Clocks

tags : Representing Time and Date, Distributed Systems, crdt, Consensus Protocols

FAQ §

About state and Time §

To read §

• “relativity means there is no such thing as simultaneity” used as an argument that synchronized clocks cannot exist.
  • This is a misunderstanding
    • the equations of special and general relativity provide exact equations for time transformations, and it is possible to define any number of sensible, globally-synchronized time clocks.
    • Consistency constraints that refer to these clocks will depend on the choice of clock
      • e.g. depending on one’s reference frame, a system might or might not provide linearizability. See Concurrency Consistency Models
    • The good news is that
      • For all intents and purposes, clocks on earth are so close to each other’s velocities and accelerations that errors are much smaller than side-channel latencies
      • Many of the algorithms for ensuring real-time bounds in asynchronous networks use causal messages to enforce a real-time order, and the resulting order is therefore invariant across all reference frames.

Monotonic? §

Lamport Clocks (Total order) §

Vector Clocks (Partial order) §

Timestamps and Databases §

• I can’t find the original post now, but there’s a good suggestion that booleans should usually be timestamps
  • UPDATE orders SET deletion_timestamp = CURRENT_TIMESTAMP() WHERE deletion_timestamp IS NULL
• See Database Locks
• See Bug story: Sorting by timestamp | Hacker News