[doc] update description about safe checkpoint (#67)

docs/formats/logmessage-cn.md

@@ -118,13 +118,17 @@
 
 ### 安全位点
 
-LogMessage 提供了 `safeTimestamp` 来表示数据接收的安全位点，也就是说早于该秒级时间戳提交的 LogMessage 均已被客户端接收。
+LogMessage 提供了 `safeTimestamp` 来表示当前数据对应的安全位点，它是一个秒级的时间戳，含义为：早于该时间戳提交的 LogMessage 均已被客户端接收。
 
-业务应用在进行数据消费时，一般还要维护一个数据处理的安全位点。在 LogMessage 中，该安全位点需要借助心跳的 `timestamp` 来实现。 LogMessage 在时间存储上有两套逻辑：
+```java
+long checkpoint = Long.parseLong(message.getSafeTimestamp());
+```
+
+对于 OceanBase 而言，该安全位点是借助心跳的 `timestamp` 来实现的。 OceanBase 的 LogMessage 在时间存储上有两套逻辑：
 - 心跳类型：`timestamp` 字段值为安全位点对应的秒级时间戳。
 - 其他类型：`timestamp` 字段值为数据变动的提交时间，而 `fileNameOffset` 字段对应最近一次心跳信息的 `timestamp`。由于 libobcdc 并不保证拉取到的数据变动是严格按照时间顺序的，因此对于 DDL、DML 类型的 LogMessage，应当使用 `fileNameOffset` 而非 `timestamp` 作为安全位点。
 
-获取当前数据对应安全位点可以使用如下代码：
+因此，OceanBase LogMessage 的安全位点也可以借助以下代码获取:
 
 ```java
 long checkpoint;

docs/formats/logmessage.md

@@ -118,13 +118,17 @@ Please refer to [LogProxyClientTest.java](../../oblogclient-logproxy/src/test/ja
 
 ### Safe Checkpoint
 
-LogMessage provides `safeTimestamp` to indicate the safe checkpoint for data reception, that is to say, LogMessage committed earlier than this timestamp has been received by the client.
+LogMessage provides `safeTimestamp` to indicate the safe checkpoint, it is a timestamp in seconds, which means all the log messages committed earlier than this timestamp have been received by the client.
 
-When a application consumes data, it generally maintains a safe checkpoint for data processing. For LogMessage, we should use HEARTBEAT `timestamp` as the safe checkpoint. LogMessage contains two kinds of timestamp:
+```java
+long checkpoint = Long.parseLong(message.getSafeTimestamp());
+```
+
+For LogMessage of OceanBase, there are two kinds of timestamp:
 - HEARTBEAT type: the value of the `timestamp` field is the timestamp corresponding to the safe checkpoint.
 - Other types: the value of the `timestamp` field is the execution time of the data change, and the `fileNameOffset` field corresponds to the latest HEARTBEAT timestamp. Since `libobcdc` does not guarantee that the fetched data changes are in timestamp order, so for DDL and DML types of LogMessage, `fileNameOffset` should be used as safe checkpoint instead of `timestamp`.
 
-The following code can be used to obtain the safe checkpoint corresponding to the current data:
+So you can also get the safe checkpoint of OceanBase LogMessage by the following code:
 
 ```java
 long checkpoint;