Hadoop学习二十六：Hadoop-Hdfs Lease源码

xuanxi

一. Lease

•  A Lease governs all the locks held by a single client.
     * For each client there's a corresponding lease, whose
     * timestamp is updated when the client periodically
     * checks in.  If the client dies and allows its lease to
     * expire, all the corresponding locks can be released.
  
• 翻译如下：Lease管理着一个client占用的所有锁。一个client对应着一个Lease，在client发送请求时，对应Lease的timestamp调整到最新时间。如果client死亡，让对应Lease过期，释放Lease管理的所有锁。
  
• 个人理解：Lease是一个文件写锁，当client需要写文件时，需要申请一个Lease。NameNode负责记录哪个文件上有Lease，Lease的客户是谁，超时时间。
  
• 成员变量
  private final String holder;//客户端名
  private long lastUpdate;//最近更新时间
  private final Collection<String> paths = new TreeSet<String>();//该客户端操作的文件集合
  
• 方法，其方法都很简单，随便举两个例子
  /** Only LeaseManager object can create a lease */
  private Lease(String holder) {
  this.holder = holder;
  renew();
  }
  /** Only LeaseManager object can renew a lease */
  private void renew() {
  this.lastUpdate = FSNamesystem.now();
  }
  //replacing oldpath with  newpath
  synchronized void changeLease(String src, String dst,
  String overwrite, String replaceBy) {
  }
  synchronized void removeLeaseWithPrefixPath(String prefix) {
  }
  
   
  

二. Monitor

•  Monitor checks for leases that have expired, and disposes of them.
  
• 每两秒调用checkLeases()。checkLeases()检查每个Lease是否过期，如果过期，调用fsnamesystem.internalReleaseLeaseOne(oldest, path);
  

三. LeaseManager

•  LeaseManager管理着所有的Lease。
  
• Lease和Monitor都是LeaseManager内部类。Lease的private构造函数保证额只有LeaseManager才能创建一个Lease。
  
• 成员变量
    private final FSNamesystem fsnamesystem;
  //Lease.holder -> Lease
  private SortedMap<String, Lease> leases = new TreeMap<String, Lease>();
  // Set of: Lease
  private SortedSet<Lease> sortedLeases = new TreeSet<Lease>();
  //pathnames -> Lease
  private SortedMap<String, Lease> sortedLeasesByPath = new TreeMap<String, Lease>();
  
• 方法，无非就是对几个集合的增删改查操作，毫无难度。需要注意的是，addLease()并没有检查文件(src)上是否已经有Lease，这个需要由LeaseManager调用者保证。
  synchronized Lease addLease(String holder, String src) {
  Lease lease = getLease(holder);
  if (lease == null) {
  lease = new Lease(holder);
  leases.put(holder, lease);
  sortedLeases.add(lease);
  } else {
  renewLease(lease);
  }
  sortedLeasesByPath.put(src, lease);
  lease.paths.add(src);
  return lease;
  }
  /**
  * Remove the specified lease and src.
  */
  synchronized void removeLease(Lease lease, String src) {
  sortedLeasesByPath.remove(src);
  }
  /**
  * Reassign lease for file src to the new holder.
  */
  synchronized Lease reassignLease(Lease lease, String src, String newHolder) {
  return addLease(newHolder, src);
  }
  /**
  * Remove the lease for the specified holder and src
  */
  synchronized void removeLease(String holder, String src) {
  }
  /**
  * Finds the pathname for the specified pendingFile
  */
  synchronized String findPath(INodeFileUnderConstruction pendingFile
  ) throws IOException {
  }
  /**
  * Renew the lease(s) held by the given client
  */
  synchronized void renewLease(String holder) {
  }
  

四. LeaseManager类图