zookeeper源码分析三:Znode(DataNode)

zookeeper的数据模型是一个有层次的命名空间，类似于文件系统。zookeeper树里面的每个节点被称为znode。每个节点是用"/"的绝对路径来表示。

数据结构与数据组织方式：

ZKDatabase

/**
 * This class maintains the in memory database of zookeeper
 * server states that includes the sessions, datatree and the
 * committed logs. It is booted up  after reading the logs
 * and snapshots from the disk.
 */
public class ZKDatabase {
    
    private static final Logger LOG = LoggerFactory.getLogger(ZKDatabase.class);
    
    /**
     * make sure on a clear you take care of 
     * all these members.
     */
    protected DataTree dataTree;
    protected FileTxnSnapLog snapLog;
    protected long minCommittedLog, maxCommittedLog;
    protected LinkedList  committedLog = new LinkedList();
 

ZKDatabase 这个类在内存中维护了zookeeper的一些状态信息，譬如会话、datatree和已提交日志信息。启动的时候读取磁盘上的日志和快照，把相关信息load到内存里面。
ZKDatabase 里面有多个重要元素组成：DataTree

DataTree

/**
 * 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.
 */
public class DataTree {
    private static final Logger LOG = LoggerFactory.getLogger(DataTree.class);

    /**
     * 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 nodes =
        new ConcurrentHashMap();

datatree维护了一个树状的数据结构。用ConcurrentHashMap维护了路径和datanode(znode)的关系:key：路径，value：datanode

datanode(znode)

/**
 * This class contains the data for a node in the data tree.
 * 

 * A data node contains a reference to its parent, a byte array as its data, 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;

DataNode parent ：父亲节点，这样维护了一个数据链的关系
byte data[]：datanode数据
Long acl：用来做访问控制
StatPersisted stat：保存在磁盘的状态

znode的几个概念

1. Watches
   zookeeper的客户端可以给znode设置Watches，当znode发生改变，会触发并清空watch，并发送一个通知给客户端
2. 数据访问
   一个client对znode的读取是整个节点的数据，写数据是完全覆盖znode。所以为了安全，每个znode有acl来控制数据访问。
   ZooKeeper不是用来设计成一个大型的或者大对象的存储。一般用来存储与管理一些配置，状态等协同的数据。znode不能存储超过1m的数据。大对象建议存储在redis，hdfs中。
3. 临时节点
   临时节点的生命周期依赖于session，当客户端与ZooKeeper集群断开连接，临时节点自动被删除，临时节点不允许有子节点。
4. 顺序节点
   可以在一个路径下面创建顺序自增的节点。自增数据是4bytes，超2147483647会溢出。
5. Zxid
   zookeeper事务id（ZooKeeper Transaction Id），zk节点状态数据的变更会收到一个时间戳格式的zxid，zxid是唯一而且是有序的，譬如zxid1比zxid2小，那么zxid1发生在zxid2之前。