Clocks and Causality - Ordering Events in Distributed Systems

Original article is at Clocks and Causality - Ordering Events in Distributed Systems

In distributed systems, where independent nodes work together to solve problems through message-passing, maintaining a consistent global state is crucial. Clocks and causality play essential roles in ordering events to ensure the proper functioning of such systems. The article discusses three types of clocks: physical clocks, logical clocks, and hybrid clocks. Physical clocks, based on physical processes, offer a global notion of time but face challenges like clock drift and network latency. Logical clocks, based on event order, provide a partial order of events but not time duration between them. Hybrid clocks combine the advantages of both physical and logical clocks.

Causality, the relationship between cause and effect, is critical in distributed systems. The happens-before relationship and concurrent events are key concepts in understanding causality. Happens-before relationships establish event order, while concurrent events, having no causal relationship, can cause conflicts requiring resolution.

Lamport timestamps use logical clocks to assign unique identifiers to events in distributed systems, maintaining the happens-before relationship between events. Each process has a local counter incremented when an event occurs or a message is sent or received. Vector clocks, an extension of Lamport timestamps, provide more accurate partial ordering of events. They use an array of counters, one for each process, to track causal relationships between events.

In conclusion, clocks and causality help order events in distributed systems, with Lamport timestamps and vector clocks being notable methods to achieve this. Understanding and implementing these concepts is vital for maintaining a consistent global state in distributed systems, ensuring their proper functioning.