code: protected String getIndexName(String indexPrefix, long timestamp) {
      return new StringBuilder(indexPrefix).append('-')
        .append(fastDateFormat.format(timestamp)).toString();
    }
docs: Gets the name of the index to use for an index request
  @param indexPrefix
  Prefix of index name to use -- as configured on the sink
  @param timestamp
  timestamp (millis) to format / use
  @return index name of the form 'indexPrefix-formattedTimestamp'