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>>();
    。。。 。。。
}