[improve][doc] Add doc for builtin functions (#18569)

Co-authored-by: tison <[email protected]>

site2/docs/functions-deploy-cluster-builtin.md

@@ -0,0 +1,16 @@
+---
+id: functions-deploy-cluster-builtin
+title: Use built-in functions
+sidebar_label: "Use built-in functions"
+---
+
+Similar to built-in connectors, the code of Java functions [packaged as NAR](functions-package-java.md) that are placed in the `functions` directory of the function worker are loaded at startup and can be referenced when creating a function.
+
+For instance if you have a built-in function with name `exclamation` in its `pulsar-io.yaml`, you can create a function instance with:
+
+```bash
+bin/pulsar-admin functions create \
+  --function-type exclamation \
+  --inputs persistent://public/default/input-1 \
+  --output persistent://public/default/output-1
+```

site2/docs/functions-deploy-cluster.md

@@ -31,4 +31,5 @@ bin/pulsar-admin functions update \
 * [Enable parallel processing](functions-deploy-cluster-parallelism.md)
 * [Enable end-to-end encryption](functions-deploy-cluster-encryption.md)
 * [Enable package management service](functions-deploy-cluster-package.md)
+* [Use built-in functions](functions-deploy-cluster-builtin.md)
 

site2/docs/functions-package-java.md

@@ -4,13 +4,25 @@ title: Package Java Functions
 sidebar_label: "Package Java Functions"
 ---
 
+There are two methods to package Java Functions, that is [uber JAR](#package-as-jar) and [NAR](#package-as-nar).
+
 :::note
 
+If you plan to package and distribute your function for others to use, you are obligated to
+license and copyright your own code properly. Remember to add the license and copyright to
+all libraries your code uses and to your distribution.
+
+If you use the [NAR](#package-as-nar) method, the NAR plugin
+automatically creates a `DEPENDENCIES` file in the generated NAR package, including the proper
+licensing and copyrights of all libraries of your function.
+
 For the runtime Java version, refer to [Pulsar Runtime Java Version Recommendation](https://github.com/apache/pulsar/blob/master/README.md#pulsar-runtime-java-version-recommendation) according to your target Pulsar version.
 
 :::
 
-To package a Java function, complete the following steps.
+## Package as JAR
+
+To package a Java function as JAR, complete the following steps.
 
 1. Create a new maven project with a pom file. In the following code sample, the value of `mainClass` is your package name.
 
@@ -106,3 +118,90 @@ To package a Java function, complete the following steps.
     07:55:03.724 [main] INFO  org.apache.pulsar.functions.runtime.ProcessRuntime - Started process successfully
     ...
    ```
+
+## Package as NAR
+
+To package a Java function as NAR, complete the following steps.
+
+1. Create a new maven project with a pom file.
+
+   ```xml
+    <?xml version="1.0" encoding="UTF-8"?>
+    <project xmlns="http://maven.apache.org/POM/4.0.0"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+        <modelVersion>4.0.0</modelVersion>
+
+        <groupId>java-function</groupId>
+        <artifactId>java-function</artifactId>
+        <version>1.0-SNAPSHOT</version>
+
+        <dependencies>
+            <dependency>
+                <groupId>org.apache.pulsar</groupId>
+                <artifactId>pulsar-functions-api</artifactId>
+                <version>@pulsar:version@</version>
+            </dependency>
+        </dependencies>
+
+        <build>
+            <plugins>
+                <plugin>
+                    <groupId>org.apache.nifi</groupId>
+                    <artifactId>nifi-nar-maven-plugin</artifactId>
+                    <version>1.2.0</version>
+                    </plugin>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-compiler-plugin</artifactId>
+                    <configuration>
+                        <release>17</release>
+                    </configuration>
+                </plugin>
+            </plugins>
+        </build>
+
+    </project>
+   ```
+
+   You must also create a `resources/META-INF/services/pulsar-io.yaml` file. In the following code sample, the value of `functionClass` is your function class name. The `name` is the one used when the Function is deployed as a [built-in](functions-deploy-cluster-builtin.md) one.
+
+   ```yaml
+   name: java-function
+   description: my java function
+   functionClass: org.example.test.ExclamationFunction
+   ```
+
+2. Package your Java function.
+
+   ```bash
+   mvn package
+   ```
+
+   After the Java function is packaged, a `target` directory is created automatically. Open the `target` directory to check if there is a NAR package similar to `java-function-1.0-SNAPSHOT.nar`.
+
+3. Copy the packaged NAR file to the Pulsar image.
+
+   ```bash
+    docker cp <path of java-function-1.0-SNAPSHOT.nar>  CONTAINER ID:/pulsar
+   ```
+
+4. Run the Java function using the following command.
+
+   ```bash
+    ./bin/pulsar-admin functions localrun \
+    --jar java-function-1.0-SNAPSHOT.nar \
+    --inputs persistent://public/default/my-topic-1 \
+    --output persistent://public/default/test-1 \
+    --tenant public \
+    --namespace default \
+    --name JavaFunction
+   ```
+
+   The following log indicates that the Java function starts successfully.
+
+   ```text
+    ...
+    07:55:03.724 [main] INFO  org.apache.pulsar.functions.runtime.ProcessRuntime - Started process successfully
+    ...
+   ```

site2/website/sidebars.json

@@ -102,7 +102,8 @@
                 "functions-deploy-cluster-resource",
                 "functions-deploy-cluster-parallelism",
                 "functions-deploy-cluster-encryption",
-                "functions-deploy-cluster-package"
+                "functions-deploy-cluster-package",
+                "functions-deploy-cluster-builtin"
               ]
             },
             "functions-deploy-trigger"