Error Handling

In one line: Every error follows one structured JSON shape with a machine-readable code, a human message, and a requestId for Sentry correlation. Operational (expected) errors are logged at warn; programmer (unexpected) errors are logged at error and alert via Sentry.

Design Principles

Never leak internals — No stack traces, DB column names, or internal IDs in client responses
Structured, predictable responses — Every error follows the same JSON shape
Machine-readable codes — Clients can programmatically handle specific errors
Human-readable messages — Users see meaningful text, not “Something went wrong”
Correlation everywhere — Every error carries a requestId for Sentry cross-referencing
Fail gracefully — Operational errors return proper HTTP codes; programmer errors trigger alerts

Standard Response Shapes

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{
  "success": false,
  "error": {
    "code": "BOOKING_SLOT_UNAVAILABLE",
    "message": "This time slot is no longer available. Please choose another.",
    "statusCode": 409,
    "requestId": "req_m8n3k5p2w",
    "retryable": false
  }
}

{
  "success": true,
  "data": { ... },
  "meta": { "page": 1, "totalPages": 5, "totalCount": 47 }
}

HTTP Status Code Map

Code	Error Code	When Used
`400`	`VALIDATION_ERROR`	Zod validation fails, malformed JSON
`401`	`UNAUTHENTICATED`	No session, expired session
`403`	`FORBIDDEN`	Authenticated but lacks permission
`404`	`NOT_FOUND`	Resource doesn’t exist
`409`	`CONFLICT` / domain-specific	Slot taken, duplicate booking
`422`	`BUSINESS_RULE_VIOLATION`	Valid data but violates business rule
`429`	`RATE_LIMITED`	Upstash Ratelimit threshold exceeded
`500`	`INTERNAL_ERROR`	Unhandled exception, DB failure
`502`	`UPSTREAM_ERROR`	Resend API down, PDF service unreachable
`503`	`SERVICE_UNAVAILABLE`	Neon DB in maintenance

Error Classification

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Logged at warn level. No Sentry alert.

Category	Examples
Validation	Invalid phone, missing required field, bad date format
Auth	Expired session, insufficient role
Business rules	Slot already booked, membership expired, max reschedules reached
Rate limits	Too many API calls
Not found	Booking ID doesn’t exist

Logged at error level. Sentry captures with full context.

Category	Examples
Unhandled exceptions	TypeError, null reference, missing env var
DB errors	Connection pool exhausted, constraint violation
External service failures	Resend 5xx, Ably connection lost, Neon timeout

Error Code Registry

All error codes are centralised in packages/errors/codes.ts — no magic strings:

export const ErrorCode = {
  // Generic
  VALIDATION_ERROR: 'VALIDATION_ERROR',
  INTERNAL_ERROR: 'INTERNAL_ERROR',
  UNAUTHENTICATED: 'UNAUTHENTICATED',
  FORBIDDEN: 'FORBIDDEN',
  NOT_FOUND: 'NOT_FOUND',
  RATE_LIMITED: 'RATE_LIMITED',
  UPSTREAM_ERROR: 'UPSTREAM_ERROR',

  // Booking domain
  BOOKING_SLOT_UNAVAILABLE: 'BOOKING_SLOT_UNAVAILABLE',
  BOOKING_ALREADY_CANCELLED: 'BOOKING_ALREADY_CANCELLED',
  BOOKING_MAX_RESCHEDULES: 'BOOKING_MAX_RESCHEDULES',
  BOOKING_CANCEL_WINDOW_PASSED: 'BOOKING_CANCEL_WINDOW_PASSED',

  // Membership domain
  MEMBERSHIP_EXPIRED: 'MEMBERSHIP_EXPIRED',
  MEMBERSHIP_INSUFFICIENT_HOURS: 'MEMBERSHIP_INSUFFICIENT_HOURS',
  MEMBERSHIP_ALREADY_ACTIVE: 'MEMBERSHIP_ALREADY_ACTIVE',

  // Gems domain
  GEMS_INSUFFICIENT_BALANCE: 'GEMS_INSUFFICIENT_BALANCE',
  GEMS_SERVICE_NOT_REDEEMABLE: 'GEMS_SERVICE_NOT_REDEEMABLE',

