def _write_log(self, version_key, meta_data, index_fields):
meta_data = meta_data or {}<EOL>meta_data.update({<EOL>'<STR_LIT>': version_key,<EOL>'<STR_LIT>': time.time(),<EOL>})<EOL>obj = log_bucket.new(data=meta_data)<EOL>obj.add_index('<STR_LIT>', version_key)<EOL>obj.add_index('<STR_LIT>', int(meta_data['<STR_LIT>']))<EOL>for field, index_type in index_fields:<EOL><INDENT>obj.add_index('<STR_LIT>' % (field, index_type), meta_data.get(field, "<STR_LIT>"))<EOL><DEDENT>obj.store()<EOL>
Creates a log entry for current object,
Args:
    version_key(str): Version_bucket key from _write_version().
    meta_data (dict): JSON serializable meta data for logging of save operation.
        {'lorem': 'ipsum', 'dolar': 5}
    index_fields (list): Tuple list for secondary indexing keys in riak (with 'bin' or 'int').
        [('lorem','bin'),('dolar','int')]

Returns: