zookeeper读书笔记十二 zookeeper应用之实现分布式锁
- client想要获得锁需要执行以下步骤
- Call create( ) with a pathname of "locknode/guid-lock-" and the sequence and ephemeral flags set. The guid is needed in case the create() result is missed. See the note below.
- Call getChildren( ) on the lock node without setting the watch flag (this is important to avoid the herd effect).
- If the pathname created in step 1 has the lowest sequence number suffix, the client has the lock and the client exits the protocol.
- The client calls exists( ) with the watch flag set on the path in the lock directory with the next lowest sequence number.
- if exists( ) returns null, go to step 2. Otherwise, wait for a notification for the pathname from the previous step before going to step 2.
- 实现分布式读写锁
获取读锁的关键判断，只要当前子节点前没有写锁的子节点，就可以获得读锁。否则要等前面的写锁对应的节点删除才能获取读锁。
获取写锁的关键判断，只要当前子节点前面有任何子节点，都不能获取锁，要等前面的子节点都删除，才能获取锁。
| Obtaining a read lock: | Obtaining a write lock: |
|---|---|
| 1. Call create( ) to create a node with pathname "guid-/read-". This is the lock node use later in the protocol. Make sure to set both the sequence and ephemeral flags. | 1. Call create( ) to create a node with pathname "guid-/write-". This is the lock node spoken of later in the protocol. Make sure to set both sequence and ephemeral flags. |
| 2. Call getChildren( ) on the lock node without setting the watch flag - this is important, as it avoids the herd effect. | 2. Call getChildren( ) on the lock node without setting the watch flag - this is important, as it avoids the herd effect. |
| 3. If there are no children with a pathname starting with "write-" and having a lower sequence number than the node created in step 1, the client has the lock and can exit the protocol. | 3. If there are no children with a lower sequence number than the node created in step 1, the client has the lock and the client exits the protocol. |
| 4. Otherwise, call exists( ), with watch flag, set on the node in lock directory with pathname staring with "write-" having the next lowest sequence number. | 4. Call exists( ), with watch flag set, on the node with the pathname that has the next lowest sequence number. |
| 5. If exists( ) returns false, goto step 2. | 5. If exists( ) returns false, goto step 2. Otherwise, wait for a notification for the pathname from the previous step before going to step 2. |
| 6. Otherwise, wait for a notification for the pathname from the previous step before going to step 2 |