[4.2] Add websocket callbacks

.ai/ARCHITECTURE.md

@@ -723,6 +723,11 @@ Special handling for Colab:
 - `background_callback_manager` - DiskcacheManager or CeleryManager
 - `on_error` - Global callback error handler
 
+**WebSocket Callbacks:**
+- `websocket_callbacks` - Enable WebSocket for all callbacks (default: `False`). Requires FastAPI backend.
+- `websocket_allowed_origins` - List of allowed origins for WebSocket connections
+- `websocket_inactivity_timeout` - Disconnect WebSocket after inactivity period in ms (default: `300000` = 5 minutes). Set to `0` to disable.
+
 ### app.run() Parameters
 
 - `host` - Server IP (default: `"127.0.0.1"`, env: `HOST`)
@@ -861,6 +866,177 @@ async def async_background(n_clicks):
 
 Both DiskcacheManager and CeleryManager support async functions via `asyncio.run()`.
 
+## WebSocket Callbacks
+
+WebSocket callbacks use a persistent WebSocket connection instead of HTTP POST for callback execution. This reduces latency and connection overhead for applications with frequent callbacks.
+
+### Requirements
+
+- **FastAPI backend required**: WebSocket callbacks only work with FastAPI
+- **SharedWorker support**: Modern browsers (not IE)
+
+### Usage
+
+**Enable globally for all callbacks:**
+```python
+from fastapi import FastAPI
+from dash import Dash
+
+server = FastAPI()
+app = Dash(__name__, server=server, websocket_callbacks=True)
+```
+
+**Enable per-callback:**
+```python
+@app.callback(
+    Output('output', 'children'),
+    Input('input', 'value'),
+    websocket=True  # Use WebSocket for this callback only
+)
+def update(value):
+    return f"Value: {value}"
+```
+
+### Configuration
+
+```python
+app = Dash(
+    __name__,
+    server=server,
+    websocket_callbacks=True,
+    websocket_inactivity_timeout=300000,  # 5 minutes (default)
+    websocket_allowed_origins=['https://example.com'],
+)
+```
+
+- **`websocket_callbacks`** - Enable WebSocket for all callbacks (default: `False`)
+- **`websocket_inactivity_timeout`** - Close WebSocket after period of inactivity in milliseconds (default: `300000` = 5 minutes). Heartbeats do not count as activity. Set to `0` to disable timeout. Connection automatically reconnects when needed.
+- **`websocket_allowed_origins`** - List of allowed origins for WebSocket connections (security)
+
+### Architecture
+
+```
+┌─────────────────────────────────────────────────────────────────────────┐
+│ Browser Tab 1                          Browser Tab 2                    │
+│ ┌─────────────┐                       ┌─────────────┐                   │
+│ │  Renderer   │                       │  Renderer   │                   │
+│ └──────┬──────┘                       └──────┬──────┘                   │
+│        │ postMessage                         │ postMessage              │
+│        └────────────┬───────────────────────┘                           │
+│                     ▼                                                   │
+│         ┌─────────────────────┐                                         │
+│         │    SharedWorker     │  (one per origin)                       │
+│         │   dash-ws-worker    │                                         │
+│         └──────────┬──────────┘                                         │
+└────────────────────│────────────────────────────────────────────────────┘
+                     │ WebSocket
+                     ▼
+┌─────────────────────────────────────────────────────────────────────────┐
+│ Server (FastAPI)                                                        │
+│   WebSocket Endpoint: /_dash-ws-callback                                │
+└─────────────────────────────────────────────────────────────────────────┘
+```
+
+**Connection & Reconnection Flow:**
+```
+Renderer                   SharedWorker                 Server
+    │                              │                        │
+    │──[CONNECT]──────────────────>│                        │
+    │                              │──[WebSocket Connect]──>│
+    │<─[CONNECTED]─────────────────│<─[Connected]───────────│
+    │                              │                        │
+    │──[CALLBACK_REQUEST]─────────>│──[callback request]───>│
+    │<─[CALLBACK_RESPONSE]─────────│<─[callback response]───│
+    │                              │                        │
+    │      (inactivity)            │    (heartbeat check)   │
+    │                              │──[close 4001]─────────>│
+    │<─[DISCONNECTED]──────────────│                        │
+    │                              │                        │
+    │──[CALLBACK_REQUEST]─────────>│──[reconnect + send]───>│
+    │<─[CALLBACK_RESPONSE]─────────│<─[response]────────────│
+```
+
+- **SharedWorker**: Single WebSocket connection shared across browser tabs
+- **Heartbeat**: Periodic ping/pong to detect dead connections (30s interval)
+- **Inactivity timeout**: Closes connection after no actual callback activity (not heartbeats)
+- **Auto-reconnect**: Reconnects automatically when a callback is triggered after timeout
+
+### Long-Running Callbacks with set_props/get_props
+
+WebSocket callbacks can stream updates to the client during execution using `set_props()` and read current component values using `ctx.get_websocket()`:
+
+```python
+import asyncio
+from dash import callback, Output, Input, set_props, ctx
+
+@callback(
+    Output('result', 'children'),
+    Input('start-btn', 'n_clicks'),
+    prevent_initial_call=True
+)
+async def long_running_task(n_clicks):
+    ws = ctx.get_websocket()
+    if not ws:
+        return "WebSocket not available"
+
+    # Stream progress updates to the client
+    for i in range(100):
+        await asyncio.sleep(0.1)
+        set_props('progress-bar', {'value': i + 1})
+        set_props('status', {'children': f'Processing step {i + 1}/100...'})
+
+    # Read current value from another component
+    current_value = await ws.get_prop('input-field', 'value')
+
+    return f"Completed! Input was: {current_value}"
+```
+
+**API:**
+- `set_props(component_id, props_dict)` - Stream prop updates immediately to client
+- `ctx.get_websocket()` - Get WebSocket interface (returns `None` if not in WS context)
+- `await ws.get_prop(component_id, prop_name)` - Read current prop value from client
+- `await ws.set_prop(component_id, prop_name, value)` - Set single prop (async version)
+- `await ws.close(code, reason)` - Close the WebSocket connection
+
+### Connection Hooks
+
+Use hooks to validate connections and messages:
+
+```python
+from dash import Dash, hooks
+
+@hooks.websocket_connect()
+async def validate_connection(websocket):
+    """Validate WebSocket connection before accepting."""
+    session_id = websocket.cookies.get("session_id")
+    if not session_id:
+        return (4001, "No session cookie")
+    if not await is_valid_session(session_id):
+        return (4002, "Invalid session")
+    return True  # Allow connection
+
+@hooks.websocket_message()
+async def validate_message(websocket, message):
+    """Validate each WebSocket message."""
+    session_id = websocket.cookies.get("session_id")
+    if not await is_session_active(session_id):
+        return (4002, "Session expired")
+    return True  # Allow message
+```
+
+**Hook Return Values:**
+- `True` (or truthy) - Allow connection/message
+- `False` - Reject with default code (4001)
+- `(code, reason)` - Reject with custom close code and reason
+
+### Key Files
+
+- `dash/dash.py` - WebSocket config in `_generate_config()`
+- `dash/dash-renderer/src/utils/workerClient.ts` - Browser-side SharedWorker client
+- `@plotly/dash-websocket-worker/src/WebSocketManager.ts` - WebSocket connection management
+- `@plotly/dash-websocket-worker/src/worker.ts` - SharedWorker entry point
+- `dash/backends/_fastapi.py` - Server-side WebSocket handler
+
 ## Security
 
