Jennifer88huang commented on a change in pull request #4485: add docs on how to
debug Pulsar Functions
URL: https://github.com/apache/pulsar/pull/4485#discussion_r291824875
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File path: site2/docs/functions-debugging.md
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@@ -0,0 +1,121 @@
+---
+id: functions-debugging
+title: Debugging Pulsar Functions
+sidebar_label: Debugging functions
+---
+
+A Pulsar Function at its core is just a function with inputs and outputs, thus
testing a Pulsar Function can be done in a similar way as testing any function.
+
+For example, if a user has the following Pulsar Function:
+
+```java
+import java.util.function.Function;
+
+public class JavaNativeExclamationFunction implements Function<String, String>
{
+ @Override
+ public String apply(String input) {
+ return String.format("%s!", input);
+ }
+}
+```
+
+The user can write a simple unit test to test this Pulsar function:
+
+```java
+@Test
+public void testJavaNativeExclamationFunction() {
+ JavaNativeExclamationFunction exclamation = new
JavaNativeExclamationFunction();
+ String output = exclamation.apply("foo");
+ Assert.assertEquals(output, "foo!");
+}
+```
+
+Consequently, if a user has a Pulsar Function that implements the
```org.apache.pulsar.functions.api.Function``` interface:
+
+```java
+import org.apache.pulsar.functions.api.Context;
+import org.apache.pulsar.functions.api.Function;
+
+public class ExclamationFunction implements Function<String, String> {
+ @Override
+ public String process(String input, Context context) {
+ return String.format("%s!", input);
+ }
+}
+```
+
+The user can write a unit test for this function as well. Just be remember to
mock out the ```Context``` parameter.
+
+For example:
+
+```java
+@Test
+public void testExclamationFunction() {
+ ExclamationFunction exclamation = new ExclamationFunction();
+ String output = exclamation.process("foo", mock(Context.class));
+ Assert.assertEquals(output, "foo!");
+}
+```
+
+## Debugging with localrun mode
+
+> Please note that there is currenlty only support for this for Pulsar
Functions written in Java. Users will need a Pulsar version > 2.4.9 to do the
following. Even though localrun is available in older version of Pulsar, it
does not have the functionally to be executed programmatically and run
functions as threads.
+
+To test in a more realistic fashion, a Pulsar Function can be run via localrun
mode which will launch a instance of the function on your local machine as a
thread.
+
+In this mode, the Pulsar Function can consume and produce actual data to a
Pulsar Cluster and mirrors how the function will actually run in a Pulsar
Cluster.
+
+Users can launch his or her function in the following manner:
+
+```java
+FunctionConfig functionConfig = new FunctionConfig();
+functionConfig.setName(functionName);
+functionConfig.setInputs(Collections.singleton(sourceTopic));
+functionConfig.setClassName(ExclamationFunction.class.getName());
+functionConfig.setRuntime(FunctionConfig.Runtime.JAVA);
+functionConfig.setOutput(sinkTopic);
+
+LocalRunner localRunner =
LocalRunner.builder().functionConfig(functionConfig).build();
+localRunner.start(true);
+```
+
+This allows users to easily debug functions using an IDE. User's can set
break points and manually set through a function to debug will real data.
+
+The code snippet below illustrates a more complete example of how to use
launch a function via localrun programmatically.
Review comment:
```suggestion
The following code example illustrates how to launch a function via the
localrun mode.
```
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