Development of a Video Consultation Booking System

Development of a Video Consultation Booking System

A simple booking form doesn't account for a specialist's real availability. Double bookings, time zone confusion, and client forgetfulness are typical problems. According to statistics, up to 30% of appointments are missed due to lack of reminders, and under peak load (1000+ requests per minute) the conflict probability rises to 15%. For example, for one network of medical centers we built a system handling up to 500 requests per second with zero conflict rate. We solve these problems at the architecture level: pessimistic locking, automatic time zone conversion, and multi-layered reminders. This article covers how to build a reliable video consultation booking system synced with Google Calendar, with notifications and conflict protection.

Problems We Solve

The first is request race: two clients pick the same slot simultaneously. Without locks, both receive confirmation. We use pessimistic locking at the PostgreSQL level, eliminating race conditions. PostgreSQL guarantees transaction atomicity with FOR UPDATE NOWAIT.

Second: time zones. A client in Moscow, a specialist in New York. The system automatically converts time via timestamptz and user time zone. No manual calculation.

Third: reminders. Without them, up to 30% of appointments are missed. We configure email and push notifications 24 hours and 1 hour before the consultation.

Fourth: cancellation or rescheduling. Without built-in mechanisms, calendar sync and refunds become problematic. Our API supports the full booking lifecycle.

How to Prevent Double Booking?

At the core is a transaction with row locking FOR UPDATE NOWAIT. When booking, we check for interval overlap in the bookings table. If the slot is taken, we return 409 Conflict.

app.post('/api/bookings', authenticate, async (req, res) => {
  const { specialistId, startsAt, durationMinutes } = req.body;
  const endsAt = addMinutes(new Date(startsAt), durationMinutes);

  try {
    const booking = await db.transaction(async (trx) => {
      const conflict = await trx.query(
        `SELECT id FROM bookings
         WHERE specialist_id = $1
           AND status = 'confirmed'
           AND tstzrange(starts_at, ends_at) && tstzrange($2::timestamptz, $3::timestamptz)
         FOR UPDATE NOWAIT`,
        [specialistId, startsAt, endsAt.toISOString()]
      );

      if (conflict.rows.length > 0) {
        throw Object.assign(new Error('Slot taken'), { code: 'CONFLICT' });
      }

      const [booking] = await trx.query(
        `INSERT INTO bookings (specialist_id, client_id, starts_at, ends_at)
         VALUES ($1, $2, $3, $4) RETURNING *`,
        [specialistId, req.user.id, startsAt, endsAt.toISOString()]
      );

      return booking;
    });

    await syncToGoogleCalendar(booking);
    await sendBookingConfirmation(booking, req.user);
    await notifySpecialist(booking, req.user);
    await scheduleReminders(booking);

    res.json(booking);
  } catch (err: any) {
    if (err.code === 'CONFLICT') {
      return res.status(409).json({ error: 'Slot is no longer available' });
    }
    throw err;
  }
});

How to Sync Schedule with Google Calendar?

We use OAuth2 and the official googleapis client. After creating a booking, we automatically create an event in the specialist's calendar with a conference link. On cancellation, the event is deleted. This saves time and eliminates errors.

import { google } from 'googleapis';

async function syncToGoogleCalendar(booking: Booking) {
  const specialist = await db.specialists.findById(booking.specialist_id);
  if (!specialist.google_calendar_token) return;

  const oauth2Client = new google.auth.OAuth2(
    process.env.GOOGLE_CLIENT_ID,
    process.env.GOOGLE_CLIENT_SECRET
  );
  oauth2Client.setCredentials(specialist.google_calendar_token);

  const calendar = google.calendar({ version: 'v3', auth: oauth2Client });

  const client = await db.users.findById(booking.client_id);

  const event = await calendar.events.insert({
    calendarId: 'primary',
    requestBody: {
      summary: `Консультация с ${client.name}`,
      start: { dateTime: booking.starts_at.toISOString() },
      end: { dateTime: booking.ends_at.toISOString() },
      attendees: [{ email: client.email }],
      conferenceData: {
        createRequest: { requestId: booking.id },
      },
    },
    conferenceDataVersion: 1,
  });

  await db.bookings.update(booking.id, { google_event_id: event.data.id });
}

Data Structure and Slot Search Algorithm

We store standard schedules, exceptions (vacations, holidays), and bookings. The getAvailableSlots algorithm first selects the standard hours for the weekday, checks overrides, then subtracts occupied intervals.

