Hadoop学习二十六：Hadoop-Hdfs Lease源码

一. Lease

1.  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.
2. 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。
3. 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。
4. 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

5. 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

1.  Monitor checks for leases that have expired, and disposes of them.
2. 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

1.  LeaseManager管理着所有的Lease。
2. 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。
3. 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>();

4. 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