Implement WebSocket streaming support for real-time observation streaming and command status updates

## Problem

The ogc-client-CSAPI implementation is an **HTTP-only client** that does not support **WebSocket streaming** for real-time data delivery. While the library excellently implements OGC API - Connected Systems Parts 1 and 2 via HTTP (98% compliance), it lacks real-time streaming capabilities for:

**1. Observation Streaming**
- **Current:** Applications must poll `/datastreams/{id}/observations` repeatedly to get new data
- **Problem:** Inefficient for high-frequency sensors (polling overhead, latency, server load)
- **Missing:** WebSocket connection to stream observations as they arrive
- **Use Case:** Real-time sensor dashboards, live monitoring systems

**2. Command Status Updates**
- **Current:** Applications must poll `/commands/{id}/status` to check command execution
- **Problem:** Delays in detecting status changes, unnecessary HTTP requests
- **Missing:** WebSocket notifications when command status changes
- **Use Case:** Control system interfaces, command execution monitoring

**3. System Event Notifications**
- **Current:** Applications must poll `/systemEvents` to discover new events
- **Problem:** Missed events between polls, high latency for critical alerts
- **Missing:** Real-time event push via WebSocket
- **Use Case:** Alert systems, system health monitoring

**Current HTTP-Only Architecture:**
```typescript
// Current pattern: Polling (inefficient)
async function monitorObservations(datastreamId: string) {
  while (true) {
    // Poll every 5 seconds
    const observations = await nav.getDatastreamObservations(datastreamId, {
      datetime: { start: lastTimestamp }
    });
    
    if (observations.features.length > 0) {
      processObservations(observations.features);
      lastTimestamp = getLatestTime(observations);
    }
    
    await sleep(5000); // Wait 5 seconds, repeat
  }
}

// Issues:
// - 12 requests per minute (720/hour)
// - 5-second latency minimum
// - Bandwidth waste (headers + empty responses)
// - Server load from constant polling
```

**Desired WebSocket Architecture:**
```typescript
// Future pattern: Streaming (efficient)
async function monitorObservations(datastreamId: string) {
  const stream = nav.streamDatastreamObservations(datastreamId);
  
  stream.on('observation', (observation) => {
    // Received immediately when server produces observation
    processObservation(observation);
  });
  
  stream.on('error', (error) => {
    console.error('Stream error:', error);
    // Automatic reconnection
  });
  
  // Benefits:
  // - 1 persistent connection (vs 720 requests)
  // - Sub-second latency (immediate delivery)
  // - Minimal bandwidth (only data, no headers)
  // - Lower server load (no polling)
}
```

**Real-World Impact Examples:**

**Scenario 1: Weather Station Dashboard (10 Sensors)**
```
Current HTTP Polling (5-second interval):
- Requests: 12/min × 10 sensors = 120 requests/min = 7,200/hour
- Bandwidth: ~50 KB/request × 7,200 = 360 MB/hour
- Latency: 0-5 seconds (average 2.5s)
- Server CPU: Constant query processing

With WebSocket Streaming:
- Connections: 10 persistent WebSockets
- Bandwidth: ~1 KB/observation (only data) = ~10 MB/hour (97% reduction)
- Latency: <100ms (immediate push)
- Server CPU: Minimal (event-driven push)
```

**Scenario 2: Industrial Control System (Valve Commands)**
```
Current HTTP Polling:
- Poll /commands/{id}/status every 1 second
- 60 requests/min to detect "executing" → "completed" transition
- Response time feedback: 0-1 second delay
- User experience: Laggy, unresponsive

With WebSocket:
- Single connection for command status updates
- Instant notification on status change
- Response time feedback: <100ms
- User experience: Smooth, responsive
```

**Scenario 3: Critical Alert System (System Events)**
```
Current HTTP Polling:
- Poll /systemEvents every 10 seconds
- Critical alert delay: 0-10 seconds (average 5s)
- Risk: Missed alerts between crashes

With WebSocket:
- Persistent connection for event push
- Critical alert delay: <100ms
- Reconnection ensures no missed events
```

---

## Context

This issue was identified during the comprehensive validation conducted January 27-28, 2026.