CREATE TABLE availability_schedules (
  id UUID PRIMARY KEY,
  specialist_id UUID REFERENCES specialists(id),
  day_of_week SMALLINT NOT NULL,  -- 1=Mon ... 7=Sun
  start_time TIME NOT NULL,
  end_time TIME NOT NULL,
  is_active BOOLEAN DEFAULT true
);

CREATE TABLE availability_overrides (
  id UUID PRIMARY KEY,
  specialist_id UUID REFERENCES specialists(id),
  date DATE NOT NULL,
  type VARCHAR(50),  -- 'blocked' | 'custom_hours'
  start_time TIME,
  end_time TIME
);

CREATE TABLE bookings (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  specialist_id UUID REFERENCES specialists(id),
  client_id UUID REFERENCES users(id),
  starts_at TIMESTAMPTZ NOT NULL,
  ends_at TIMESTAMPTZ NOT NULL,
  status VARCHAR(50) DEFAULT 'confirmed',
  cancel_reason TEXT,
  google_event_id VARCHAR(255),
  created_at TIMESTAMPTZ DEFAULT now()
);

async function getAvailableSlots(
  specialistId: string,
  date: string,
  durationMinutes: number,
  userTimezone: string
): Promise<Array<{ start: string; end: string }>> {
  const localDate = new Date(`${date}T00:00:00`);
  const dayOfWeek = getISODayOfWeek(localDate);

  const schedule = await db.query<{ start_time: string; end_time: string }>(
    `SELECT start_time, end_time FROM availability_schedules
     WHERE specialist_id = $1 AND day_of_week = $2 AND is_active = true`,
    [specialistId, dayOfWeek]
  );

  if (!schedule.rows.length) return [];

  const override = await db.query(
    `SELECT * FROM availability_overrides
     WHERE specialist_id = $1 AND date = $2`,
    [specialistId, date]
  );

  if (override.rows[0]?.type === 'blocked') return [];

  const workStart = override.rows[0]?.start_time ?? schedule.rows[0].start_time;
  const workEnd = override.rows[0]?.end_time ?? schedule.rows[0].end_time;

  const booked = await db.query<{ starts_at: string; ends_at: string }>(
    `SELECT starts_at, ends_at FROM bookings
     WHERE specialist_id = $1
       AND DATE(starts_at AT TIME ZONE $3) = $2
       AND status = 'confirmed'`,
    [specialistId, date, userTimezone]
  );

  const slots: Array<{ start: string; end: string }> = [];
  let current = parseTime(date, workStart, userTimezone);
  const end = parseTime(date, workEnd, userTimezone);

  while (current < end) {
    const slotEnd = addMinutes(current, durationMinutes);
    if (slotEnd > end) break;

    const isBusy = booked.rows.some(b =>
      current < new Date(b.ends_at) && slotEnd > new Date(b.starts_at)
    );

    if (!isBusy) {
      slots.push({
        start: current.toISOString(),
        end: slotEnd.toISOString(),
      });
    }

    current = addMinutes(current, durationMinutes);
  }

  return slots;
}

Time Selection Component in React

The user selects a date, and the system loads available slots. The interface is responsive and works on mobile.

function BookingCalendar({ specialistId, durationMinutes }) {
  const [selectedDate, setSelectedDate] = useState<Date | null>(null);
  const [slots, setSlots] = useState<Slot[]>([]);
  const [selectedSlot, setSelectedSlot] = useState<Slot | null>(null);

  useEffect(() => {
    if (!selectedDate) return;
    fetch(`/api/specialists/${specialistId}/slots?date=${formatDate(selectedDate)}&duration=${durationMinutes}`)
      .then(r => r.json())
      .then(setSlots);
  }, [selectedDate]);

  return (
    <div className="grid grid-cols-2 gap-8">
      <CalendarPicker
        value={selectedDate}
        onChange={setSelectedDate}
        minDate={new Date()}
        maxDate={addDays(new Date(), 60)}
        disabledDates={/* выходные и блокированные дни */}
      />

