All of the read operations in ZooKeeper - getData(), getChildren(), and exists() - have the option of setting a watch as a side effect. Here is ZooKeeper‘s definition of a watch: a watch event is one-time trigger, sent to the client that set the watch, which occurs when the data for which the watch was set changes. There are three key points to consider in this definition of a watch:

• One-time trigger（如果被节点被删除，再创建的节点需要重新绑定watcher）

  One watch event will be sent to the client when the data has changed. For example, if a client does a getData("/znode1", true) and later the data for /znode1 is changed or deleted, the client will get a watch event for /znode1. If /znode1 changes again, no watch event will be sent unless the client has done another read that sets a new watch.

• Sent to the client

  This implies that an event is on the way to the client, but may not reach the client before the successful return code to the change operation reaches the client that initiated the change. Watches are sent asynchronously异步 to watchers. ZooKeeper provides an ordering guarantee（有序的）: a client will never see a change for which it has set a watch until it first sees the watch event. Network delays or other factors may cause different clients to see watches and return codes from updates at different times. The key point is that everything seen by the different clients will have a consistent order.

• The data for which the watch was set

  This refers to the different ways a node can change. It helps to think of ZooKeeper as maintaining two lists of watches: data watches and child watches. getData() and exists() set data watches. getChildren() sets child watches. Alternatively, it may help to think of watches being set according to the kind of data returned. getData() and exists() return information about the data of the node, whereas getChildren() returns a list of children. Thus, setData() will trigger data watches for the znode being set (assuming the set is successful). A successful create() will trigger a data watch for the znode being created and a child watch for the parent znode. A successful delete() will trigger both a data watch and a child watch (since there can be no more children) for a znode being deleted as well as a child watch for the parent znode.

Watches are maintained locally at the ZooKeeper server to which the client is connected. This allows watches to be lightweight to set, maintain, and dispatch. When a client connects to a new server, the watch will be triggered for any session events. Watches will not be received while disconnected from a server. When a client reconnects, any previously registered watches will be reregistered and triggered if needed（客户端重新连接上时，之前发生的watch事件将会触发）. In general this all occurs transparently. There is one case where a watch may be missed: a watch for the existence of a znode not yet created will be missed if the znode is created and deleted while disconnected（如果在断开连接期间，watch的节点被创建又被删除，那么watch事件会丢失）.

Semantics of Watches

We can set watches with the three calls that read the state of ZooKeeper: exists, getData, and getChildren. The following list details the events that a watch can trigger and the calls that enable them:

• Created event:

  Enabled with a call to exists.

• Deleted event:

  Enabled with a call to exists, getData, and getChildren.

• Changed event:

  Enabled with a call to exists and getData.

• Child event:

  Enabled with a call to getChildren.

What ZooKeeper Guarantees about Watches

With regard to watches, ZooKeeper maintains these guarantees:

• Watches are ordered with respect to other events, other watches, and asynchronous replies. The ZooKeeper client libraries ensures that everything is dispatched in order.

• A client will see a watch event for a znode it is watching before seeing the new data that corresponds to that znode.

• The order of watch events from ZooKeeper corresponds to the order of the updates as seen by the ZooKeeper service.

Things to Remember about Watches

• Watches are one time triggers; if you get a watch event and you want to get notified of future changes, you must set another watch.

• Because watches are one time triggers and there is latency between getting the event and sending a new request to get a watch you cannot reliably see every change that happens to a node in ZooKeeper. Be prepared to handle the case where the znode changes multiple times between getting the event and setting the watch again.也就是说在两次watch之间node实际上可能发生多次变更  (You may not care, but at least realize it may happen.)

• A watch object, or function/context pair, will only be triggered once for a given notification. For example, if the same watch object is registered for an exists and a getData call for the same file and that file is then deleted, the watch object would only be invoked once with the deletion notification for the file.

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• When you disconnect from a server (for example, when the server fails), you will not get any watches until the connection is reestablished. For this reason session events are sent to all outstanding watch handlers. Use session events to go into a safe mode: you will not be receiving events while disconnected, so your process should act conservatively in that mode.（谨慎处理与server断开连接的情况）