lib/sync-cache.js
function get(key, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.get(key, cb);
}
function get(key, cb) {
  if (!syncConfig.useCache || !redisClient) {
    return cb && cb();
  }
  return redisClient.get(key, cb);
}
Read data from cache
@param {String} key
@param {Function} cb