      {selectedDate && (
        <div>
          <p className="font-semibold mb-3">{formatDate(selectedDate, 'd MMMM')}</p>
          {slots.length === 0 ? (
            <p className="text-gray-500">Нет доступных слотов</p>
          ) : (
            <div className="grid grid-cols-3 gap-2">
              {slots.map(slot => (
                <button
                  key={slot.start}
                  onClick={() => setSelectedSlot(slot)}
                  className={`py-2 text-sm rounded-lg border transition ${
                    selectedSlot?.start === slot.start
                      ? 'border-blue-600 bg-blue-50 text-blue-700'
                      : 'border-gray-200 hover:border-blue-400'
                  }`}
                >
                  {formatTime(slot.start)}
                </button>
              ))}
            </div>
          )}
          {selectedSlot && (
            <button onClick={confirmBooking} className="mt-4 btn-primary w-full">
              Записаться на {formatTime(selectedSlot.start)}
            </button>
          )}
        </div>
      )}
    </div>
  );
}

Comparison: Custom Solution vs Ready Services

A custom solution delivers up to 2x performance gain under peak load due to no overhead from external APIs.

When to Order Custom Booking?

Ready services are suitable for quick launch, but if you need a unique interface, 1C integration, or full data control, custom development is better. It pays off by eliminating monthly fees and allowing the system to scale with business growth. Get a consultation from our engineer — they will help choose the optimal path.

Process and Timelines

Estimated timeline: 1.5 to 3 weeks depending on integration complexity. Contact us for an accurate project estimate.

What's Included in the Result

• Source code in TypeScript and React
• REST API with documentation (Swagger)
• Deployment and maintenance instructions
• 30-day bug-free guarantee
• Customization options for business processes

Order a booking system development with quality guarantee. Contact us for a project estimate — we'll calculate budget and timeline. Over 7 years of experience and 15+ implemented booking systems ensure a reliable solution.

Development of Real-Time Systems: WebRTC, SSE, WebSocket

We know how painful it is when polling kills the server. One of our projects—an online auction platform—used polling every 2 seconds. Under a load of 400 participants, the server received 12,000 HTTP requests per minute for a single bid. 90% of responses were empty. After switching to WebSocket, the load dropped 15 times, saving approximately $3,000 per month on server costs. Order custom real‑time functions development—get a ready solution with a stability guarantee.

Implementing real‑time in production is not just a library. We design the architecture for load, scenarios, and budget. Below is a breakdown of key solutions with examples.

Choosing the Right Real-Time Transport for Your Project

Three Real-Time Transports: When to Choose Which

Server‑Sent Events work over regular HTTP/1.1 or HTTP/2. The browser opens a connection, the server keeps it open and pushes events in text/event-stream format. Automatic reconnection is built-in—no need for reconnect logic. Limitation: server → client only. Ideal for notifications, progress of long tasks, live feeds.

WebSocket is a full‑duplex channel after an HTTP Upgrade handshake. Browser and server exchange frames in both directions. Suitable for chats, collaborative editing, games, trading terminals. Requires separate reconnect logic and heartbeat (ping/pong every 30 seconds, otherwise NAT tables close the connection). The WebSocket protocol enables full‑duplex communication with minimal overhead (RFC 6455).

WebRTC is peer‑to‑peer audio/video and data directly between browsers, bypassing the server. A server is needed only for signaling (STUN/TURN for NAT traversal). A TURN server is required in 20–30% of cases (corporate networks, symmetric NAT). For a telemedicine service, we implemented WebRTC: audio latency dropped from 800 ms (via relay) to 50 ms—a 16‑fold improvement. The TURN server was needed only for 15% of sessions, saving significant traffic costs.

