Wire protocols

One process serves both protocols and REST from one port.

Socket.IO — /socket.io/

Engine.IO v4 / Socket.IO v5, as spoken by the official Fluid and Routerlicious drivers. An event frame such as 42["op",…] decomposes as:

Field	Meaning
4	Engine.IO MESSAGE packet type
2	Socket.IO EVENT packet type
["op", …payload]	JSON array: event name, then arguments

Observed semantics, pinned by the golden fixtures:

• The Fluid clientId is the Engine.IO sid.
• maxPayload in the Engine.IO open, maxMessageSize in IConnected, and serviceConfiguration.maxMessageSize all carry the single configured frame cap (default 16777216).
• The Engine.IO ping enforces pingInterval + pingTimeout as a deadline; a missing pong disconnects.
• 42["close"] is emitted on server-initiated close.
• IConnected claims omit jti and the user’s name (the claims encoder emits only id).
• Join/leave ops have clientId: null, csn/rsn -1, and data as a JSON string.
• Timestamps are whole seconds times 1000 — always ≡ 0 (mod 1000).

Phoenix Channels V2 — /socket/websocket

The protocol spoken by levee-driver and levee-client. Every frame is a five-element JSON array:

[join_ref, ref, topic, event, payload]

• Document topics take the form document:{tenant}:{documentId}.
• Joining is two-phase: phx_join → phx_reply (ok/empty), then a connect_document push.
• Heartbeats run on the reserved topic phoenix.
• A join with vsn=1.0.0 is rejected 403 before the WebSocket upgrade (see Divergence notes).

Signals

• A legacy {content} broadcast signal reaches everyone including the sender (plain broadcast, not broadcast-from).
• A v2 contentBatches signal can target a single client and reaches only that client; v2 signal content objects come back key-sorted, while legacy string content passes through verbatim.

REST

Served from the same port: document creation and sessions, delta (op) retrieval, token minting (when UNDERTOW_TOKEN_MINT_SECRET is set), a /health endpoint, and git-like Historian storage endpoints backed by the configured storage backend.