add({url, generatorId, resource}) {
        debug('add', resource.data.id, url);

        if (this.urls[resource.data.id]) {
            const error = new errors.InternalServerError({
                message: 'This should not happen.',
                code: 'URLSERVICE_RESOURCE_DUPLICATE'
            });
            if (process.env.NODE_ENV.startsWith('test')) {
                logging.warn({
                    message: 'Duplicate URL',
                    err: error
                });
            } else {
                logging.error(error);
            }

            this.removeResourceId(resource.data.id);
        }

        this.urls[resource.data.id] = {
            url: url,
            generatorId: generatorId,
            resource: resource
        };

        // @NOTE: Notify the whole system. Currently used for sitemaps service.
        events.emit('url.added', {
            url: {
                relative: url,
                absolute: urlUtils.createUrl(url, true)
            },
            resource: resource
        });
    }
@description Add a url to the system.
@param {Url} options