**Related Validation Issues:** [#20](https://github.com/Sam-Bolling/CSAPI-Live-Testing/issues/20) (OGC Standards Compliance)

**Work Item ID:** 42 from [Remaining Work Items](https://github.com/Sam-Bolling/CSAPI-Live-Testing/blob/main/docs/remaining-work-items.md)

**Repository:** https://github.com/OS4CSAPI/ogc-client-CSAPI

**Validated Commit:** `a71706b9592cad7a5ad06e6cf8ddc41fa5387732`

---

## Detailed Findings

### 1. Current HTTP-Only Architecture (Issue #20 Validation)

**From Issue #20 Validation Report:**
> **Gap 3: WebSocket Streaming Not Implemented ❌**
> 
> **Status:** Explicitly out of scope  
> **Impact:** NONE (Design decision)
> 
> **Reasoning:**
> - This is an HTTP client library
> - WebSocket streaming is a separate protocol
> - OGC CSAPI Part 2 supports both HTTP and WebSocket
> - HTTP implementation is complete and compliant
> - WebSocket would require different client architecture
> 
> **Compliance Impact:** NONE - HTTP endpoints are fully compliant

**Key Finding:** WebSocket streaming was intentionally excluded to focus on HTTP compliance, but is a valuable enhancement for real-time applications.

---

### 2. OGC API - Connected Systems Part 2 WebSocket Support

**OGC 23-002r1 Specification:**

**Section 8.4: Real-Time Observation Streaming**
- Servers MAY provide WebSocket endpoint for observation streaming
- Pattern: `wss://server/datastreams/{id}/observations`
- Protocol: Send observations as JSON messages when available
- Client subscribes once, receives continuous stream

**Section 9.4: Command Status Updates**
- Servers MAY provide WebSocket endpoint for command status
- Pattern: `wss://server/commands/{id}/status`
- Protocol: Push status updates (executing, completed, failed)
- Avoids polling for long-running commands

**Section 12.3: System Event Notifications**
- Servers MAY provide WebSocket endpoint for system events
- Pattern: `wss://server/systemEvents`
- Protocol: Push events as they occur
- Enables real-time alerting and monitoring

**Current Implementation:**
- ✅ HTTP endpoints: Fully compliant (98% compliance from Issue #20)
- ❌ WebSocket endpoints: Not implemented
- ❌ Real-time streaming: Not available

---

### 3. Existing Infrastructure Analysis

**TypedCSAPINavigator Architecture** (from Issue #16):

**File:** `src/ogc-api/csapi/typed-navigator.ts` (~320 lines)
- High-level API for fetching and parsing resources
- Built on CSAPINavigator (URL building)
- Uses `_fetch()` method for HTTP requests
- No WebSocket support

**Current Fetch Pattern:**
```typescript
// typed-navigator.ts
private async _fetch(
  url: string,
  options: TypedFetchOptions = {}
): Promise<Response> {
  const fetchFn = options.fetch || fetch;
  const headers: Record<string, string> = {
    ...options.headers,
  };
  
  // HTTP only - no WebSocket support
  const response = await fetchFn(url, { headers });
  
  if (!response.ok) {
    throw new Error(
      `HTTP ${response.status}: ${response.statusText} (${url})`
    );
  }

  return response;
}
```

**Key Insight:** Current architecture uses standard `fetch()` for HTTP. WebSocket would require:
- Separate connection management (persistent connections)
- Event-driven API (instead of request/response)
- Reconnection logic (handle disconnects)
- Message parsing (JSON frames → parsed observations)

---

### 4. CSAPINavigator URL Building (from Issue #13)

**File:** `src/ogc-api/csapi/navigator.ts` (2,091 lines)

**Existing URL Methods:**
```typescript
// These exist for HTTP, need WebSocket equivalents
getDatastreamObservationsUrl(datastreamId: string, options: ObservationsQueryOptions): string
getCommandStatusUrl(commandId: string, controlStreamId?: string): string
getSystemEventsUrl(options: SystemEventsQueryOptions): string
```

**WebSocket URL Pattern:**
- HTTP: `https://server/datastreams/{id}/observations`
- WebSocket: `wss://server/datastreams/{id}/observations`
- Same path, different protocol (http → ws, https → wss)

**Implementation Strategy:**
```typescript
// New methods needed in CSAPINavigator
getDatastreamObservationsStreamUrl(datastreamId: string): string {
  // Convert HTTP URL to WebSocket URL
  const httpUrl = this.getDatastreamObservationsUrl(datastreamId);
  return httpUrl.replace(/^http/, 'ws'); // http→ws, https→wss
}

getCommandStatusStreamUrl(commandId: string, controlStreamId?: string): string {
  const httpUrl = this.getCommandStatusUrl(commandId, controlStreamId);
  return httpUrl.replace(/^http/, 'ws');
}

getSystemEventsStreamUrl(): string {
  const httpUrl = this.getSystemEventsUrl();
  return httpUrl.replace(/^http/, 'ws');
}
```

---

### 5. Parsers for Real-Time Data (from Issue #10)

**From Issue #10 Validation Report:**
> **File:** `src/ogc-api/csapi/parsers/index.ts`
> 
> Parsers exist for all resource types:
> - ObservationParser - Parses observation features
> - SystemEventParser - Parses system events
> - CommandParser - Parses commands (includes status)
> 
> **Test Coverage:** 79 tests, 97.63% coverage

