apache-zookeeper - Zookeeper，数据版本控制

Question

场景如下:

1. Znode 创建:create/config 12345(例如创建于12/12/12)
2. 更新此配置，设置/config 34567(例如在 2013 年 12 月 12 日修改)
3. 一个月后再次修改配置，设置/config 567889(例如1/1/13修改)

问题:

1. “获取”(或维护)版本历史记录的最佳方式是什么与'/config'相关联，即有没有办法让我得到存储在节点中的全部数据历史？
2. 将当前配置值(即 567889)还原为原始值 12345 的最佳方法是什么？ (通过爬取节点历史数据)

我需要一些帮助来解决这个问题。谢谢

Answer 1

我不认为有一种简单的方法可以做到这一点。以下是我将如何实现这一点:

 1. divide the configurations in two znodes [say, /Config and /Config/data-*]
  - /config/data-* are the data nodes which will store the configuration
  - /config will store the history of all the configs so far
 2. /Config/data-* is sequential node
       So, every znode will have a strictly increasing number appended to it.
       For example, /config/data-1, /config/data-2, and so on. Your configuration object
       will be stored in data nodes.
         /config/data-1 -> 12345
         /config/data-2 -> 34567
         /config/data-3 -> 56789
 3. /config will look like this: 
    ["/config/data-1","/config/data-2","/config/data-3"] or just 
    ["1", "2", "3"]

这是写入算法:

 1. create a data node with new config. Which will return the the actual path.
    say /config/data-4
 2. now try to do conditional update on /config with "/config/data-4" or just "4" appended to
    the existing data i.e. 
    ["/config/data-1","/config/data-2","/config/data-3","/config/data-4"] or just
    ["1", "2", "3","4"]
    - if update succeeds, end.
    - else, someone else was simultaneously trying to update the config and won the race.
     So, either quit or try again starting with step1.

注意:如果更新失败，您将在树中拥有有效的孤立节点，这些节点可以通过自动方式定期清理

阅读将是:

1. call sync [if consistency is important]
2. read /config
3. pick the path which is at the last index in the array retrieved from /config
4. read the config from that path

要回滚，要么从存储在/config 中的列表中删除最后一个条目，要么将列表的最后但第二个条目附加到末尾