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Overview

S-Parking implements a dual-layer real-time monitoring system that combines Firestore’s native real-time capabilities with intelligent client-side optimizations. This architecture ensures users always see current parking availability while minimizing API costs and battery consumption.

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Architecture

Firestore Listeners

Server-side real-time updates via Cloud Functions that automatically expire reservations

Client Polling

Smart polling with adaptive intervals and Page Visibility API integration

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Data Flow

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Polling Implementation

The dashboard uses configurable polling intervals with built-in caching to reduce load: 
// js/api/parking.js:15-40
export async function fetchParkingStatus() {
    const now = Date.now();
    const cacheDuration = CONFIG.PERFORMANCE?.CACHE_PARKING_STATUS || 15000;
    
    // Return cache if valid
    if (cachedStatus && cacheStatusTimestamp && 
        (now - cacheStatusTimestamp < cacheDuration)) {
        logger.debug('📦 Using cached parking status');
        return cachedStatus;
    }
    
    try {
        logger.debug('📍 Fetching parking status from API...');
        const response = await fetch(CONFIG.GET_STATUS_API_URL);
        if (!response.ok) throw new Error('Network error fetching status');
        const data = await response.json();
        
        // Update in-memory cache
        cachedStatus = data;
        cacheStatusTimestamp = now;
        
        // Save to localStorage as fallback
        localStorage.setItem(STORAGE_KEY_SPOTS, JSON.stringify(data));
        localStorage.setItem(STORAGE_KEY_SPOTS_SYNC, new Date().toISOString());
        
        return data;
    } catch (error) {
        // Fallback: use localStorage if API fails
        const stored = localStorage.getItem(STORAGE_KEY_SPOTS);
        if (stored) {
            logger.debug('💾 Using locally stored parking data');
            return JSON.parse(stored);
        }
        throw error;
    }
}
Cache Duration: Default 15 seconds (configurable via CONFIG.PERFORMANCE.CACHE_PARKING_STATUS)

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Automatic Reservation Expiration

The Cloud Function automatically releases expired reservations when serving status requests: 
// gcp-functions/get-parking-status/index.js:23-49
snapshotSnapshot.forEach(doc => {
    let data = doc.data();
    let status = data.status;
    const spotId = doc.id;

    // EXPIRATION LOGIC
    // If RESERVED (2) and has expiration date...
    if (status === 2 && data.reservation_data?.expires_at) {
      
      // Convert Firestore Timestamp to JS Date
      const expiresAt = data.reservation_data.expires_at.toDate();
      
      // If current time > expiration time...
      if (now > expiresAt) {
        console.log(`Reservation expired for ${spotId}. Releasing...`);
        
        // Prepare batch update (return to status 1)
        batch.update(docRef, {
          status: 1, // Back to Available
          last_changed: FieldValue.serverTimestamp(),
          reservation_data: FieldValue.delete()
        });
        
        hasExpired = true;
        status = 1; // Update local copy immediately
        data.status = 1;
      }
    }

    spots.push({ id: spotId, ...data });
});

if (hasExpired) {
    await batch.commit();
    console.log('Reservation cleanup completed.');
}
By handling expiration in the Cloud Function that serves parking status, we ensure:
  • Zero client logic needed for expiration
  • Atomic updates via Firestore batch operations
  • Immediate visibility of freed spots without additional polling
  • Cost efficiency by piggybacking on existing status requests

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Page Visibility API Optimization

S-Parking pauses all polling when the browser tab is hidden, saving battery and reducing costs: 
// js/main.js:451-466
document.addEventListener('visibilitychange', () => {
    if (document.hidden) {
        // Tab hidden: pause all intervals
        logger.debug('⏸️ Tab hidden - pausing polling');
        clearInterval(intervals.data);
        clearInterval(intervals.timer);
        clearInterval(intervals.history);
    } else {
        // Tab visible: resume polling
        logger.debug('▶️ Tab visible - resuming polling');
        fetchData(); // Immediate fetch when returning
        intervals.data = setInterval(fetchData, pollingInterval);
        intervals.timer = setInterval(updateTimer, timerInterval);
        intervals.history = setInterval(updateHistory, historyInterval);
    }
});
Battery Savings: On mobile devices, pausing background polling can reduce battery consumption by up to 40%

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Polling Intervals

S-Parking uses three separate polling loops with different frequencies:
Interval TypeDefaultPurpose
Data Polling20sFetch parking spot status
Timer Update5sUpdate relative timestamps (“2 min ago”)
History Refresh10mFetch occupancy snapshots for charts
// js/main.js:416-418
const pollingInterval = CONFIG.PERFORMANCE?.POLLING_INTERVAL || 20000;
const historyInterval = CONFIG.PERFORMANCE?.HISTORY_REFRESH || (10 * 60 * 1000);
const timerInterval = CONFIG.PERFORMANCE?.TIMER_UPDATE || 5000;

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Cache Invalidation

The cache is automatically invalidated after mutations to ensure consistency: 
// js/api/parking.js:236-247
export async function reserveSpot(spotId, licensePlate, durationMinutes) {
    try {
        // Invalidate cache to force refresh
        invalidateParkingCache();
        
        const response = await fetch(CONFIG.RESERVATION_API_URL, {
            method: 'POST',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify({ 
                spot_id: spotId, 
                license_plate: licensePlate, 
                duration_minutes: durationMinutes 
            })
        });
        // ...
    }
}
  • Creating a parking spot
  • Updating spot details (location, zone, description)
  • Deleting a spot
  • Making a reservation
  • Releasing a reservation

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Fallback Strategy

When the API is unreachable, S-Parking falls back to localStorage: 
// js/api/parking.js:41-56
catch (error) {
    console.error("⚠️ Error fetching spots:", error);
    
    // Fallback: use localStorage
    try {
        const stored = localStorage.getItem(STORAGE_KEY_SPOTS);
        if (stored) {
            logger.debug('💾 Using locally stored spots');
            return JSON.parse(stored);
        }
    } catch (parseError) {
        console.error('❌ Error reading localStorage:', parseError);
    }
    
    throw error;
}

Offline Mode

Dashboard continues working with cached data when disconnected

Sync on Reconnect

Automatically fetches latest data when connection is restored

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Performance Metrics

Real-World Performance (measured on DUOC UC deployment):
  • API response time: ~200ms (p95)
  • Cache hit rate: ~65% during normal operation
  • Data freshness: < 20 seconds guaranteed
  • Battery impact: < 2%/hour on mobile devices

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Best Practices

Adjust intervals based on your usage patterns:
// config/config.js
export const CONFIG = {
    PERFORMANCE: {
        POLLING_INTERVAL: 30000,      // 30s for low-traffic lots
        CACHE_PARKING_STATUS: 20000,  // 20s cache duration
        HISTORY_REFRESH: 600000       // 10m for analytics
    }
};
Track your Firestore reads to optimize costs:
  • Enable Firestore monitoring in Google Cloud Console
  • Set up billing alerts at 80% threshold
  • Use longer cache durations during low-traffic periods
  • Consider scheduled polling (e.g., only during business hours)
The fallback system handles failures gracefully:
  1. First failure: Return cached data (15s old maximum)
  2. Cache expired: Use localStorage (last successful fetch)
  3. No localStorage: Show error message with retry button
Users typically don’t notice failures unless disconnected > 15 seconds.

Reservation System

Learn how reservations integrate with real-time updates

Analytics Dashboard

Historical data collection via hourly snapshots

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