Clocks

Physical Clock count number of seconds elapsed. This clocks has problem with instrument measurement accuracy. Classical clocks use Quartz clocks with low accuracy (1 ppm = 1 microsecond/second = 86 ms/day = 32 s/year), atomic clocks (1 second in 3 million years) but costs ~ £ 20_000. For synchronizations use NTP Servers.

Logical clock count events, e.g. messages sent (relevant in distributed systems). This clocks requires communication between notes.

• 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.