ADR 0003: Hybrid Logical Clocks (HLC) for Distributed Consistency

Status

Accepted

Context

In a distributed environment, system clocks (wall clocks) are never perfecty synchronized. Relying on them for ordering invalidations leads to race conditions and stale data due to clock skew.

Decision

Use Hybrid Logical Clocks (HLC) to version tags. HLC combines wall clock time with a logical counter to ensure causal ordering and monotonicity across nodes. Implement a Ratchet mechanism to sync local clocks with the highest seen version.

Consequences

• Positive: Causal consistency is preserved even with skewed system clocks.
• Positive: Robustness against lost invalidation messages through "passive resync" (self-healing).
• Negative: Slightly larger storage requirements (64-bit timestamps per dependency segment).