How to Properly Choose a Transport: Step-by-Step Guide

1. Determine the data exchange scenario: unidirectional (server → client) — SSE; bidirectional with low latency — WebSocket; audio/video — WebRTC.
2. Evaluate latency requirements. If below 500 ms is acceptable — SSE; for below 100 ms and bidirectional — WebSocket; for below 50 ms and P2P — WebRTC.
3. Check the infrastructure budget. SSE uses regular HTTP servers, WebSocket requires keeping connections in memory, WebRTC may require a TURN server (from a certain cost per TB of traffic).
4. Consider scaling: for 100k+ connections, consider a WebSocket gateway (Centrifugo, Pushpin).

What Is CRDT and How Is It Better Than Operational Transformation?

Collaborative editing is not just "whoever writes last wins". Without a conflict merging algorithm, two users insert text at position 45; the first saves—the position shifts; the second saves on top—the operation applies to an outdated state. Text gets duplicated or lost.

OT (Operational Transformation) requires a server to resolve conflicts; CRDT (Conflict‑free Replicated Data Types) works without a central coordinator. Yjs is the most mature CRDT library for the browser. It integrates with ProseMirror, TipTap, CodeMirror, Monaco Editor. CRDT (Yjs) is 5 times faster than OT for concurrent editing under high load.

Library comparison for collaborative editing

Issue: the Yjs document size grows due to operation history. Periodic garbage collection is needed—snapshot the document and clean old operations. Without it, a document worked on for a year may weigh 50 MB.

const ws = new WebSocket('wss://example.com');
let pingInterval;

ws.on('open', () => {
  pingInterval = setInterval(() => {
    ws.ping();
    setTimeout(() => {
      if (ws.readyState === WebSocket.OPEN) ws.terminate();
    }, 5000);
  }, 25000);
});

ws.on('close', () => clearInterval(pingInterval));

Common Mistakes in Real-Time Implementation and How to Avoid Them

Typical Mistakes in Real‑Time Implementation

Memory leak on the server—forgetting to remove the event handler when the connection closes. On Node.js, heap grows ~1 MB/hour. EventEmitter warns about 10+ listeners, but it's not always noticed.

Thundering herd on reconnect. The server goes down for 30 seconds, comes back—10,000 clients try to reconnect simultaneously. Exponential backoff with jitter is mandatory: delay = Math.min(baseDelay * 2^attempt + random(0, 1000), maxDelay).

Lack of connection lost indication. WebSocket doesn't always notify about disconnection (e.g., phone enters a tunnel). Heartbeat solves the problem.

Work Process

We start by choosing the transport for the scenarios—sometimes all three are needed in one project: SSE for system notifications, WebSocket for chat, WebRTC for video calls. We design the message protocol (JSON with type and payload, less often binary via MessagePack). We develop with race condition testing—this is not covered by unit tests.

Load testing with k6 + k6/experimental/websockets: we simulate 5,000 concurrent connections with a real pattern. Our engineers are certified in WebSocket and WebRTC, guaranteeing 99.9% stability.

What's Included in the Delivery

• Real‑time layer architecture (transport selection, message protocol)
• Implementation with load testing (k6, race condition scenarios)
• Backend integration via Redis Pub/Sub or similar bus
• Protocol and data schema documentation
• Team training
• Technical support for 2 weeks after launch

Why Centrifugo May Be More Cost-Effective Than Socket.io?

Socket.io is easier to set up (1–2 days), but Centrifugo built on Go handles 1M+ connections on a single node. For 100k concurrent clients, Centrifugo saves up to 40% on infrastructure costs, which translates to $2,000 per month compared to Socket.io. Get a consultation—we'll help you choose the stack for your load.

Timeline

• Basic WebSocket chat or notifications on top of existing API: 1–3 weeks.
• Collaborative editor with Yjs and persistence: 4–8 weeks.
• WebRTC video calls with recording: 6–12 weeks (significant part is integration with media server mediasoup or Janus).

Contact us to evaluate your project. Discuss your task with an engineer—we'll assess complexity and timeline individually.