Logical clock

A logical clock is a mechanism for capturing chronological and causal relationships in a distributed system. Distributed systems may have no physically synchronous global clock, so a logical clock allows global ordering on events from different processes in such systems. The first implementation, the Lamport timestamps, was proposed by Leslie Lamport in 1978 (Turing Award in 2013).

Local vs global time

In logical clock systems each process has two data structures: logical local time and logical global time. Logical local time is used by the process to mark its own events, and logical global time is the local information about global time. A special protocol is used to update logical local time after each local event, and logical global time when processes exchange data.[1]

Applications

Logical clocks are useful in computation analysis, distributed algorithm design, individual event tracking, and exploring computational progress.

Algorithms

Some noteworthy logical clock algorithms are:

  • Lamport timestamps, which are monotonically increasing software counters.
  • Vector clocks, that allow for partial ordering of events in a distributed system.
  • Version vectors, order replicas, according to updates, in an optimistic replicated system.
  • Matrix clocks, an extension of vector clocks that also contains information about other processes' views of the system.
gollark: *FTP*? Seriously?
gollark: Depends how bored I was feeling.
gollark: Or ignore them.
gollark: If they did have that, I would give them something fun like my SSH honeypot server.
gollark: Yes, I would just ignore them.

References
  1. Chapter 3: Logical Time // Ajay Kshemkalyani and Mukesh Singhal, Distributed Computing: Principles, Algorithms, and Systems, Cambridge University Press, 2008
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