Playground

Simulate consensus in your browser

Crabka's metadata quorum is a native KRaft implementation. Its core is deterministic: it reaches the same state from the same sequence of events. This makes the consensus engine testable, and it also lets the engine run in your browser. The engine on this page is the engine that runs in the broker.

The playground below compiles the deterministic consensus core to WebAssembly and drives it from JavaScript through custom in-memory transports. The peers do not use TCP. They exchange messages over an in-memory bus that the page controls. You can inject partitions, drop, reorder, or duplicate messages, and produce records. You then watch the cluster elect a leader, lose the leader, and recover. There is no backend.

Press Play to watch a fresh cluster hold its first election. Then cut off the leader with Partition leader. The majority elects a new leader at a higher epoch. To heal the partition, click the dimmed node. The stale leader then steps down, because the engine never allows two leaders in one epoch.

Every action drives the real KIP-595/996 state machine, not a scripted animation.

How it works

flowchart LR
  UI[Browser UI] -->|trigger faults| Core[WASM consensus core]
  Core <-->|messages| Transport[JS-controlled in-memory bus]
  Transport --> Trace[Event trace]
  Trace --> SVG[SVG render]
  SVG --> UI

The UI drives the consensus core that is compiled to WASM. The core sends and receives peer messages over a JS-controlled in-memory bus. Every step appends to an event trace. The page renders the trace to SVG and shows it in the UI as an interactive timeline.

The engine already had the necessary boundary. The consensus state machine is a pure, sans-IO on_event(event, log, now) -> [Action] function. It never reads the clock, opens a socket, or writes to disk. This page drives the same deterministic multi-node simulator that the integration tests and the generated failure diagrams use. Here you supply the events.

The core is in a dependency-light leaf crate, crabka-kraft-core, with the voter-set value types in crabka-voters. The crate carries no tokio, no filesystem, and no crypto, so it compiles cleanly for wasm32-unknown-unknown. A thin wasm-bindgen shim, crabka-playground, gives JavaScript access to the simulator. The page supplies the clock and the transports.

The async engine, the real KIP-595 wire, and the on-disk log are in crabka-raft. That crate re-exports the same core, so the algorithm on this page is byte-for-byte the algorithm that the broker runs.

The failure-scenario reference shows the same partition, reordering, and duplication scenarios as generated sequence diagrams.