**Key Finding:** Parsers already exist for real-time data. WebSocket implementation can reuse:
- `ObservationParser.parse()` for streamed observations
- `SystemEventParser.parse()` for event notifications
- `CommandParser.parse()` for status updates

**No new parsers needed** - just integrate existing parsers with WebSocket messages.

---

### 6. Performance Benefits Analysis

**Polling vs Streaming Comparison:**

| Metric | HTTP Polling (5s) | WebSocket Streaming | Improvement |
|--------|-------------------|---------------------|-------------|
| **Requests/Hour** | 720 | 1 (connection) | 99.86% reduction |
| **Bandwidth** | ~360 MB | ~10 MB | 97% reduction |
| **Latency** | 0-5s (avg 2.5s) | <100ms | 96% reduction |
| **Server CPU** | High (constant queries) | Low (event push) | 80%+ reduction |
| **Client CPU** | High (polling loop) | Low (event listener) | 90% reduction |
| **Battery (Mobile)** | High (periodic wake) | Low (push only) | 85% reduction |

**Network Traffic Breakdown:**

**Polling (1 minute, 1 datastream, 5s interval):**
```
12 requests × (200 byte headers + 500 byte body) = 8.4 KB
Empty responses: 8 × 200 byte headers = 1.6 KB
Total: 10 KB/min = 600 KB/hour
```

**Streaming (1 minute, 1 datastream):**
```
1 WebSocket handshake: 500 bytes
10 observations × 50 bytes each = 500 bytes
Total: 1 KB/min = 60 KB/hour (90% reduction)
```

**Scalability Impact (100 concurrent clients):**

**Polling:**
- 72,000 requests/hour
- Database queries: 72,000/hour
- Server threads: Burst loads every 5 seconds

**Streaming:**
- 100 WebSocket connections
- Database queries: Only when data produced (~1,000/hour)
- Server threads: Constant low utilization

---

### 7. Use Case Requirements

**Real-Time Dashboard Use Case:**
```
Requirements:
- Display 20 sensor datastreams in real-time
- Update frequency: 1 observation/second per sensor
- User expects <500ms latency for dashboard updates
- Must run on mobile devices (battery constraint)

Current HTTP Solution:
- Poll each datastream every 1 second
- 20 datastreams × 60 requests/min = 1,200 requests/min
- Latency: 0-1 second (poor UX)
- Battery drain: High (constant polling)
- FAILS latency and battery requirements

WebSocket Solution:
- 20 WebSocket connections (or multiplexed into 1)
- Observations pushed as they arrive
- Latency: <100ms (excellent UX)
- Battery: Low (push notifications)
- MEETS all requirements
```

**Industrial Control Use Case:**
```
Requirements:
- Send valve control command
- Monitor command execution status
- Provide real-time feedback to operator
- Detect command completion within 100ms

Current HTTP Solution:
- Issue command via POST
- Poll /commands/{id}/status every 500ms
- Best-case latency: 0-500ms (average 250ms)
- Worst-case: Missed status if polling paused
- FAILS 100ms requirement

WebSocket Solution:
- Issue command via POST (or WebSocket)
- Subscribe to command status stream
- Receive status updates immediately
- Latency: <100ms
- MEETS requirement
```

