Logical Clocks
Practice this topic in a realistic system design interview
Physical clocks answer the question, "What time did this happen?"
Logical clocks answer a different question: "What could have caused this event?"
That difference is the whole reason logical clocks exist.
Many distributed-system bugs are not really about calendar time. They are about order.
For example:
- Did this read happen after the write it should see?
- Did this update know about the previous update?
- Did this service receive a message after the sender actually sent it?
Physical timestamps cannot answer those questions reliably by themselves, because machine clocks drift and disagree. Logical clocks give systems a way to reason about cause and effect without trusting perfectly synchronized clocks.
This chapter explains happens-before, concurrent events, causal order versus total order, the clock condition, and the main logical clock designs: Lamport timestamps, vector clocks, and hybrid logical clocks.