org.javagroups.protocols
Class NAKACK

java.lang.Object
  |
  +--org.javagroups.stack.Protocol
        |
        +--org.javagroups.protocols.NAKACK

public class NAKACK

extends Protocol

Negative AcKnowledgement layer (NAKs), paired with positive ACKs. The default is to send a message using NAKs: the sender sends messages with monotonically increasing seqnos, receiver requests retransmissions of missing messages (gaps). When a SWITCH_NAK_ACK event is received, the mode is switched to using NAK_ACKS: the sender still uses monotonically increasing seqnos, but the receiver acknowledges every message. NAK and NAK_ACK seqnos are the same, when switching the mode, the current seqno is reused. Both NAK and NAK_ACK messages use the current view ID in which the message is sent to queue messages destined for an upcoming view, or discard messages sent in a previous view. Both modes reset their seqnos to 0 when receiving a view change. The NAK_ACK scheme is used for broadcasting view changes.

The third mode is for out-of-band control messages (activated by SWITCH_OUT_OF_BAND): this mode does neither employ view IDs, nor does it use the same seqnos as NAK and NAK_ACK. It uses its own seqnos, unrelated to the ones used by NAK and NAK_ACK, and never resets them. In combination with the sender's address, this makes every out-of-band message unique. Out-of-band messages are used for example for broadcasting FLUSH messages.

Once a mode is set, it remains in effect until exactly 1 message has been sent, afterwards the default mode NAK is used again. The following communication between 2 peers exists (left side is initiator, right side receiver):

   send_out_of_band
   -------------->       synchronous     (1)
   <-------------
        ack


      send_nak
   -------------->       asynchronous    (2)


    send_nak_ack
   -------------->       synchronous     (3)
   <--------------
        ack


      retransmit
   <--------------       asynchronous    (4)


   

When a message is sent, it will contain a header describing the type of the message, and containing additional data, such as sequence number etc. When a message is received, it is fed into either the OutOfBander or NAKer, depending on the header's type.

Note that in the synchronous modes, ACKs are sent for each request. If a reliable unicast protocol layer exists somewhere underneath this layer, then even the ACKs are transmitted reliably, thus increasing the number of messages exchanged. However, since it is envisaged that ACK/OUT_OF_BAND are not used frequently, this problem is currently not addressed.

Author:

Bela Ban

Field Summary

Fields inherited from class org.javagroups.stack.Protocol

down_handler, down_prot, down_queue, down_thread, down_thread_prio, observer, props, stack, up_handler, up_prot, up_queue, up_thread, up_thread_prio

Constructor Summary

NAKACK()

Method Summary

void	down(Event evt) Callback.
java.lang.String	getName()
void	init() Do some initial tasks
java.util.Vector	providedDownServices() List of events that are provided to layers below (they will be handled when sent down from below).
java.util.Vector	providedUpServices() List of events that are provided to layers above (they will be handled when sent down from above).
boolean	setProperties(java.util.Properties props) Configures the protocol initially.
void	stop() This method is called on a Channel.disconnect().
void	up(Event evt) Callback.

Methods inherited from class org.javagroups.stack.Protocol

destroy, getDownProtocol, getDownQueue, getProperties, getUpProtocol, getUpQueue, passDown, passUp, receiveDownEvent, receiveUpEvent, requiredDownServices, requiredUpServices, setDownProtocol, setObserver, setPropertiesInternal, setProtocolStack, setUpProtocol, start, startDownHandler, startUpHandler, stopInternal

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

NAKACK

public NAKACK()

Method Detail

init

public void init()
          throws java.lang.Exception

Do some initial tasks

Overrides:

init in class Protocol

Throws:

java.lang.Exception - Thrown if protocol cannot be initialized successfully. This will cause the ProtocolStack to fail, so the channel constructor will throw an exception

stop

public void stop()

Description copied from class: Protocol

This method is called on a Channel.disconnect(). Stops work (e.g. by closing multicast socket). Will be called from top to bottom. This means that at the time of the method invocation the neighbor protocol below is still working. This method will replace the STOP, STOP_OK, CLEANUP and CLEANUP_OK events. The ProtocolStack guarantees that when this method is called all messages in the down queue will have been flushed

Overrides:

stop in class Protocol

getName

public java.lang.String getName()

Specified by:

getName in class Protocol

providedUpServices

public java.util.Vector providedUpServices()

Description copied from class: Protocol

List of events that are provided to layers above (they will be handled when sent down from above).

Overrides:

providedUpServices in class Protocol

Returns:

Vector (of Integers)

providedDownServices

public java.util.Vector providedDownServices()

Description copied from class: Protocol

List of events that are provided to layers below (they will be handled when sent down from below).

Overrides:

providedDownServices in class Protocol

Returns:

Vector (of Integers)

up

public void up(Event evt)

Callback. Called by superclass when event may be handled.

Do not use passUp() in this method as the event is passed up by default by the superclass after this method returns !

Overrides:

up in class Protocol

Returns:

boolean Defaults to true. If false, event will not be passed up the stack.

down

public void down(Event evt)

Callback. Called by superclass when event may be handled.

Do not use passDown in this method as the event is passed down by default by the superclass after this method returns !

Overrides:

down in class Protocol

setProperties

public boolean setProperties(java.util.Properties props)

Description copied from class: Protocol

Configures the protocol initially. A configuration string consists of name=value items, separated by a ';' (semicolon), e.g.:

 "loopback=false;unicast_inport=4444"
 

Overrides:

setProperties in class Protocol