ZooKeeper 数据持久化---ZKDatabase
ZKDatabase 主要维持了一个 DataTree的内存数据结构，通过序列化到本地磁盘的快照文件，以及Redo日志文件。
核心是DataTree。
先看看这棵树的节点： DataNode.java
public interface Record { //这个接口应该是类似于java里的Serializable 接口
public void serialize(OutputArchive archive, String tag)
throws IOException;
public void deserialize(InputArchive archive, String tag)
throws IOException;
}
DataTree上的节点结构：
/**
* A data node contains a reference to its parent,
* a byte array as its data, at most 1M
* an array of ACLs,
* a stat object,
* and a set of its children's paths.
*/
public class DataNode implements Record {
/** the parent of this datanode */
DataNode parent;
/** the data for this datanode */
byte data[];
// the acl map long for this datanode. the datatree has the map
Long acl;
//the stat for this node that is persisted to disk.
public StatPersisted stat;
/**
* the list of children for this node. note that the list of children string
* does not contain the parent path -- just the last part of the path. This
* should be synchronized on except deserializing (for speed up issues).
*/
private Set<String> children = null;
......
}
再先先看DataTree 的描述：
/**
* This class maintains the tree data structure. It doesn't have any networking
* or client connection code in it so that it can be tested in a stand alone way.
*
* The tree maintains two parallel data structures: a hashtable that maps from
* full paths to DataNodes and a tree of DataNodes. All accesses to a path is
* through the hashtable. The tree is traversed only when serializing to disk.
*/
DataTree 主要数据成员：
public class DataTree {
/**
* This hashtable provides a fast lookup to the datanodes. The tree is the
* source of truth and is where all the locking occurs
*/
private final ConcurrentHashMap<String, DataNode> nodes =
new ConcurrentHashMap<String, DataNode>();
private final WatchManager dataWatches = new WatchManager();
private final WatchManager childWatches = new WatchManager();
/** the root of zookeeper tree */
private static final String rootZookeeper = "/";
/** the zookeeper nodes that acts as the management and status node **/
private static final String procZookeeper = Quotas.procZookeeper; //“/zookeeper”
/** this will be the string thats stored as a child of root */
private static final String procChildZookeeper = procZookeeper.substring(1);
/**
* the zookeeper quota node that acts as the quota management node for
* zookeeper
*/
private static final String quotaZookeeper = Quotas.quotaZookeeper;// "/zookeeper/quota"
/** this will be the string thats stored as a child of /zookeeper */
private static final String quotaChildZookeeper = quotaZookeeper
.substring(procZookeeper.length() + 1);
/**
* the path trie that keeps track fo the quota nodes in this datatree
*/
private final PathTrie pTrie = new PathTrie();
/**
* This hashtable lists the paths of the ephemeral nodes of a session.
*/
private final Map<Long, HashSet<String>> ephemerals =
new ConcurrentHashMap<Long, HashSet<String>>();
。。。 。。。
}
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