docs: function requireValidCacheClient (cacheClient) {
      // require object for cache client
      if (!cacheClient || typeof cacheClient !== 'object') {
          throw new Error('cacheClient error: cacheClient must be object')
      }
      // check for all required methods
      _.each(requireMethods, method => {
          if (typeof cacheClient[method] !== 'function') {
              throw new Error ('cacheClient error: cacheClient must provied '+method+' method')
          }
      })
  }