 ### XSS Protection

.github/workflows/testing.yml

@@ -19,6 +19,7 @@ jobs:
       backend_cb_changed: ${{ steps.filter.outputs.backend_paths }}
       dcc_paths_changed: ${{ steps.filter.outputs.dcc_related_paths }}
       html_paths_changed: ${{ steps.filter.outputs.html_related_paths }}
+      websocket_changed: ${{ steps.filter.outputs.websocket_paths }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -48,6 +49,16 @@ jobs:
             backend_paths:
               - 'dash/backends/**'
               - 'tests/backend_tests/**'
+            websocket_paths:
+              - 'dash/backends/_fastapi.py'
+              - 'dash/_callback.py'
+              - 'dash/_callback_context.py'
+              - 'dash/_hooks.py'
+              - 'dash/dash.py'
+              - '@dash-websocket-worker/**'
+              - 'dash/dash-renderer/src/**'
+              - 'tests/websocket/**'
+              - 'requirements/**'
 
   lint-unit:
     name: Lint & Unit Tests (Python ${{ matrix.python-version }})
@@ -366,7 +377,7 @@ jobs:
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version: '24'
           cache: 'npm'
 
       - name: Install Node.js dependencies
@@ -377,6 +388,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           cache: 'pip'
+          cache-dependency-path: requirements/*.txt
 
       - name: Download built Dash packages
         uses: actions/download-artifact@v4
@@ -387,43 +399,13 @@ jobs:
       - name: Install Dash packages
         run: |
           python -m pip install --upgrade pip wheel
-          python -m pip install "setuptools<78.0.0"
-          python -m pip install "selenium==4.32.0"
+          python -m pip install "setuptools<80.0.0"
           find packages -name dash-*.whl -print -exec sh -c 'pip install "{}[async,ci,testing,dev,celery,diskcache,fastapi,quart]"' \;
 
