/// max_age_ seconds in the past.
  bool Lookup(const std::string& key, T* value) {
    if (max_age_ == 0) {
      return false;
    }
    absl::MutexLock lock(&mu_);
    return LookupLocked(key, value);
  }

  typedef std::function<void(const std::string&, T*, TF_Status*)> ComputeFunc;

  /// Look up the entry with key `key` and copy it to `value` if found. If not