapped inside the ReplicationMessage object.
      Returns:
      The object contained in the message
    • getType

      public int getType()
      Used to get the type of this ReplicationMessage.
      Returns:
      The type of the message.
    • readExternal

      public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException
      Used to restore the contents of this object.
      Specified by:
      readExternal in interface Externalizable
      Parameters:
      in - the stream to read data from in order to restore the object.
      Throws:
      IOException - If an exception occurs while reading from the InputStream.
      ClassNotFoundException - Class of a serialized object cannot be found.
    • writeExternal

      public void writeExternal(ObjectOutput out) throws IOException
      Used to save the contents of this Object.
      Specified by:
      writeExternal in interface Externalizable
      Parameters:
      out - the stream to write the object to.
      Throws:
      IOException - if an exception occurs while writing to the OutputStream.