Ghost/core/server/web/shared/middlewares/maintenance.js

const config = require('../../../config');
const common = require('../../../lib/common');
const urlService = require('../../../services/url');

module.exports = function maintenance(req, res, next) {
    if (config.get('maintenance').enabled) {
        return next(new common.errors.MaintenanceError({
            message: common.i18n.t('errors.general.maintenance')
        }));
    }

    if (!urlService.hasFinished()) {
        return next(new common.errors.MaintenanceError({
            message: common.i18n.t('errors.general.maintenanceUrlService')
        }));
    }

    next();
};