extended by org.jgroups.stack.Protocol
      extended by org.jgroups.protocols.CAUSAL

public class CAUSAL
extends Protocol

Implements casual ordering layer using vector clocks.

Causal protocol layer guarantees that if message m0 multicasted by a process group member p0 causes process group member p1 to multicast message m1 then all other remaining process group members in a current view will receive messages in order m0 followed by m1.

First time encountered, causal order seems very similar to FIFO order but there is an important distinction. While FIFO order gurantees that if process group member p0 multicasts m0 followed by m1 the messages will be delivered in order m0,m1 to all other group members, causal order expands this notion of an order from a single group member "space" to a whole group space i.e if p0 sends message m0 which causes member p1 to send message m1 then all other group members are guaranteed to receive m0 followed by m1.

Causal protocol layer achieves this ordering type by introducing sense of a time in a group using vector clocks. The idea is very simple. Each message is labeled by a vector, contained in a causal header, representing the number of prior causal messages received by the sending group member. Vector time of [3,5,2,4] in a group of four members [p0,p1,p2,p3] means that process p0 has sent 3 messages and has received 5,2 and 4 messages from a member p1,p2 and p3 respectively.

Each member increases its counter by 1 when it sends a message. When receiving message mi from a member pi , (where pi != pj) containing vector time VT(mi), process pj delays delivery of a message mi until:

for every k:1..n VT(mi)[k] == VT(pj)[k] + 1 if k=i, VT(mi)[k] <= VT(pj)[k] otherwise

After the next causal message is delivered at process group pj, VT(pj) is updated as follows:

for every k:1...n VT(pj)[k] == max(VT(mi)[k],VT(pj)[k])

Note that this protocol is experimental and has never been tested extensively !

$Id:,v 1.23 2009/09/06 13:51:07 belaban Exp $
Vladimir Blagojevic

Nested Class Summary
static class CAUSAL.CausalHeader
static class CAUSAL.CausalNewViewHeader
static class CAUSAL.MissingIndexesMessage
Field Summary
Fields inherited from class org.jgroups.stack.Protocol
down_prot, log, name, stack, stats, up_prot
Constructor Summary
          Default constructor.
Method Summary
 java.lang.Object down(Event evt)
          Process a downward event.
 java.lang.Object up(Event evt)
          Process an upward event.
Methods inherited from class org.jgroups.stack.Protocol
destroy, downThreadEnabled, dumpStats, enableStats, getConfigurableObjects, getDownProtocol, getLevel, getName, getProperties, getProtocolStack, getThreadFactory, getTransport, getUpProtocol, init, printStats, providedDownServices, providedUpServices, requiredDownServices, requiredUpServices, resetStats, setDownProtocol, setLevel, setProperties, setPropertiesInternal, setProperty, setProtocolStack, setUpProtocol, start, statsEnabled, stop, upThreadEnabled
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public CAUSAL()
Default constructor.

Method Detail


public java.lang.Object down(Event evt)
Process a downward event.

down in class Protocol
evt - The event.


public java.lang.Object up(Event evt)
Process an upward event.

up in class Protocol
evt - The event.

Copyright © 1998-2009 Bela Ban / Red Hat. All Rights Reserved.