---

## Proposed Solution

### 1. WebSocket Stream Client Class

Create new `WebSocketStreamClient` class for managing WebSocket connections:

```typescript
// New file: src/ogc-api/csapi/websocket-stream-client.ts

import { EventEmitter } from 'events';

export interface StreamOptions {
  reconnect?: boolean;           // Auto-reconnect on disconnect (default: true)
  reconnectDelay?: number;       // Delay before reconnect (default: 1000ms)
  maxReconnectAttempts?: number; // Max reconnect attempts (default: Infinity)
  heartbeatInterval?: number;    // Ping interval (default: 30000ms)
}

export interface StreamMessage<T = unknown> {
  type: 'data' | 'error' | 'status';
  timestamp: Date;
  data: T;
}

export class WebSocketStreamClient<T> extends EventEmitter {
  private ws: WebSocket | null = null;
  private url: string;
  private options: Required<StreamOptions>;
  private reconnectAttempts = 0;
  private heartbeatTimer: NodeJS.Timeout | null = null;
  
  constructor(url: string, options: StreamOptions = {}) {
    super();
    this.url = url;
    this.options = {
      reconnect: options.reconnect ?? true,
      reconnectDelay: options.reconnectDelay ?? 1000,
      maxReconnectAttempts: options.maxReconnectAttempts ?? Infinity,
      heartbeatInterval: options.heartbeatInterval ?? 30000,
    };
  }
  
  // Connect to WebSocket
  connect(): void {
    this.ws = new WebSocket(this.url);
    
    this.ws.onopen = () => {
      this.reconnectAttempts = 0;
      this.emit('connected');
      this.startHeartbeat();
    };
    
    this.ws.onmessage = (event) => {
      try {
        const message: StreamMessage<T> = JSON.parse(event.data);
        this.emit('message', message);
        
        if (message.type === 'data') {
          this.emit('data', message.data);
        } else if (message.type === 'error') {
          this.emit('error', new Error(message.data as string));
        }
      } catch (error) {
        this.emit('error', error);
      }
    };
    
    this.ws.onerror = (error) => {
      this.emit('error', error);
    };
    
    this.ws.onclose = () => {
      this.stopHeartbeat();
      this.emit('disconnected');
      
      if (this.options.reconnect && this.reconnectAttempts < this.options.maxReconnectAttempts) {
        this.reconnectAttempts++;
        setTimeout(() => this.connect(), this.options.reconnectDelay);
      }
    };
  }
  
  // Disconnect from WebSocket
  disconnect(): void {
    this.stopHeartbeat();
    if (this.ws) {
      this.ws.close();
      this.ws = null;
    }
  }
  
  // Send message (for bidirectional streams)
  send(data: unknown): void {
    if (this.ws && this.ws.readyState === WebSocket.OPEN) {
      this.ws.send(JSON.stringify(data));
    } else {
      throw new Error('WebSocket not connected');
    }
  }
  
  // Heartbeat to keep connection alive
  private startHeartbeat(): void {
    this.heartbeatTimer = setInterval(() => {
      if (this.ws && this.ws.readyState === WebSocket.OPEN) {
        this.ws.send(JSON.stringify({ type: 'ping' }));
      }
    }, this.options.heartbeatInterval);
  }
  
  private stopHeartbeat(): void {
    if (this.heartbeatTimer) {
      clearInterval(this.heartbeatTimer);
      this.heartbeatTimer = null;
    }
  }
  
  // Check if connected
  get connected(): boolean {
    return this.ws !== null && this.ws.readyState === WebSocket.OPEN;
  }
}
```

---

### 2. Extend CSAPINavigator with Stream URL Methods

Add WebSocket URL methods to CSAPINavigator:

```typescript
// Modify: src/ogc-api/csapi/navigator.ts

// Add new methods for WebSocket URLs
export class CSAPINavigator {
  // ... existing methods ...
  
  /**
   * Get WebSocket URL for streaming datastream observations
   * @see https://docs.ogc.org/is/23-002r1/23-002r1.html#_real_time_observation_streaming
   */
  getDatastreamObservationsStreamUrl(datastreamId: string): string {
    this._checkResourceAvailable('datastreams');
    const httpUrl = this.getDatastreamObservationsUrl(datastreamId);
    return this._toWebSocketUrl(httpUrl);
  }
  
  /**
   * Get WebSocket URL for streaming command status updates
   * @see https://docs.ogc.org/is/23-002r1/23-002r1.html#_command_status_streaming
   */
  getCommandStatusStreamUrl(commandId: string, controlStreamId?: string): string {
    this._checkResourceAvailable('commands');
    const httpUrl = this.getCommandStatusUrl(commandId, controlStreamId);
    return this._toWebSocketUrl(httpUrl);
  }
  
  /**
   * Get WebSocket URL for streaming system events
   * @see https://docs.ogc.org/is/23-002r1/23-002r1.html#_system_event_streaming
   */
  getSystemEventsStreamUrl(options: SystemEventsQueryOptions = {}): string {
    this._checkResourceAvailable('systemEvents');
    const httpUrl = this.getSystemEventsUrl(options);
    return this._toWebSocketUrl(httpUrl);
  }
  
  /**
   * Convert HTTP URL to WebSocket URL
   * @private
   */
  private _toWebSocketUrl(httpUrl: string): string {
    return httpUrl.replace(/^http(s)?/, 'ws$1');
  }
}
```

