Software Engineering Glossary

Hybrid Logical Clock

Also known as: HLC Hybrid Time

Definition

A hybrid logical clock, or HLC, is a clock that mixes the wall clock time with a small counter. The result is a 64 bit number that always goes up, stays close to real time, and keeps causal order across nodes even when wall clocks drift or jump backwards.

Key Takeaways

  • An HLC gives you the causal ordering of a logical clock and the human readable feel of a real timestamp in one value.
  • Each timestamp is a pair of physical time and a counter. The counter goes up if physical time stalls or moves backwards, so the clock never goes down.
  • Two events with the same HLC are either truly concurrent or causally ordered correctly, even with bounded clock skew.
  • HLCs are common in modern databases that want serializable transactions without a TrueTime style hardware setup.

How It Works

  1. Each node keeps a local HLC made of a physical time and a counter.
  2. On a local event, set the physical time to the max of the wall clock and the current HLC physical time. If the physical time did not move, bump the counter.
  3. On a message receive, merge the incoming HLC. Take the max of the physical times and bump the counter so the result is bigger than both.
  4. Send the HLC with every outgoing message so causal order travels with normal traffic.

Where It Is Used

  • CockroachDB uses HLCs as commit timestamps and to drive uncertainty windows for serializable reads.
  • YugabyteDB and TiDB use HLC based timestamps for cross shard consistency.
  • MongoDB’s clusterTime is an HLC used for causal consistency across replica sets.