Architecture

The Booking Kit is organized into three layers that can be used independently or together.

System layers

┌─────────────────────────────────────────────────────┐
│                   Your Next.js App                   │
├──────────┬──────────────────┬────────────────────────┤
│  @thebookingkit/ui               │     API Routes          │
│  21 React components       │     (your code)         │
│  Copy-paste, you own them  │                         │
├──────────┴──────────────────┤                        │
│        @thebookingkit/core        │                        │
│  Slot engine, scheduling    │                        │
│  math, business logic       │                        │
├─────────────────────────────┼────────────────────────┤
│        @thebookingkit/db          │    Adapter Interfaces   │
│  Drizzle schema, 23 tables  │    Auth · Email · Jobs  │
│  PostgreSQL 15+             │    Calendar · Payment   │
└─────────────────────────────┴────────────────────────┘

1. Logic layer (@thebookingkit/core)

Pure TypeScript with zero framework dependencies. Contains:

• Slot engine — Three-step computation pipeline (RRULE → overrides → bookings)
• Timezone utilities — UTC normalization and local display
• RRULE parser — iCalendar recurrence expansion with EXDATE support
• Booking limits — Daily/weekly/monthly caps
• Team scheduling — Round-robin, collective, managed, fixed strategies
• Payments — Cancellation policies, fee computation
• Workflows — Trigger-condition-action evaluation
• Webhooks — HMAC signing, verification, retry logic
• Auth middleware — Provider ownership and customer access assertions
• Adapter interfaces — TypeScript interfaces for external services

2. Data layer (@thebookingkit/db)

Drizzle ORM schema with 23 PostgreSQL tables:

• Full TypeScript type inference from schema definitions
• Custom SQL migrations for btree_gist, audit triggers, and GDPR anonymization
• EXCLUDE USING gist constraint for database-level double-booking prevention
• SERIALIZABLE transactions with automatic retry (withSerializableRetry())

3. UI layer (@thebookingkit/ui)

21 React components following the shadcn/ui convention:

• Copy source code into your project — you own it completely
• Built on react-day-picker, react-big-calendar, react-hook-form
• All components accept className and style props for customization

Slot computation pipeline

The core algorithm follows a three-step model:

RRULE rules  →  Base windows  →  Apply overrides  →  Subtract bookings  →  Available slots
                 (expand)         (mask)               (filter + buffer)

1. Base Layer: Expand availability_rules via RRULE into time windows for the requested date range
2. Mask Layer: Apply availability_overrides — block dates, shorten hours, or add extra hours
3. Filter Layer: Subtract existing bookings and their buffer time to produce available slots

Adapter pattern

External dependencies are abstracted behind TypeScript interfaces. Swap implementations without changing business logic:

Adapter	Purpose	Default	Alternatives
AuthAdapter	Authentication	NextAuth.js	Supabase Auth, Clerk, Lucia
EmailAdapter	Transactional email	Resend	SendGrid, AWS SES, Postmark
CalendarAdapter	Calendar sync	Google Calendar	Outlook, CalDAV
JobAdapter	Background jobs	Inngest	Trigger.dev, BullMQ
PaymentAdapter	Payment processing	Stripe	—
StorageAdapter	Secret storage	Env vars	Vault, KMS
SmsAdapter	SMS notifications	—	Twilio, MessageBird

// Example: implement the EmailAdapter interface
const myEmailAdapter: EmailAdapter = {
  async sendEmail(options: SendEmailOptions): Promise<EmailResult> {
    // Your implementation
    return { messageId: "abc", status: "sent" };
  },
};

// Pass it to notification functions
await sendConfirmationEmail(payload, myEmailAdapter);

Double-booking prevention

The Booking Kit prevents double-bookings at two levels:

1. Database constraint: EXCLUDE USING gist with btree_gist extension ensures no two confirmed bookings overlap for the same provider
2. Application-level: SERIALIZABLE transactions with withSerializableRetry() — catches serialization failures (SQLSTATE 40001) and retries up to 3 times with jittered exponential backoff

This means double-bookings are impossible even under high concurrent load.