---

### 3. High-Level Streaming API in TypedCSAPINavigator

Add streaming methods to TypedCSAPINavigator:

```typescript
// Modify: src/ogc-api/csapi/typed-navigator.ts

import { WebSocketStreamClient, StreamOptions } from './websocket-stream-client';
import type { ObservationFeature, SystemEventFeature, CommandFeature } from './geojson/features';

export class TypedCSAPINavigator {
  // ... existing methods ...
  
  /**
   * Stream observations from a datastream in real-time
   * @param datastreamId - ID of the datastream
   * @param options - Stream options (reconnect, etc.)
   * @returns WebSocket stream client
   */
  streamDatastreamObservations(
    datastreamId: string,
    options: StreamOptions = {}
  ): WebSocketStreamClient<ObservationFeature> {
    const url = this.navigator.getDatastreamObservationsStreamUrl(datastreamId);
    const stream = new WebSocketStreamClient<ObservationFeature>(url, options);
    
    // Parse incoming observations
    stream.on('message', (message) => {
      if (message.type === 'data') {
        try {
          const observation = this.observationParser.parse(message.data);
          stream.emit('observation', observation);
        } catch (error) {
          stream.emit('parseError', error);
        }
      }
    });
    
    return stream;
  }
  
  /**
   * Stream command status updates in real-time
   * @param commandId - ID of the command
   * @param controlStreamId - Optional control stream ID
   * @param options - Stream options
   * @returns WebSocket stream client
   */
  streamCommandStatus(
    commandId: string,
    controlStreamId?: string,
    options: StreamOptions = {}
  ): WebSocketStreamClient<CommandFeature> {
    const url = this.navigator.getCommandStatusStreamUrl(commandId, controlStreamId);
    const stream = new WebSocketStreamClient<CommandFeature>(url, options);
    
    // Parse incoming status updates
    stream.on('message', (message) => {
      if (message.type === 'data') {
        try {
          const command = this.commandParser.parse(message.data);
          stream.emit('status', command);
        } catch (error) {
          stream.emit('parseError', error);
        }
      }
    });
    
    return stream;
  }
  
  /**
   * Stream system events in real-time
   * @param options - Query options and stream options
   * @returns WebSocket stream client
   */
  streamSystemEvents(
    options: SystemEventsQueryOptions & StreamOptions = {}
  ): WebSocketStreamClient<SystemEventFeature> {
    const { reconnect, reconnectDelay, maxReconnectAttempts, heartbeatInterval, ...queryOptions } = options;
    const streamOptions: StreamOptions = { reconnect, reconnectDelay, maxReconnectAttempts, heartbeatInterval };
    
    const url = this.navigator.getSystemEventsStreamUrl(queryOptions);
    const stream = new WebSocketStreamClient<SystemEventFeature>(url, streamOptions);
    
    // Parse incoming events
    stream.on('message', (message) => {
      if (message.type === 'data') {
        try {
          const event = this.systemEventParser.parse(message.data);
          stream.emit('event', event);
        } catch (error) {
          stream.emit('parseError', error);
        }
      }
    });
    
    return stream;
  }
}
```

---

### 4. Usage Examples

**Example 1: Stream Observations**
```typescript
import { TypedCSAPINavigator } from 'ogc-client-csapi';

const nav = new TypedCSAPINavigator(collection);

// Start streaming observations
const stream = nav.streamDatastreamObservations('sensor-temp-123', {
  reconnect: true,           // Auto-reconnect on disconnect
  reconnectDelay: 2000,      // Wait 2s before reconnect
  maxReconnectAttempts: 10,  // Try 10 times
});

// Listen for observations
stream.on('observation', (obs) => {
  console.log('New observation:', {
    time: obs.properties.phenomenonTime,
    value: obs.properties.result,
  });
  updateDashboard(obs);
});

// Listen for connection events
stream.on('connected', () => {
  console.log('Stream connected');
});

stream.on('disconnected', () => {
  console.log('Stream disconnected, will reconnect...');
});

stream.on('error', (error) => {
  console.error('Stream error:', error);
});

// Connect
stream.connect();

// Later: disconnect
stream.disconnect();
```

**Example 2: Monitor Command Execution**
```typescript
// Issue a command
const commandUrl = nav.issueCommandUrl('valve-control-456');
const response = await fetch(commandUrl, {
  method: 'POST',
  body: JSON.stringify({ action: 'open' }),
});
const command = await response.json();
const commandId = command.id;

// Stream status updates
const statusStream = nav.streamCommandStatus(commandId, 'valve-control-456');

statusStream.on('status', (cmd) => {
  console.log('Command status:', cmd.properties.executionStatus);
  
  if (cmd.properties.executionStatus === 'completed') {
    console.log('Command completed!');
    statusStream.disconnect();
  } else if (cmd.properties.executionStatus === 'failed') {
    console.error('Command failed:', cmd.properties.error);
    statusStream.disconnect();
  }
});

statusStream.connect();
```

**Example 3: Real-Time Event Monitoring**
```typescript
// Monitor critical system events
const eventStream = nav.streamSystemEvents({
  eventType: 'alert',  // Filter for alerts only
  reconnect: true,
});

eventStream.on('event', (event) => {
  if (event.properties.severity === 'critical') {
    // Send push notification
    sendAlert({
      title: event.properties.label,
      message: event.properties.description,
      timestamp: event.properties.eventTime,
    });
  }
});

eventStream.connect();
```

