ADR 0001: Vector clocks as the default causal ordering primitive

Status

Accepted.

Context

The system needs to track causal ordering between events across multiple processes. Lamport scalar clocks provide a total order but not enough information to detect concurrent events; vector clocks are the standard trade-off between Lamport's compactness and matrix clocks' expressiveness.

Decision

Vector clocks are the default. Lamport clocks are available for compatibility and matrix clocks for advanced garbage-collection analysis.

Consequences

• Detects concurrent events correctly (Lamport cannot).
• Memory cost is O(P) per event where P is the number of processes.
• Matrix clocks' GC capability is documented but not enforced by default.

Alternatives considered

• Lamport only: Rejected — cannot answer "are A and B concurrent?" queries.
• Matrix only: Rejected — 2× memory cost with no benefit for our scale.
• Hybrid (Lamport + vector): Considered — adds complexity without enabling new features.