Logical vs physical clock optimistic locking

Vlad Mihalcea's Blog

Introduction

In my previous post I demonstrated why optimistic locking is the only viable solution for application-level transactions. Optimistic locking requires a version column that can be represented as:

  • a physical clock (a timestamp value taken from the system clock)
  • a logical clock (an incrementing numeric value)

This article will demonstrate why logical clocks are better suited for optimistic locking mechanisms.

System time

The system time is provided by the operating system internal clocking algorithm. The programmable interval timer periodically sends an interrupt signal (with a frequency of 1.193182 MHz). The CPU receives the time interruption and increments a tick counter.

Both Unix and Window record time as the number of ticks since a predefined absolute time reference (an epoch). The operating system clock resolution varies from 1ms (Android) to 100ns (Windows) and to 1ns (Unix).

Monotonic time

To order events, the version must advance monotonically. While incrementing…

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