Hybrid Logical Clock (HLC)
A clock that combines physical time (wall clock) with a logical counter, so you get timestamps that are both close to real time AND guaranteed to respect causal order. Used in CockroachDB, MongoDB, and YugabyteDB to order events without atomic clocks.
Problem: pure physical clocks drift apart (clock skew) → can give wrong ordering; pure logical clocks (Lamport) lack a connection to real time. HLC takes the best of both: the timestamp is max(physical time, latest seen timestamp) + a logical counter that breaks ties. Result: timestamps that are monotonic, capture causality (if A caused B, B has a higher stamp), and stay within a known distance of real time. Win over Spanner's TrueTime: it requires no special hardware (GPS/atomic clocks). The basis for consistent snapshots and MVCC in distributed SQL databases. Related to Lamport clock and Spanner TrueTime.