[GitHub] [pulsar] Jennifer88huang commented on a change in pull request #4587: [Doc] Add two methods (log topic & CLI) for debugging functions

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Jennifer88huang commented on a change in pull request #4587: [Doc] Add two methods (log topic & CLI) for debugging functions
URL: https://github.com/apache/pulsar/pull/4587#discussion_r297123869
 
 

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 File path: site2/docs/functions-debugging.md
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 @@ -124,3 +130,322 @@ To use localrun like above programmatically please addd the following dependency
 For complete code samples, see [here](https://github.com/jerrypeng/pulsar-functions-demos/tree/master/debugging)
 
 In the future, debugging with localrun mode for Pulsar Functions written in other languages will be supported.
+
+## Use log topic
+
+The Pulsar Functions allows you to output the log information defined in functions to a specified log topic. Consumers can be configured to consume messages from a specified log topic to check the log information.
+
+![Pulsar Functions core programming model](assets/pulsar-functions-overview.png)
+
+**Example** 
+
+```text
+import org.apache.pulsar.functions.api.Context;
+import org.apache.pulsar.functions.api.Function;
+import org.slf4j.Logger;
+
+public class LoggingFunction implements Function<String, Void> {
+    @Override
+    public void apply(String input, Context context) {
+        Logger LOG = context.getLogger();
+        String messageId = new String(context.getMessageId());
+
+        if (input.contains("danger")) {
+            LOG.warn("A warning was received in message {}", messageId);
+        } else {
+            LOG.info("Message {} received\nContent: {}", messageId, input);
+        }
+
+        return null;
+    }
+}
+```
+
+As shown in the example below, you can get the logger via `context.getLogger()` and assign the logger to the `LOG` variable of `slf4j`, so you can define your desired log information in a function using the `LOG` variable. You also need to specify the topic to which the log information is produced.
 
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