org.jgroups.protocols

## Class SMACK

- java.lang.Object
- - org.jgroups.stack.Protocol
  - - org.jgroups.protocols.SMACK

All Implemented Interfaces:
  AckMcastSenderWindow.RetransmitCommand

    public class SMACK
    extends Protocol
    implements AckMcastSenderWindow.RetransmitCommand

Simple Multicast ACK protocol. A positive acknowledgment-based protocol for reliable delivery of multicast messages, which does not need any group membership service. Basically works as follows:

- Sender S sends multicast message M
- When member P receives M, it sends back a unicast ack to S
- When S receives the ack from P, it checks whether P is in its membership list. If not, P will be added. This is necessary to retransmit the next message sent to P.
- When S sends a multicast message M, all members are added to a retransmission entry (containing all members to which the message was sent), which is added to a hashmap (keyed by seqno). Whenever an ack is received from receiver X, X will be removed from the retransmission list for the given seqno. When the retransmission list is empty, the seqno will be removed from the hashmap.
- A retransmitter thread in the sender periodically retransmits (either via unicast, or multicast) messages for which no ack has been received yet
- When a max number of (unsuccessful) retransmissions have been exceeded, all remaining members for that seqno are removed from the local membership, and the seqno is removed from the hashmap, ceasing all retransmissions

Advantage of this protocol: no group membership necessary, fast.

Author:
  Bela Ban Aug 2002
  Fix membershop bug: start a, b, kill b, restart b: b will be suspected by a.

### Nested Class Summary

Nested Classes
`static class` `SMACK.SmackHeader`

### Field Summary

### Fields inherited from class org.jgroups.stack.Protocol

`down_prot, ergonomics, id, log, name, stack, stats, up_prot`

### Constructor Summary

Constructors
Constructor and Description
`SMACK()`

### Method Summary

Methods
`java.lang.Object` `down(Event evt)` An event is to be sent down the stack.
`void` `retransmit(long seqno, Message msg, Address dest)` Retranmit the given msg
`void` `stop()` This method is called on a `Channel.disconnect()`.
`java.lang.Object` `up(Event evt)` An event was received from the layer below.

### Methods inherited from class org.jgroups.stack.Protocol

`destroy, downThreadEnabled, dumpStats, enableStats, getConfigurableObjects, getDownProtocol, getId, getLevel, getName, getProperties, getProtocolStack, getSocketFactory, getThreadFactory, getTransport, getUpProtocol, getValue, init, isErgonomics, printStats, providedDownServices, providedUpServices, requiredDownServices, requiredUpServices, resetStats, setDownProtocol, setErgonomics, setId, setLevel, setProperties, setPropertiesInternal, setProperty, setProtocolStack, setSocketFactory, setUpProtocol, setValue, setValues, start, statsEnabled, upThreadEnabled`

### Methods inherited from class java.lang.Object

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

### Constructor Detail

#### SMACK

    public SMACK()

### Method Detail

#### 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`

#### up

    public java.lang.Object up(Event evt)

Description copied from class: `Protocol`
An event was received from the layer below. Usually the current layer will want to examine the event type and - depending on its type - perform some computation (e.g. removing headers from a MSG event type, or updating the internal membership list when receiving a VIEW_CHANGE event). Finally the event is either a) discarded, or b) an event is sent down the stack using `down_prot.down()` or c) the event (or another event) is sent up the stack using `up_prot.up()`.

Overrides:
  `up` in class `Protocol`

#### down

    public java.lang.Object down(Event evt)

Description copied from class: `Protocol`
An event is to be sent down the stack. The layer may want to examine its type and perform some action on it, depending on the event's type. If the event is a message MSG, then the layer may need to add a header to it (or do nothing at all) before sending it down the stack using `down_prot.down()`. In case of a GET_ADDRESS event (which tries to retrieve the stack's address from one of the bottom layers), the layer may need to send a new response event back up the stack using `up_prot.up()`.

Overrides:
  `down` in class `Protocol`

#### retransmit

    public void retransmit(long seqno,
                  Message msg,
                  Address dest)

Description copied from interface: `AckMcastSenderWindow.RetransmitCommand`
Retranmit the given msg

Specified by:
  `retransmit` in interface `AckMcastSenderWindow.RetransmitCommand`
Parameters:
  `seqno` - the sequence number associated with the message
  `msg` - the msg to retransmit (it should be a copy!)
  `dest` - the msg destination