Hadoop学习二十六：Hadoop-Hdfs Lease源码
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 withnewpath
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
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