[GitHub] [skywalking] wu-sheng commented on a change in pull request #4555: Correlation protocol implement

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wu-sheng commented on a change in pull request #4555: Correlation protocol implement
URL: https://github.com/apache/skywalking/pull/4555#discussion_r396327981
 
 

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 File path: docs/en/setup/service-agent/java-agent/Application-toolkit-correlation.md
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 @@ -0,0 +1,33 @@
+## Cross Process Correlation
+
+## Introduce
+This plugin is help user to transport custom data in the tracing context. [Follow this to get protocol description.](../../../protocols/Skywalking-Cross-Process-Correlation-Headers-Protocol-v1.md)
+
+## How to use it
+* Dependency the toolkit, such as using maven or gradle
+```xml
+   <dependency>
+      <groupId>org.apache.skywalking</groupId>
+      <artifactId>apm-toolkit-trace</artifactId>
+      <version>${skywalking.version}</version>
+   </dependency>
+```
+
+* Use `CorrelationContext.set()` API to setting custom data.
+```java
+CorrelationSettingResult settingResult = CorrelationContext.set("customKey", "customValue");
+```
+_Sample codes only_
+
+1. Add `CorrelationContext.set` to setting your custom data.
+1. Key and value only support `String` type.
+1. Please follow [the agent configuration](README.md#table-of-agent-configuration-properties) to get more setting limit.
+
+* Use `CorrelationContext.get()` API to get custom data.
+```java
+CorrelationContext.get("customKey");
+```
+_Sample codes only_
+
+1. `CorrelationContext.get` to get custom data.
+1. Return empty string if cannot found the custom key.
 
 Review comment:
   I think in the JDK 1.8, the recommended way is returning `option`. We can't have that because agent used to be JDK1.6 compatible, but we are not anymore. The new API should follow the JDK1.8 sytle.

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