---

## Acceptance Criteria

### WebSocketStreamClient Class (15 criteria)
- [ ] Implement `WebSocketStreamClient<T>` class with TypeScript generics
- [ ] Extend EventEmitter for event-driven API
- [ ] `connect()` method establishes WebSocket connection
- [ ] `disconnect()` method closes WebSocket connection
- [ ] Auto-reconnect on disconnect (configurable, default: true)
- [ ] Exponential backoff for reconnection attempts (default: 1s, max: 60s)
- [ ] Max reconnection attempts (configurable, default: Infinity)
- [ ] Heartbeat/ping mechanism to keep connection alive (default: 30s)
- [ ] Parse incoming JSON messages
- [ ] Emit 'connected' event on connection established
- [ ] Emit 'disconnected' event on connection closed
- [ ] Emit 'data' event for data messages
- [ ] Emit 'error' event for errors
- [ ] `send()` method for bidirectional communication
- [ ] `connected` property returns connection state

### CSAPINavigator Stream URLs (6 criteria)
- [ ] `getDatastreamObservationsStreamUrl(datastreamId)` returns WebSocket URL
- [ ] `getCommandStatusStreamUrl(commandId, controlStreamId?)` returns WebSocket URL
- [ ] `getSystemEventsStreamUrl(options?)` returns WebSocket URL
- [ ] WebSocket URLs use `ws://` or `wss://` protocol
- [ ] WebSocket URLs preserve path and query parameters from HTTP URLs
- [ ] Private `_toWebSocketUrl(httpUrl)` helper converts http(s) → ws(s)

### TypedCSAPINavigator Streaming Methods (12 criteria)
- [ ] `streamDatastreamObservations(datastreamId, options)` returns stream client
- [ ] `streamCommandStatus(commandId, controlStreamId?, options)` returns stream client
- [ ] `streamSystemEvents(options)` returns stream client
- [ ] Stream clients emit parsed typed features (ObservationFeature, CommandFeature, SystemEventFeature)
- [ ] Stream clients reuse existing parsers (ObservationParser, CommandParser, SystemEventParser)
- [ ] Stream clients emit 'observation', 'status', 'event' events (high-level)
- [ ] Stream clients emit 'parseError' event on parsing failures
- [ ] Stream options passed through to WebSocketStreamClient
- [ ] Query options (for systemEvents) converted to URL parameters
- [ ] Documentation links to OGC 23-002r1 streaming sections
- [ ] TypeScript types for stream options and events
- [ ] Examples in JSDoc comments

### Testing (20 criteria)

**Unit Tests (10 tests):**
- [ ] WebSocketStreamClient connects to WebSocket URL
- [ ] WebSocketStreamClient reconnects on disconnect (when enabled)
- [ ] WebSocketStreamClient stops reconnecting after max attempts
- [ ] WebSocketStreamClient sends heartbeat pings
- [ ] WebSocketStreamClient parses JSON messages
- [ ] WebSocketStreamClient emits correct events
- [ ] CSAPINavigator converts HTTP URLs to WebSocket URLs
- [ ] TypedCSAPINavigator creates stream clients with correct URLs
- [ ] TypedCSAPINavigator integrates parsers with streams
- [ ] Stream clients emit parse errors on invalid JSON

**Integration Tests (5 tests):**
- [ ] Stream observations from mock WebSocket server
- [ ] Stream command status updates from mock server
- [ ] Stream system events from mock server
- [ ] Handle disconnect and reconnect in integration test
- [ ] Handle parse errors in integration test

**Manual Tests (5 tests):**
- [ ] Test against OGC reference implementation (if WebSocket supported)
- [ ] Test against OpenSensorHub (if WebSocket supported)
- [ ] Monitor network traffic (verify bandwidth reduction)
- [ ] Test on mobile device (verify battery impact)
- [ ] Test with high-frequency data (1000+ obs/sec)

### Documentation (8 criteria)
- [ ] Add "Real-Time Streaming" section to README
- [ ] Document WebSocket support and requirements
- [ ] Provide streaming examples (observations, commands, events)
- [ ] Document reconnection behavior and configuration
- [ ] Document server requirements (OGC 23-002r1 WebSocket endpoints)
- [ ] Document fallback to HTTP polling when WebSocket unavailable
- [ ] Document performance benefits (latency, bandwidth, battery)
- [ ] Document browser/Node.js compatibility (WebSocket API)

### Browser/Environment Support (4 criteria)
- [ ] Works in browsers (native WebSocket API)
- [ ] Works in Node.js (ws package or compatible)
- [ ] Polyfill detection and error message if WebSocket unavailable
- [ ] Graceful degradation to HTTP polling if server doesn't support WebSocket

---

## Implementation Notes

### Files to Create

**New File: `src/ogc-api/csapi/websocket-stream-client.ts` (~300-400 lines)**
- `WebSocketStreamClient<T>` class
- `StreamOptions` interface
- `StreamMessage<T>` interface
- Event emitter integration
- Reconnection logic
- Heartbeat mechanism

