@Override
  public String push(final Request entry, CrawlProfile profile, final RobotsTxt robots)
      throws IOException, SpaceExceededException {
    assert entry != null;
    final byte[] hash = entry.url().hash();
    synchronized (this) {
      // double-check
      if (this.has(hash)) return "double occurrence in urlFileIndex";

      // increase dom counter
      if (profile != null) {
        int maxPages = profile.domMaxPages();
        if (maxPages != Integer.MAX_VALUE && maxPages > 0) {
          String host = entry.url().getHost();
          profile.domInc(host);
        }
      }

      // add to index
      Index depthStack = getStack(entry.depth());
      final int s = depthStack.size();
      depthStack.put(entry.toRow());
      assert s < depthStack.size()
          : "hash = " + ASCII.String(hash) + ", s = " + s + ", size = " + depthStack.size();
      assert depthStack.has(hash) : "hash = " + ASCII.String(hash);
    }
    return null;
  }