5.2 Causal Ordering of Messages

DITRIBUTED SYSTEM NOTES,IOE,TU,BCA

CAUSAL ORDERING

Causal ordering of messages in distributed systems ensures that messages are delivered in an order that respects the causal relationships between events. This concept is crucial for maintaining consistency and coherence in systems where operations are dependent on the order of events. 

Definition

Causal ordering ensures that if an event AAA causally affects an event BBB, then AAA is delivered before BBB to all nodes in the system. This concept is based on the "happens-before" relationship defined by Lamport.

Happens-before Relationship

• Local Events: If event AAA occurs before event BBB on the same process, then A→BA.
• Message Sending and Receiving: If event A is the sending of a message and event B is the receipt of that message, then A→BA.
• Transitivity: If A→B  and B→C , then A→C .

Algorithms for Causal Ordering

Vector Clocks

Vector clocks are a common method used to implement causal ordering.