text: import json
  
  from functools import wraps
  
  from flask import (
      Blueprint,
      jsonify,
      request,
      current_app
  )
  
  from app.celery.tasks import update_letter_notifications_statuses, record_daily_sorted_counts
  from app.v2.errors import register_errors
  from app.notifications.utils import autoconfirm_subscription
  from app.schema_validation import validate
  from app.config import QueueNames
  
  letter_callback_blueprint = Blueprint('notifications_letter_callback', __name__)
  register_errors(letter_callback_blueprint)
  
  
  dvla_sns_callback_schema = {
      "$schema": "http://json-schema.org/draft-04/schema#",
      "description": "sns callback received on s3 update",
      "type": "object",
      "title": "dvla internal sns callback",
      "properties": {
          "Type": {"enum": ["Notification", "SubscriptionConfirmation"]},
          "MessageId": {"type": "string"},
          "Message": {"type": ["string", "object"]}
      },
      "required": ["Type", "MessageId", "Message"]
  }
  
  
  def validate_schema(schema):
      def decorator(f):
          @wraps(f)
          def wrapper(*args, **kw):
              validate(request.get_json(force=True), schema)
              return f(*args, **kw)
          return wrapper
      return decorator
  
  
  @letter_callback_blueprint.route('/notifications/letter/dvla', methods=['POST'])
  @validate_schema(dvla_sns_callback_schema)
  def process_letter_response():
      req_json = request.get_json(force=True)
      current_app.logger.debug('Received SNS callback: {}'.format(req_json))
      if not autoconfirm_subscription(req_json):
          # The callback should have one record for an S3 Put Event.
          message = json.loads(req_json['Message'])
          filename = message['Records'][0]['s3']['object']['key']
          current_app.logger.info('Received file from DVLA: {}'.format(filename))
  
          if filename.lower().endswith('rs.txt') or filename.lower().endswith('rsp.txt'):
              current_app.logger.info('DVLA callback: Calling task to update letter notifications')
              update_letter_notifications_statuses.apply_async([filename], queue=QueueNames.NOTIFY)
              record_daily_sorted_counts.apply_async([filename], queue=QueueNames.NOTIFY)
  
      return jsonify(
          result="success", message="DVLA callback succeeded"
      ), 200
  