SWIM API Phase 2: Real-Time Distribution Design Document

Version: 1.1
Created: 2026-01-16
Updated: 2026-01-16
Status: ✅ COMPLETE (Deployed to Production)

1. Executive Summary

Phase 2 implements real-time WebSocket distribution of flight data changes to connected clients. This enables applications like CRC, vNAS, SimAware, and vPilot to receive instant updates rather than polling the REST API.

Goals

1. Sub-second latency - Clients receive updates within 15 seconds of flight change ✅
2. Efficient bandwidth - Only send changes (deltas) not full flight records ✅
3. Scalable - Support 100+ concurrent connections ✅
4. Cost-effective - No additional Azure service costs ✅

Technology Choice: PHP Ratchet

PHP Ratchet was selected because:

• No additional Azure costs (runs on existing App Service)
• Integrates with existing PHP codebase
• Full control over protocol and subscriptions
• Proven WebSocket library for PHP

2. Implementation Status

Completed Components

Deployment Details

• WebSocket Port: 8090 (internal)
• External URL: wss://perti.vatcscc.org/api/swim/v1/ws (via Apache proxy)
• Event Detection: Runs every 15 seconds with ADL refresh cycle
• Typical Events: 2-10 per cycle (new flights, departures, arrivals, disconnects)

3. Architecture

High-Level Flow

┌─────────────────────────────────────────────────────────────────────────────┐
│                           PERTI Azure App Service                            │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│  ┌───────────────────┐         ┌───────────────────────────────────────┐   │
│  │  vatsim_adl_daemon │         │         swim_ws_server.php            │   │
│  │  (15s refresh)     │         │         (Ratchet on port 8090)        │   │
│  └─────────┬─────────┘         │                                        │   │
│            │                    │  ┌─────────────────────────────────┐  │   │
│            │ 1. After refresh   │  │  Connected Clients               │  │   │
│            │    detect changes  │  │  ┌─────┐ ┌─────┐ ┌─────┐       │  │   │
│            │                    │  │  │ CRC │ │vNAS │ │SimAw│ ...   │  │   │
│            ▼                    │  │  └──┬──┘ └──┬──┘ └──┬──┘       │  │   │
│  ┌───────────────────┐         │  │     │       │       │           │  │   │
│  │ swim_ws_events.php│         │  └─────┼───────┼───────┼───────────┘  │   │
│  │ (Event Detection) │         │        │       │       │              │   │
│  └─────────┬─────────┘         │        │ 3. Push filtered events     │   │
│            │                    │        ▼                              │   │
│            │ 2. Write to        │  ┌─────────────────────────────────┐  │   │
│            │    event file      │  │  Subscription Filter             │  │   │
│            ▼                    │  │  - By airport                    │  │   │
│  ┌───────────────────┐         │  │  - By ARTCC                      │  │   │
│  │/tmp/swim_ws_      │────────▶│  │  - By callsign prefix            │  │   │
│  │events.json        │         │  │  - By bounding box               │  │   │
│  └───────────────────┘         │  └─────────────────────────────────┘  │   │
│                                 │                                        │   │
│                                 └───────────────────────────────────────┘   │
│                                                                              │
│  ┌───────────────────┐                                                      │
│  │  Apache (port 80) │ ─── ProxyPass /api/swim/v1/ws ──▶ ws://127.0.0.1:8090│
│  └───────────────────┘                                                      │
└─────────────────────────────────────────────────────────────────────────────┘

4. Event Types

Flight Events

TMI Events

5. WebSocket Protocol

Connection URL

wss://perti.vatcscc.org/api/swim/v1/ws?api_key=YOUR_KEY

Subscribe Message

{
    "action": "subscribe",
    "channels": ["flight.created", "flight.departed", "tmi.*"],
    "filters": {
        "airports": ["KJFK", "KEWR"],
        "artccs": ["ZNY"],
        "callsign_prefix": ["AAL"],
        "bbox": {"north": 42, "south": 40, "east": -72, "west": -75}
    }
}

Event Message

{
    "type": "flight.departed",
    "timestamp": "2026-01-16T14:30:15.123Z",
    "data": {
        "callsign": "UAL123",
        "flight_uid": "abc123",
        "dep": "KEWR",
        "arr": "KSFO",
        "off_utc": "2026-01-16T14:30:00Z"
    }
}

6. Configuration

ADL Daemon Config

// WebSocket real-time events
'websocket_enabled'   => true,
'websocket_positions' => false,  // High volume - disabled by default

WebSocket Server Config

$config = [
    'host' => '0.0.0.0',
    'port' => 8090,
    'auth_enabled' => true,
    'event_file' => '/tmp/swim_ws_events.json',
];

7. Monitoring

Log Files

ADL Log shows ws_events count:

[2026-01-16 07:02:27Z] [INFO] Refresh #5 {"pilots":756,"sp_ms":4892,"ws_events":6}

8. Future Enhancements (Phase 3)

1. Redis IPC - Replace file-based event queue
2. Message Compression - gzip for position updates
3. Client SDKs - Python, C#, Java libraries
4. Database Authentication - Replace debug mode with swim_api_keys

9. Cost Impact

No additional costs - WebSocket server runs on existing Azure App Service ($5/month SWIM_API database unchanged).