-      - name: Install Google Chrome
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y google-chrome-stable
-
-      - name: Install ChromeDriver
-        run: |
-          echo "Determining Chrome version..."
-          CHROME_BROWSER_VERSION=$(google-chrome --version)
-          echo "Installed Chrome Browser version: $CHROME_BROWSER_VERSION"
-          CHROME_MAJOR_VERSION=$(echo "$CHROME_BROWSER_VERSION" | cut -f 3 -d ' ' | cut -f 1 -d '.')
-          echo "Detected Chrome Major version: $CHROME_MAJOR_VERSION"
-          if [ "$CHROME_MAJOR_VERSION" -ge 115 ]; then
-            echo "Fetching ChromeDriver version for Chrome $CHROME_MAJOR_VERSION using CfT endpoint..."
-            CHROMEDRIVER_VERSION_STRING=$(curl -sS "https://googlechromelabs.github.io/chrome-for-testing/LATEST_RELEASE_${CHROME_MAJOR_VERSION}")
-            if [ -z "$CHROMEDRIVER_VERSION_STRING" ]; then
-              echo "Could not automatically find ChromeDriver version for Chrome $CHROME_MAJOR_VERSION via LATEST_RELEASE. Please check CfT endpoints."
-              exit 1
-            fi
-            CHROMEDRIVER_URL="https://edgedl.me.gvt1.com/edgedl/chrome/chrome-for-testing/${CHROMEDRIVER_VERSION_STRING}/linux64/chromedriver-linux64.zip"
-          else
-            echo "Fetching ChromeDriver version for Chrome $CHROME_MAJOR_VERSION using older method..."
-            CHROMEDRIVER_VERSION_STRING=$(curl -sS "https://chromedriver.storage.googleapis.com/LATEST_RELEASE_${CHROME_MAJOR_VERSION}")
-            CHROMEDRIVER_URL="https://chromedriver.storage.googleapis.com/${CHROMEDRIVER_VERSION_STRING}/chromedriver_linux64.zip"
-          fi
-          echo "Using ChromeDriver version string: $CHROMEDRIVER_VERSION_STRING"
-          echo "Downloading ChromeDriver from: $CHROMEDRIVER_URL"
-          wget -q -O chromedriver.zip "$CHROMEDRIVER_URL"
-          unzip -o chromedriver.zip -d /tmp/
-          sudo mv /tmp/chromedriver-linux64/chromedriver /usr/local/bin/chromedriver || sudo mv /tmp/chromedriver /usr/local/bin/chromedriver
-          sudo chmod +x /usr/local/bin/chromedriver
-          echo "/usr/local/bin" >> $GITHUB_PATH
-        shell: bash
+      - name: Setup Chrome and ChromeDriver
+        uses: browser-actions/setup-chrome@v1
+        with:
+          chrome-version: stable
 
       - name: Build/Setup test components
         run: npm run setup-tests.py
@@ -558,6 +540,67 @@ jobs:
           path: components/dash-table/test-reports/
           retention-days: 7
 
+  websocket-tests:
+    name: WebSocket Tests (Python ${{ matrix.python-version }})
+    needs: [build, changes_filter]
+    if: |
+      (github.event_name == 'push' && (github.ref == 'refs/heads/master' || github.ref == 'refs/heads/dev')) ||
+      needs.changes_filter.outputs.websocket_changed == 'true'
+    timeout-minutes: 30
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.9", "3.12"]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+          cache: 'npm'
+
+      - name: Install Node.js dependencies
+        run: npm ci
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: 'pip'
+          cache-dependency-path: requirements/*.txt
+
+      - name: Download built Dash packages
+        uses: actions/download-artifact@v4
+        with:
+          name: dash-packages
+          path: packages/
+
+      - name: Install Dash packages
+        run: |
+          python -m pip install --upgrade pip wheel
+          python -m pip install "setuptools<80.0.0"
+          find packages -name dash-*.whl -print -exec sh -c 'pip install "{}[ci,testing,dev,fastapi]"' \;
+
+      - name: Setup Chrome and ChromeDriver
+        uses: browser-actions/setup-chrome@v1
+        with:
+          chrome-version: stable
+
+      - name: Build/Setup test components
+        run: npm run setup-tests.py
+
+      - name: Run WebSocket tests
+        run: |
+          mkdir wstests
+          cp -r tests wstests/tests
+          cd wstests
+          touch __init__.py
+          pytest --headless --nopercyfinalize tests/websocket -v -s
+
   test-main:
     name: Main Dash Tests (Python ${{ matrix.python-version }}, Group ${{ matrix.test-group }})
     needs: build

@plotly/dash-websocket-worker/README.md

@@ -0,0 +1,3 @@
+# Dash websocket worker
+
+Worker for websocket based callbacks.

@plotly/dash-websocket-worker/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "@plotly/dash-websocket-worker",
+  "version": "1.0.0",
+  "description": "SharedWorker for WebSocket-based Dash callbacks",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "scripts": {
+    "build": "webpack --mode production",
+    "build:dev": "webpack --mode development",
+    "watch": "webpack --mode development --watch",
+    "clean": "rm -rf dist"
+  },
+  "files": [
+    "dist"
+  ],
+  "keywords": [
+    "dash",
+    "websocket",
+    "sharedworker"
+  ],
+  "author": "Plotly",
+  "license": "MIT",
+  "devDependencies": {
+    "typescript": "^5.0.0",
+    "webpack": "^5.0.0",
+    "webpack-cli": "^5.0.0",
+    "ts-loader": "^9.0.0"
+  }
+}