Clocks, CRDTs and gossip

The Clocks and Gossip & SWIM tabs are simulators. Pick a scenario and press Start. Each step does something, plays until it has happened and pauses so you can read what happened and why, with links to the papers. Next step moves on, Back rewinds (the simulator is deterministic, so it replays the earlier steps), ⏭ jumps to the end of the step and Step ⇥ advances to the next single event. Space plays and pauses; → is the next step. Each tab has a playground with no script. Speed scales everything (at 1× a quarter of a simulated second per real second); Hop time is how long a message takes to cross the diagram at 1× — while one is on its way the simulated clock slows down — and Sim clock turns that off. Neither changes what happens. ⛶ Full screen (or f) gives the diagram the whole window with the narration in a side column you can hide; Escape leaves.

Clocks. A space-time diagram: one line per process, true time running to the right, events as dots (grey local, blue send, green receive, orange a write), messages as arrows; a dashed amber stretch is a message held back by causal delivery. Above or below each event is its timestamp — Show picks Lamport, vector, HLC (l.c) or all three. On the left each process’s physical clock offset and drift; on the right its clocks now. Tap an event to see its causal past (amber) and future (pink); tap a second to compare: happened before or concurrent, with the Lamport, vector and HLC timestamps side by side. Tap a process, or use Do something, to add local events, writes, messages and broadcasts, shift a clock, turn causal delivery on, or slow a process’s links. Panels: Events (every timestamp, and each process’s physical clock reading), Last writer wins (which write of a key survives under physical, Lamport and HLC timestamps, and whether a causally later write was lost), HLC vs physical time (the largest l − pt seen against ε, the largest gap between two clocks).

CRDTs. Pick a type. Three replicas, A, B and C, each with its own state; the links between them can be cut. Change any replica (increment, set, add, remove, or type in the text box for RGA), then Sync — one gossip round: every replica merges the state of each replica it can reach — or ⇠ merge one replica into another, or tick auto-sync. Each card shows the value and the full state (tags and tombstones included); the line below says whether the replicas have converged, and the merge laws are checked on the current states. For text, each character’s id (Lamport counter
replica) is shown, and deleted characters stay as struck-out tombstones.

Gossip & SWIM. A group of members around a circle. Each card shows the member’s incarnation number, whom it is probing, and a row of dots: what it believes about every member (green alive, amber suspect, red dead). Messages: ping, ack, ping-req M4 (please ping M4 for me), ack M4 (relay); a +2 means two membership updates ride along. Tap a member to kill, pause (a long GC), isolate or restart it, or cut single links. Panels: every member’s view, and detection times. Below the simulator, a chart runs push, pull and push-pull rumour spreading on n nodes and compares the rounds with theory.

Related: Raft and Paxos, simulated, partitioning and replication, distributed transactions, Kafka, simulated, probability, graph algorithms.

Links can be written by hand. Add a tab after #: clocks (the default), crdts or swim. A link’s settings replace this page’s saved settings (a button restores yours); a scenario brings its own settings, and parameters given with it override them.

Clock scenarios:

SWIM scenarios (add #swim):

Examples: every timestamp at once, the same pause without suspicion: a false positive, three-way collaborative text, OR-Set: add wins.