Parallel Programming: Vector Clocks

Groups of nodes act together, can send messages (multicast) to a group, and the messages are received by all the nodes that are in the group (Sandén, 2011).  If there is a broadcast, all nodes in the system get the same message.  In a multicast, the messages can reach the nodes in a group in a different order: First In First Out order, casual order, or total order (atomic multicast if it is reliable).

Per Sandén (2011), a multicast can occur if the source is a member of the group, but it cannot span across groups in causal order. Two-phase, total order multicast systems can look like a vector clock but they are not, and each message sends or receive will increment on this system by one as they talk between the systems.

Below is an example of a vector clock:

To G1

  • m1 suggested time at 6 and  m2 suggested time at 11
  • m1 commit time at 7 and m2 commit time at 12

To G2

  • m1 suggested time at 7 and  m2 suggested time at 12
  • m1 commit time at 8 and m2 commit time at 13vectorclock