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websocket-connection

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Deep WebSocket connection knowledge: FoxgloveWebSocketPlayer state machine, WorkerSocketAdapter postMessage protocol, the Foxglove WebSocket protocol handshake, RAF-based state emission, and reconnection.

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WebSocket Connection Skill

The real-time data path from a robot/simulation/bridge to visualization.

Architecture

Foxglove WebSocket Server (robot/bridge)
    │  WebSocket protocol
    ▼
WorkerSocketAdapter (Worker thread)
    │  postMessage (raw, not Comlink)
    ▼
FoxgloveWebSocketPlayer (main thread)
    │
    ▼
MessagePipeline → Panels

Core Files

FileRole
packages/suite-base/src/players/FoxgloveWebSocketPlayer/index.tsPlayer implementation for WebSocket connections
packages/suite-base/src/players/FoxgloveWebSocketPlayer/WorkerSocketAdapter.tsOffloads WebSocket I/O to a Worker
packages/suite-base/src/players/FoxgloveWebSocketPlayer/worker.tsWorker-side WebSocket handling

Foxglove WebSocket Protocol

Connection Lifecycle

  1. Server sends serverInfo (capabilities, session ID)
  2. Server sends advertise with available channels (topic + schema)
  3. Client sends subscribe with channel IDs
  4. Server streams messageData binary frames
  5. Client can publish messages back to the server

Key Operations

  • serverInfo — server capabilities, session ID
  • advertise / unadvertise — channel availability (topics can appear/disappear)
  • subscribe / unsubscribe — client topic selection
  • messageData — binary message payload with channel ID + timestamp
  • time — server clock synchronization
  • parameterValues — server parameters

FoxgloveWebSocketPlayer

Message Processing

  • Messages arrive as binary frames via WebSocket
  • Deserialized on the main thread (unlike file-based players, which decode in a Worker)
  • Accumulated in a parsedMessages queue
  • Queue flushed on requestAnimationFrame → emits state to the pipeline

Connection Management

  • Auto-reconnect with exponential backoff
  • Handles advertise/unadvertise dynamically
  • Subscription changes are sent immediately to the server (no debounce)

State Emission

  • requestAnimationFrame-driven flush of the message queue
  • Coalesces all messages received between frames into one state update (max ~60 updates/sec)
  • Prevents UI thrashing during high-frequency bursts

WorkerSocketAdapter

  • The WebSocket connection lives in a dedicated Worker
  • Avoids main-thread blocking during TLS handshake / large frame parsing
  • Communicates with the main thread via raw postMessage (not Comlink)
  • Binary frames are transferred (zero-copy) using Transferable ArrayBuffers

Performance Notes

  1. Main-thread deserialization — current limitation; high message rates can drop frames
  2. RAF-based flush — batches messages per animation frame
  3. Binary transfer — ArrayBuffer transferred from the Worker (zero-copy)
  4. Subscription filtering — only subscribed topics are sent → reduces bandwidth
  5. Backpressure — if the main thread can't keep up, messages queue in the Worker

Skills Reference

  • For Worker patterns: load web-workers skill
  • For deserialization of incoming frames: load deserialization skill