Api Design and Versioning Questions

// javascript
function mapErrorToResponse(err, requestId) {
  // Default safe response
  const safe = {
    httpStatus: 500,
    body: {
      code: 'internal_server_error',
      message: 'An unexpected error occurred.',
      details: null,
      requestId
    }
  };

  if (!err || typeof err !== 'object') return safe;

  // Prefer well-known class names or custom 'type'/'code' fields
  const type = err.name || err.type || err.code || '';

  // Helper to avoid leaking sensitive info
  const make = (httpStatus, code, message, details = null) => ({
    httpStatus,
    body: { code, message, details, requestId }
  });

  switch (type) {
    case 'ValidationError':
    case 'BadRequestError':
      // err.details may be an array of field errors; keep only safe shapes
      return make(400, 'validation_error', 'Request validation failed.', Array.isArray(err.details) ? err.details : null);

    case 'AuthError':
    case 'UnauthorizedError':
      return make(401, 'unauthorized', 'Authentication required or invalid credentials.', null);

    case 'ForbiddenError':
      return make(403, 'forbidden', 'You do not have permission to perform this action.', null);

    case 'RateLimitError':
      // Include retry info if available but not internal counters
      return make(429, 'rate_limited', 'Too many requests. Retry later.', err.retryAfter ? { retryAfter: err.retryAfter } : null);

    case 'NotFoundError':
      return make(404, 'not_found', err.message || 'Resource not found.', null);

    // common mapped library errors
    case 'SyntaxError':
      return make(400, 'bad_request', 'Malformed request body.', null);

    default:
      // For unknown errors, log full details server-side (stack, internal codes)
      // logger.error({ err, requestId }, 'Unhandled error');
      return safe;
  }
}

{
  "limit": 3,
  "offset": 6,
  "total": 1234,
  "items": [
    {"id":"tx_1007","amount":12.34,"timestamp":"2025-11-30T12:01:00Z"},
    {"id":"tx_1008","amount":5.00,"timestamp":"2025-11-30T11:59:00Z"},
    {"id":"tx_1009","amount":7.50,"timestamp":"2025-11-30T11:58:00Z"}
  ],
  "hasMore": true
}

{
  "limit": 3,
  "items": [
    {"id":"tx_2001","amount":9.99,"timestamp":"2025-12-01T09:00:00Z"},
    {"id":"tx_2000","amount":20.00,"timestamp":"2025-12-01T08:59:00Z"},
    {"id":"tx_1999","amount":3.50,"timestamp":"2025-12-01T08:58:00Z"}
  ],
  "nextCursor": "eyJub3Rlc3QiOiJ0eF8xOTk5IiwidGltZSI6IjIwMjUtMTItMDFUMDg6NTg6MDBaIn0=", 
  "hasMore": true
}

components:
  schemas:
    TransactionsCursorResponse:
      type: object
      properties:
        limit:
          type: integer
          example: 3
        items:
          type: array
          items:
            $ref: '#/components/schemas/Transaction'
        nextCursor:
          type: string
          description: Opaque cursor to fetch the next page; null if no more results.
          example: "eyJub3Rlc3QiOiJ0eF8xOTk5In0="
        hasMore:
          type: boolean
          example: true
    Transaction:
      type: object
      properties:
        id:
          type: string
        amount:
          type: number
        timestamp:
          type: string
          format: date-time

const crypto = require('crypto');
const etagCache = new Map(); // key -> {etag, fingerprint, generatedAt}

function fingerprintForResource(resourceId) {
  // lightweight change-detection: lastModified timestamp or version number
  return db.getVersion(resourceId); // small scalar
}

function computeETag(bodyBuffer) {
  // strong ETag: hash of final bytes
  return '"' + crypto.createHash('sha256').update(bodyBuffer).digest('base64') + '"';
}

// middleware
app.get('/items/:id', async (req, res) => {
  const id = req.params.id;
  const fp = await fingerprintForResource(id);
  const cacheKey = `${id}:${fp}`;
  let entry = etagCache.get(cacheKey);

  if (!entry) {
    const resource = await loadResource(id); // may be large
    const bodyJson = JSON.stringify(resource);
    const bodyBuffer = Buffer.from(bodyJson, 'utf8');
    const etag = computeETag(bodyBuffer);
    entry = { etag, bodyBuffer, generatedAt: Date.now() };
    etagCache.set(cacheKey, entry);
  }

  const ifNoneMatch = req.headers['if-none-match'];
  if (ifNoneMatch && matches(ifNoneMatch, entry.etag)) {
    res.status(304).end();
    return;
  }

  res.setHeader('ETag', entry.etag);
  res.type('application/json');
  res.send(entry.bodyBuffer);
});