Hybrid Logical Clock (HLC) Visualizer

Interactive space-time diagram for Hybrid Logical Clocks (Demirbas et al., 2014). Each node has a drifting physical clock. HLC timestamps (l, c) maintain causality while staying close to physical time — unlike Lamport clocks λ which only track ordering.

Double-click a column for a local event  ·  Drag across columns to send a message

Physical clocks
Alice 0 0.9× (slow)
Bob 0 1.0× (ref)
Carol 0 1.1× (fast)
How to interact
double-click in a lane to create a local event
drag from one lane and drop onto another to send a message
HLC Algorithm
// send or local event:
l′ = max(l, pt)
if l′=l: c = c + 1
else: c = 0; l = l′
// receive (l_m, c_m):
l′ = max(l, l_m, pt)
if l′=l=l_m: c=max(c,c_m)+1
elif l′=l: c = c + 1
elif l′=l_m: c = c_m + 1
else: c = 0
l = l′
Consistent Snapshot
Press ◎ Snapshot (enabled after t=1) to capture a consistent cut at threshold T = Bob's PT − 1.
For each node the cut selects its latest event with HLC l < T. The zigzag line joins those three events — one per node.
Why it's consistent: if a message was sent by Alice and received by Bob, and Bob's receive event is inside the cut (lrecv < T), then Alice's send event must also be inside the cut — because HLC enforces lsend ≤ lrecv. A message can never "cross" the cut from future into past.
This is impossible to guarantee with physical clocks alone (drifting clocks can reorder events). HLC's l component gives a single comparable timestamp across all nodes, making the cut threshold precise and causally safe.
Legend
Local event
Send event
Receive event
(l, c) = HLC timestamp λ = Lamport clock pt = physical time (ticks)