code: @Override
      public void initialize(long epoch) {
          // InMemoryStorage does not use epochs; we don't do anything with it.
          Preconditions.checkArgument(epoch > 0, "epoch must be a positive number. Given %s.", epoch);
          Preconditions.checkState(this.initialized.compareAndSet(false, true), "InMemoryStorage is already initialized.");
      }
docs: region Storage Implementation