  // Offer domain
  OFFER_EXPIRED: 'OFFER_EXPIRED',
  OFFER_NOT_APPLICABLE: 'OFFER_NOT_APPLICABLE',
} as const

AppError Class

// packages/errors/app-error.ts

export class AppError extends Error {
  readonly code: ErrorCode
  readonly statusCode: number
  readonly isOperational: boolean
  readonly retryable: boolean
  readonly details?: unknown

  constructor(params: {
    code: ErrorCode
    message: string
    statusCode: number
    isOperational?: boolean  // default: true
    retryable?: boolean      // default: false
    details?: unknown
    cause?: Error
  }) { ... }
}

// Convenience factories
export function notFound(message = 'Resource not found') { ... }
export function forbidden(message = '...') { ... }
export function conflict(code: ErrorCode, message: string) { ... }
export function badRequest(message: string, details?: unknown) { ... }
export function serviceUnavailable(service: string, cause?: Error) { ... }

API Route Pattern

All API routes are wrapped with withErrorHandler:

// apps/web/src/app/api/bookings/route.ts
export const POST = withErrorHandler(async (req: Request) => {
  const body = await req.json()
  const parsed = createBookingSchema.safeParse(body)

  if (!parsed.success) {
    throw new AppError({
      code: 'VALIDATION_ERROR',
      message: 'Invalid request data',
      statusCode: 400,
      details: parsed.error.flatten().fieldErrors,
    })
  }

  const result = await createBooking(parsed.data)
  return Response.json({ success: true, data: result }, { status: 201 })
})

Business Layer Pattern

Business logic throws AppError — never catches it. The handler layer above decides what to do:

// packages/business/booking/create.ts
export async function createBooking(input: CreateBookingInput) {
  const isAvailable = await checkSlotAvailable(input.branchId, input.date, input.time)
  if (!isAvailable) {
    throw conflict('BOOKING_SLOT_UNAVAILABLE', 'This time slot is no longer available.')
  }
  // ...
}

Request ID Correlation

Every request gets a unique ID attached in middleware:

const requestId = `req_${nanoid(12)}`
requestHeaders.set('x-request-id', requestId)
response.headers.set('x-request-id', requestId)

This ID appears in every error response, every Sentry event, and every log line — making it trivial to trace a specific request across all systems.

Sentry Integration

Error Type	Sentry Level	Alert
Unhandled exception (500)	`error`	Slack + Email immediately
External service failure (502/503)	`warning`	Slack if >3 in 5 min
Business rule violation (4xx)	Not sent	—
Validation error (400)	Not sent	—
Rate limit exceeded (429)	Not sent	—

Rate Limiting

All rate limiting runs in Next.js middleware via @upstash/ratelimit backed by Upstash Redis. Applied before auth checks or any business logic.

Endpoint	Window	Limit
`POST /api/bookings`	1 min	5
`POST /api/leads`	1 min	3
`GET /api/availability`	10s	10
`GET /api/services`	10s	20
`GET /api/health`	10s	30

Client-Side Error Handling

The client switches on the error code to decide what the user sees: React Error Boundaries catch rendering errors at the route level. Each route group has its own error.tsx that reports to Sentry and shows a retry button.

Overview

Product

Features

Engineering

System design

Engineering standards

API reference

Application surface

Platform

Quality & security

Operations

Reference

Error Handling

Error Handling

Design Principles

Standard Response Shapes

HTTP Status Code Map

Error Classification

Error Code Registry

AppError Class

API Route Pattern

Business Layer Pattern

Request ID Correlation

Sentry Integration

Rate Limiting

Client-Side Error Handling

​Error Handling

​Design Principles

​Standard Response Shapes

​HTTP Status Code Map

​Error Classification

​Error Code Registry

​AppError Class

​API Route Pattern

​Business Layer Pattern

​Request ID Correlation

​Sentry Integration

​Rate Limiting

​Client-Side Error Handling

Error Handling

Design Principles

Standard Response Shapes

HTTP Status Code Map

Error Classification

Error Code Registry

AppError Class

API Route Pattern

Business Layer Pattern

Request ID Correlation

Sentry Integration

Rate Limiting

Client-Side Error Handling