Apache mahout 源码阅读笔记--DataModel之FileDataModel

* <p>
 * 增量更新的方式, This class will also look for update "delta" files in the same
 * directory, with file names that start the same way (up to the first period).
 * These files have the same format, and provide updated data that supersedes
 * what is in the main data file. This is a mechanism that allows an application
 * to push updates to {@link FileDataModel} without re-copying the entire data
 * file.
 * 
 * 同一个目录下,数字来区分
 * Finds update delta files in the same directory as the data file. This finds
 * any file whose name starts the same way as the data file (up to first period)
 * but isn‘t the data file itself. For example, if the data file is
 * /foo/data.txt.gz, you might place update files at /foo/data.1.txt.gz,
 * /foo/data.2.txt.gz, etc.
 * </p>
 *
 * <p>
 * 表示删除的语法, 偏好为空 One small format difference exists. Update files must also be
 * able to express deletes. This is done by ending with a blank preference
 * value, as in "123,456,".
 * </p>
 *
 * <p>
 * 增量更新的文件中，删除和更新不能混合使用 Note that it‘s all-or-nothing -- all of the items in the
 * file must express no preference, or the all must. These cannot be mixed. Put
 * another way there will always be the same number of delimiters on every line
 * of the file!
 * </p>