**New File: `src/ogc-api/csapi/websocket-stream-client.spec.ts` (~200-300 lines)**
- Unit tests for WebSocketStreamClient
- Mock WebSocket server
- Reconnection tests
- Error handling tests

**New File: `tests/websocket-streaming.integration.spec.ts` (~150-200 lines)**
- Integration tests with mock server
- Observation streaming test
- Command status streaming test
- System event streaming test

### Files to Modify

**Modify: `src/ogc-api/csapi/navigator.ts`**
- Add `getDatastreamObservationsStreamUrl()` method
- Add `getCommandStatusStreamUrl()` method
- Add `getSystemEventsStreamUrl()` method
- Add `_toWebSocketUrl()` helper method
- ~50-70 lines added

**Modify: `src/ogc-api/csapi/typed-navigator.ts`**
- Add `streamDatastreamObservations()` method
- Add `streamCommandStatus()` method
- Add `streamSystemEvents()` method
- Integrate parsers with stream clients
- ~120-150 lines added

**Modify: `src/ogc-api/csapi/typed-navigator.spec.ts`**
- Add tests for streaming methods
- Mock WebSocketStreamClient
- ~80-100 lines added

**Modify: `README.md`**
- Add "Real-Time Streaming" section
- Add streaming examples
- Document configuration options
- ~150-200 lines added

### Implementation Phases

**Phase 1: WebSocketStreamClient (8-10 hours)**
- Implement core WebSocket wrapper
- Connection management
- Reconnection logic
- Heartbeat mechanism
- Unit tests

**Phase 2: Navigator Stream URLs (3-4 hours)**
- Add stream URL methods
- HTTP to WebSocket conversion
- Tests

**Phase 3: TypedNavigator Integration (6-8 hours)**
- High-level streaming API
- Parser integration
- Event handling
- Unit tests

**Phase 4: Integration Testing (5-6 hours)**
- Mock WebSocket server
- End-to-end streaming tests
- Error scenarios
- Reconnection scenarios

**Phase 5: Documentation and Examples (4-5 hours)**
- README documentation
- Code examples
- Performance guidance
- Server requirements

**Total Estimated Effort:** 26-33 hours

### Dependencies

**Requires:**
- Issue #13 (CSAPINavigator) - ✅ Complete (provides URL building)
- Issue #16 (TypedCSAPINavigator) - ✅ Complete (provides high-level API)
- Issue #10 (Parsers) - ✅ Complete (provides observation/event parsers)

**External Dependencies:**
- WebSocket API (native in browsers, `ws` package in Node.js)
- Server must support OGC 23-002r1 WebSocket endpoints

**Optional Dependencies:**
- Work Item #40 (Client-side caching) - Could cache streamed data
- Work Item #41 (Cache invalidation) - Could invalidate cache on stream updates

### Caveats

**Server Support Required:**
- Server MUST implement OGC 23-002r1 WebSocket endpoints
- Not all OGC CSAPI servers support WebSocket (HTTP is more common)
- Client should detect WebSocket support and fall back to HTTP polling

**WebSocket Detection:**
```typescript
async function detectWebSocketSupport(baseUrl: string): Promise<boolean> {
  try {
    // Try to connect to WebSocket endpoint
    const wsUrl = baseUrl.replace(/^http/, 'ws') + '/test';
    const ws = new WebSocket(wsUrl);
    
    return new Promise((resolve) => {
      ws.onopen = () => {
        ws.close();
        resolve(true);
      };
      ws.onerror = () => {
        resolve(false);
      };
      setTimeout(() => {
        ws.close();
        resolve(false);
      }, 5000);
    });
  } catch {
    return false;
  }
}
```

**Fallback Strategy:**
```typescript
// Try WebSocket first, fall back to polling
let stream: WebSocketStreamClient | null = null;

const hasWebSocket = await detectWebSocketSupport(baseUrl);

if (hasWebSocket) {
  stream = nav.streamDatastreamObservations(datastreamId);
  stream.connect();
} else {
  // Fall back to HTTP polling
  setInterval(async () => {
    const observations = await nav.getDatastreamObservations(datastreamId, {
      datetime: { start: lastTimestamp }
    });
    processObservations(observations.features);
  }, 5000);
}
```

**Browser Compatibility:**
- WebSocket API: IE10+, all modern browsers
- EventEmitter: Use `eventemitter3` package for browser compatibility
- Node.js: Use `ws` package for WebSocket client

**Connection Limits:**
- Browsers limit concurrent WebSocket connections (~200-255)
- Consider multiplexing multiple streams over single connection
- Server may limit connections per client

**Security:**
- Use `wss://` (WebSocket Secure) in production
- Validate server certificate (same as HTTPS)
- Handle authentication (bearer tokens, cookies)

**Performance:**
- Each WebSocket = ~2-4 KB memory overhead
- High-frequency streams (>100 msg/sec) may need throttling
- Consider batching messages on server side

### Testing Requirements

**Unit Tests:**
- Mock WebSocket API
- Test connection lifecycle
- Test reconnection logic
- Test message parsing
- Test error handling

**Integration Tests:**
- Mock WebSocket server (using `ws` package)
- Test full streaming flow
- Test multiple simultaneous streams
- Test reconnection scenarios
- Test parse errors

**Manual Tests:**
- Test with live OGC server (if available)
- Monitor browser DevTools Network tab
- Measure latency (time from server send to client receive)
- Measure bandwidth (bytes transferred)
- Test on mobile device (battery impact)

**Performance Tests:**
- Stress test with high-frequency data (1000+ obs/sec)
- Memory leak detection (long-running streams)
- Reconnection stress (rapid connect/disconnect)

---

## Priority Justification

**Priority: Low**

**Why Low Priority:**
1. **Server Dependency:** Requires server support for OGC 23-002r1 WebSocket endpoints
2. **HTTP Works:** Current HTTP implementation is fully functional and compliant (98%)
3. **Niche Use Case:** Real-time streaming needed for specific applications (dashboards, control systems)
4. **Complexity:** WebSocket adds connection management, reconnection logic, state handling
5. **Testing Challenges:** Requires mock WebSocket server, harder to test than HTTP

**Why Still Important:**
1. **Real-Time Applications:** Critical for dashboards, monitoring, control systems
2. **Performance:** 97% bandwidth reduction, 96% latency reduction vs polling
3. **User Experience:** Sub-second updates vs multi-second delays with polling
4. **OGC Standard:** Part of OGC 23-002r1 specification (even if optional)
5. **Competitive Feature:** Many IoT platforms offer WebSocket streaming

**Impact if Not Addressed:**
- ⚠️ Real-time applications must use inefficient HTTP polling
- ⚠️ Higher bandwidth costs (97% more data)
- ⚠️ Higher latency (25× slower updates)
- ⚠️ Higher server load (720× more requests)
- ⚠️ Higher battery usage on mobile (85% more)
- ✅ **HTTP functionality still works** (no feature loss)

**When to Prioritize Higher:**
- Building real-time dashboard or monitoring application
- Server supports OGC 23-002r1 WebSocket endpoints
- Performance/bandwidth is critical concern
- Mobile battery life is important
- Competing with platforms that offer real-time streaming

**Effort Estimate:** 26-33 hours
- WebSocketStreamClient: 8-10 hours
- Navigator URLs: 3-4 hours
- TypedNavigator integration: 6-8 hours
- Integration tests: 5-6 hours
- Documentation: 4-5 hours

**ROI Analysis:**
- **High ROI** for real-time applications with WebSocket servers
- **Low ROI** for typical applications using HTTP polling
- **No ROI** if server doesn't support WebSocket
- **Best ROI** for high-frequency data (sensors updating >1/sec)

**Recommendation:** Implement when building real-time application with confirmed server WebSocket support. Otherwise, HTTP polling is sufficient for most use cases.

Metric	HTTP Polling (5s)	WebSocket Streaming	Improvement
Requests/Hour	720	1 (connection)	99.86% reduction
Bandwidth	~360 MB	~10 MB	97% reduction
Latency	0-5s (avg 2.5s)	<100ms	96% reduction
Server CPU	High (constant queries)	Low (event push)	80%+ reduction
Client CPU	High (polling loop)	Low (event listener)	90% reduction
Battery (Mobile)	High (periodic wake)	Low (push only)	85% reduction

Implement WebSocket streaming support for real-time observation streaming and command status updates #65

Description

Problem

Context

Detailed Findings

1. Current HTTP-Only Architecture (Issue #20 Validation)

2. OGC API - Connected Systems Part 2 WebSocket Support

3. Existing Infrastructure Analysis

4. CSAPINavigator URL Building (from Issue #13)

5. Parsers for Real-Time Data (from Issue #10)

6. Performance Benefits Analysis

7. Use Case Requirements

Proposed Solution

1. WebSocket Stream Client Class

2. Extend CSAPINavigator with Stream URL Methods

3. High-Level Streaming API in TypedCSAPINavigator

4. Usage Examples

Acceptance Criteria

WebSocketStreamClient Class (15 criteria)

CSAPINavigator Stream URLs (6 criteria)

TypedCSAPINavigator Streaming Methods (12 criteria)

Testing (20 criteria)

Documentation (8 criteria)

Browser/Environment Support (4 criteria)

Implementation Notes

Files to Create

Files to Modify

Implementation Phases

Dependencies

Caveats

Testing Requirements

Priority Justification

Metadata

Metadata

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Labels

Projects

Milestone

Relationships

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