flynn/docs/architecture/GATEWAY_SESSIONS_AND_QUEUE.md

Gateway Sessions and Queueing (Agent Execution Model)

This document explains how the gateway maps WebSocket clients onto durable sessions, and how work is serialised per session so agent execution stays coherent under concurrent requests.

If you only want the protocol surface, see docs/api/PROTOCOL.md.

Key Ideas

• A WebSocket client gets a connectionId.
• Each connection is attached to a sessionId.
• Agent work is queued per sessionId (FIFO), not per connection.
• Sessions persist in SQLite via SessionManager even if clients disconnect.
• Once dequeued, message routing may execute the native orchestrator path or an optional external backend path (claude_code, opencode, codex, gemini, pi_embedded) depending on agent/backend config.
• Runtime backend mode can be overridden manually via /runtime command fast-path (status, activate pi, deactivate pi, use config) and is persisted in preferences (/backend remains a compatibility alias).
• Backend routing outcomes are auditable via backend.route / backend.success / backend.fallback, which enables offline canary evaluation without changing gateway protocol methods.

Component Map

flowchart LR
  subgraph CFG[Config + Runtime Policy]
    QP[server.queue policy\nmode/cap/overflow/overrides]
    BM[backend runtime mode\nconfig_default|force_native|force_pi_embedded]
  end

  subgraph GW[Gateway Process]
    WS[WebSocket connection\n(connectionId)]
    GS[GatewayServer]
    LQ[LaneQueue\nper-session FIFO]
    SB[SessionBridge\nconnectionId -> sessionId -> AgentOrchestrator]
    AQ[AuditLogger\nqueue.preempt events]
  end

  subgraph CORE[Flynn Core]
    SM[SessionManager\nin-memory cache + SQLite]
    SS[SessionStore\nSQLite tables]
    AO[AgentOrchestrator / External Backends]
  end

  WS --> GS
  QP --> GS
  BM --> GS
  GS --> LQ
  GS --> SB
  LQ --> AQ

  SB --> AO
  SB --> SM
  SM --> SS

Session IDs (What Actually Gets Stored)

The durable session ID stored by SessionManager is:

<frontend>:<userId>

For the gateway:

• SessionBridge.connect() assigns a connectionId (UUID).
• It defaults the connection's sessionId to ws:<connectionId>.
• It then calls SessionManager.getSession('ws', sessionId).

That means gateway sessions are stored as:

• ws:ws:<connectionId>

This is expected: the gateway adds its own namespace, and the session manager namespaces again by frontend.

Key files:

• src/gateway/session-bridge.ts
• src/session/manager.ts

Per-Session FIFO Queueing (LaneQueue)

agent.send uses a lane ID derived from the session:

• lane = SessionBridge.getSessionId(connectionId) (preferred)
• fallback lane = connectionId (only if session lookup fails)

Within a lane:

• Only one request executes at a time.
• Later requests queue (FIFO) and start after the active request finishes.

Across lanes:

• Independent sessions run in parallel.

Key files:

• src/gateway/lane-queue.ts
• src/gateway/handlers/agent.ts

Cancellation Semantics

agent.cancel performs two separate actions:

1. Cancels any queued (not-yet-started) work in the lane (LaneQueue.cancel(laneId)).
2. Requests cancellation of the active agent operation (AgentOrchestrator.cancel() via SessionBridge.cancel()).

Important:

• Cancellation is best-effort for the currently running work: it stops at the next safe point in the agent loop.
• Queued work is deterministically rejected.

Key files:

• src/gateway/handlers/agent.ts
• src/backends/native/orchestrator.ts