Entter

Offline-first event check-in — QR payloads validated at the client, attendance persisted async, duplicates blocked at two independent layers.

Entter handles ticket sales, digital credential generation, and QR-based attendance tracking. The scanner client validates each QR signature locally using SubtleCrypto and a public key — no server round-trip, no dependency on venue Wi-Fi. Scans queue in IndexedDB when offline and replay with exponential backoff on reconnect. On the server, a Redis distributed lock and a PostgreSQL UNIQUE constraint on (event_id, participant_id) act as two independent duplicate barriers: Redis catches concurrent scans from multiple devices; the constraint covers Redis restarts, network partitions, and anything that bypasses the cache layer.

[entter] DevConf SP · 1 200 registered · 3 check-in devices

  ✓  001  Lucas Andrade      <3ms  local      Workshop A — 09:00
  ✓  002  Marina Ferreira    <3ms  local      Keynote — 10:00
  ✓  003  Rafael Costa       <3ms  local      Workshop B — 09:00
  ⚡  network lost — offline mode active
  ✓  004  Priya Mendes       <3ms  local  (queued)
  ✓  005  João Moura         <3ms  local  (queued)
  ·  reconnecting ···
  ✓  synced 89 scans — 0 duplicates · 1 111 remaining

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The check-in moment

Most event platforms assume a stable connection at the gate — a reasonable assumption until 300 attendees arrive at once and the venue Wi-Fi collapses. The usual failure mode is scans that stall, screens that freeze, and staff falling back to printed lists.

Entter treats the connection as optional:

• QR payloads are self-contained. Each QR encodes a signed JWT with the participant's name, ticket tier, and event slug. The check-in client holds the event's public key and validates the signature locally. The attendee's name appears instantly. The server is never in the critical path for a scan.
• IndexedDB is the buffer. Every validated scan is written to IndexedDB synchronously. When online, scans flush to the API immediately. When offline, they queue and replay in order on reconnect — staff never stop scanning because the network dropped.
• Two duplicate barriers. Redis catches concurrent duplicates across multiple devices in real time with a distributed lock. The PostgreSQL UNIQUE constraint catches everything Redis misses. Both layers are necessary because each covers a different failure mode.

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Core architecture

Layer	Mechanism	Why it matters
QR validation	ECDSA signature verified client-side with SubtleCrypto	Sub-3ms response, no server round-trip, works fully offline
Offline sync	IndexedDB queue + exponential backoff replay	Connectivity is a performance concern, not a correctness one
Duplicate prevention	Redis lock (fast path) + PostgreSQL UNIQUE (hard guarantee)	Two independent layers cover different failure modes
Credential layout	Percentage-based coordinates, Sharp for rendering	Badge alignment survives any change in template dimensions or export resolution
Credential delivery	BullMQ job queue → email + WhatsApp	Generation is async; the payment webhook returns immediately
Real-time dashboard	WebSocket counters pushed per scan	Organizers see live attendance without polling

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Architecture

flowchart TD
    A[Landing Page] --> B[Checkout / Asaas]
    B --> C{Payment confirmed?}
    C -->|Webhook| D[Create participant + sign QR]
    D --> E[BullMQ: generate credential]
    E --> F[Email / WhatsApp delivery]

    G[Check-in client] --> H{Online?}
    H -->|Yes| I[POST /attendance]
    H -->|No| J[IndexedDB queue]
    J -->|Reconnect| K[Batch sync + exponential backoff]

    I --> L[Redis distributed lock]
    K --> L
    L -->|Pass| M[PostgreSQL UNIQUE]
    M --> N[WebSocket: live counters → dashboard]

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Full system spec — modules, schema, API reference, security model: docs/ARQUITETURA_credenciamento_eventos.md

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Stack

Layer	Technologies
Frontend	Next.js 15, TypeScript, Tailwind CSS
Backend	NestJS, TypeScript, Prisma
Database	PostgreSQL, Redis
Queues	BullMQ
Auth	JWT + HTTP-only cookies
Payments	Asaas
Image processing	Sharp
PDF generation	pdf-lib
QR scanning	BarcodeDetector API, ZXing fallback
Real-time	WebSockets

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Getting started

git clone https://github.com/Castro-arch/Entter.git
cd Entter
npm install

Backend

cp backend/.env.example backend/.env
npm run --workspace backend prisma:migrate
npm run --workspace backend start:dev
# → http://localhost:3000

Frontend

cp frontend/.env.example frontend/.env.local
npm run --workspace frontend dev
# → http://localhost:3001

The frontend authenticates via HTTP-only cookies. Set FRONTEND_URL in the backend .env accordingly.

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Roadmap

Shipped

• Authentication and event management
• Ticket sales with Asaas payment integration
• Credential generation with drag-and-drop editor
• QR check-in with offline-first sync
• Live attendance dashboard via WebSockets
• Certificate generation and delivery
• Public event pages and organizer dashboard

Planned

• AWS S3 for credential and certificate storage
• Organizer roles and permissions
• Event reports and export
• Mobile check-in application
• Email template customization

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Contributing

Bug reports, ideas, and pull requests are welcome.

1. Fork the repository
2. Create a feature branch
3. Commit your changes and open a pull request

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License

MIT — see LICENSE.