doc(fixes): add state & update kubernetes tool doc

docs/zh_CN/rules/state_and_fault_tolerance.md

@@ -0,0 +1,46 @@
+# State
+
+Kuiper supports stateful rule stream. There are two kinds of states in Kuiper:
+1. Internal state for window operation and rewindable source
+2. User state exposed to extensions with stream context, check [state storage](../extension/overview.md#state-storage).
+
+# Fault Tolerance
+
+By default, all the states reside in memory only which means that if the stream exits abnormally, the states will disappear.
+
+In order to make state fault tolerance, Kuipler need to checkpoint the state into persistent storage which will allow a recovery after failure.
+
+## Enable Checkpointing
+
+Set the rule option qos to 1 or 2 will enable the checkpointing. Configure the checkpoint interval by setting the checkpointInterval option.
+
+When things go wrong in a stream processing application, it is possible to have either lost, or duplicated results. For the 3 options of qos, the behavior will be:
+
+1. At-most-once(0): Kuiper makes no effort to recover from failures
+2. At-least-once(1): Nothing is lost, but you may experience duplicated results
+3. Exactly-once(2): Nothing is lost or duplicated 
+
+Given that Kuiper recovers from faults by rewinding and replaying the source data streams, when the ideal situation is described as exactly once does not mean that every event will be processed exactly once. Instead, it means that every event will affect the state being managed by Kuiper exactly once.
+
+If you don’t need "exactly once", you can gain some performance by configuring Kuiper to use AT_LEAST_ONCE.
+
+## Exactly Once End to End
+
+### Source consideration
+
+To have an end to end qos of the stream, the source must be rewindable. That means after recovery, the source can be reverted to the checkpointed offset and resend data from that so that the whole stream can be replayed from the last failure.
+
+For extended source, the user must implement the api.Rewindable interface as well as the default api.Source interface. Kuiper will handle the rewind internally.
+
+```go
+type Rewindable interface {
+	GetOffset() (interface{}, error)
+	Rewind(offset interface{}) error
+}
+```
+
+### Sink consideration
+
+We cannot guarantee the sink to receive a data exactly once. If failures happen during the period of checkpointing, some states which have sent to the sink may not be checkpointed. And those states will be replayed as they are not restored because of not being checkpointed. In this case, the sink may receive them more than once. 
+
+To implement exactly-once, the user will have to implement deduplication tailored to fit the various sinking system.

tools/kubernetes/README.md

@@ -2,14 +2,15 @@
 
 ### 1.1 Program description:
 
-​    本程序用于监控并处理命令文件夹中的文件。当程序发现命令文件夹下存在新创建的文件或已更新的文件时，程序将加载这些文件并执行文件中的命令，之后程序将处理过的文件名记录在命令文件夹同级目录的 .history 文件中。 .history 的数据格式如下：
+The program is used for monitoring and processing files under folder. If program find any newly added or updated files, the program will read the file and execute the commands in the file. For the files are processed, the program saves the file name into .history file, which is at the same folder for the command file. The content of .history is as following:
 
 ```json
 [
   {
-        "name":"sample.json",	//已处理过文件的文件名
-        "loadTime":1594362344 //处理文件时的时间戳
-    }]
+     "name":"sample.json",	//The file name that has been processed
+     "loadTime":1594362344 //The timestamp of processing file
+  }
+]
 ```
 
 ### 1.2 Command file format and meaning: