You are viewing documentation for an older version of Confluent Platform. For the latest, click here.

Implement a User-defined Function (UDF and UDAF)

  • Apache Maven
  • Confluent Platform installed locally
  • Internet connectivity for downloading Confluent POM files

Create a user-defined function (UDF) or a user-defined aggregation function (UDAF) by following these steps:

  1. Create the KSQL extensions directory that contains your UDF and UDAF packages.
  2. Create Java source and project files for your implementation.
  3. Build the package for your UDF or UDAF.
  4. Use your custom function in a KSQL query or statement.

For more information on custom functions, see KSQL Custom Function Reference (UDF and UDAF).

Create the KSQL Extensions Directory

When you create a custom user-defined function (UDF), you implement it in Java and deploy it as a JAR to the KSQL extensions directory. By default, this directory doesn’t exist, so you need to create it and assign it in the KSQL Server configuration properties.

Create the KSQL extensions directory, <path-to-confluent>/etc/ksql/ext:

mkdir confluent-5.1.4/etc/ksql/ext

Edit the configuration file in <path-to-confluent>/etc/ksql to add the fully qualified path to the ext directory:



Use the fully qualified path or the relative path from <path-to-confluent>/bin, which is ../etc/ksql/ext. KSQL Server won’t load extensions if the path begins with ~.

Create the Source and Project Files

The following steps shows how to implement your UDF in a Java class and build it by defining a Maven POM file.

  1. Create a root directory for your UDF’s source code and project files.
  2. Create the source code directory, which has a path that corresponds with the package name.
  3. Create the Java source code file in the source code directory.
  4. Create a Project Object Model (POM) file that defines how Maven builds the source code.

Create a Project Root Directory

Create the directory that holds your UDF or UDAF project:

mkdir ksql-udf-demo && cd ksql-udf-demo

Create the Source Code Directory

From the root directory for your UDF, create the source code directory. In this example, the package name is

mkdir -p src/main/java/my/company/ksql/udfdemo

Create the Java Source Code File

The following Java code defines four overloads for a multiply function. The UdfDescription and Udf annotations tell KSQL Server to load the Multiply class and look for methods to add to its list of available functions. For more information, see KSQL Custom Function Reference (UDF and UDAF).

Copy the following code into a new file, named


import io.confluent.ksql.function.udf.Udf;
import io.confluent.ksql.function.udf.UdfDescription;

@UdfDescription(name = "multiply", description = "multiplies 2 numbers")
public class Multiply {

  @Udf(description = "multiply two non-nullable INTs.")
  public long multiply(final int v1, final int v2) {
    return v1 * v2;

  @Udf(description = "multiply two non-nullable BIGINTs.")
  public long multiply(final long v1, final long v2) {
    return v1 * v2;

  @Udf(description = "multiply two nullable BIGINTs. If either param is null, null is returned.")
  public Long multiply(final Long v1, final Long v2) {
    return v1 == null || v2 == null ? null : v1 * v2;

  @Udf(description = "multiply two non-nullable DOUBLEs.")
  public double multiply(final double v1, double v2) {
    return v1 * v2;

Save the file to the source code directory that you created in the previous step, src/main/java/my/company/ksql/udfdemo.

Create the POM File

In the root directory for your custom UDF implementation, create the Project Object Model (POM) file for the Maven build, and name it pom.xml:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns=""

    <!-- Specify the package details for the custom UDF -->

    <!-- Specify the repository for Confluent dependencies -->

    <!-- Specify build properties -->

    <!-- Specify the ksql-udf dependency -->
        <!-- KSQL dependency is needed to write your own UDF -->

    <!-- Build boilerplate -->

            <!-- Package all dependencies as one jar -->


For production environments, we strongly recommend that you write comprehensive tests to cover your custom functions.

Build the UDF Package

Use Maven to build the package and create a JAR. Copy the JAR to the KSQL extensions directory.

In the root folder for your UDF, run Maven to build the package:

mvn clean package

After a great deal of build info, your output should resemble:

[INFO] --- maven-assembly-plugin:2.5.2:single (assemble-all) @ ksql-udf-demo ---
[INFO] Building jar: /home/my-home-dir/ksql-udf-demo/target/ksql-udf-demo-1.0-jar-with-dependencies.jar
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 17.511 s
[INFO] Finished at: 2018-12-17T22:07:08Z
[INFO] Final Memory: 26M/280M
[INFO] ------------------------------------------------------------------------

The Maven build creates a directory named target and saves the build output there. Copy the JAR file, ksql-udf-demo-1.0-jar-with-dependencies.jar, from the target directory to the ext directory of your KSQL installation. For example, if your Confluent Platform installation is at /home/my-home-dir/confluent-5.1.4, copy the JAR to /home/my-home-dir/confluent-5.1.4/etc/ksql/ext.

cp target/ksql-udf-demo-1.0-jar-with-dependencies.jar <path-to-confluent>/etc/ksql/ext

The custom UDF is deployed and ready to run.

Use Your Custom UDF in a KSQL Query

When your custom UDF is deployed in the KSQL extensions directory, it’s loaded automatically when you start KSQL Server, and you can use it like you use the other KSQL functions.


KSQL loads UDFs and UDAFs only on startup, so when you make changes to your UDF code and re-deploy the JAR, you must restart KSQL Server to get the latest version of your UDF.

Start Confluent Platform and KSQL Server:

<path-to-confluent>/bin/confluent start ksql-server

Start the KSQL CLI:

LOG_DIR=./ksql_logs <path-to-confluent>/bin/ksql

In the KSQL CLI, list the available functions to ensure that KSQL Server loaded the MULTIPLY user-defined function:


Your output should resemble:

 Function Name     | Type
 ABS               | SCALAR
 ...               |
 ...               |
 SUM               | AGGREGATE
 ...               |

Inspect the details of the MULTIPLY function:


Your output should resemble:

Name        : MULTIPLY
Overview    : multiplies 2 numbers
Type        : scalar
Jar         : /home/my-home-dir/confluent-5.1.4|/etc/ksql/ext/ksql-udf-demo-1.0-jar-with-dependencies.jar
Variations  :

    Variation   : MULTIPLY(BIGINT, BIGINT)
    Returns     : BIGINT
    Description : multiply two nullable BIGINTs. If either param is null, null is

    Variation   : MULTIPLY(DOUBLE, DOUBLE)
    Returns     : DOUBLE
    Description : multiply two non-nullable DOUBLEs.

    Variation   : MULTIPLY(INT, INT)
    Returns     : BIGINT
    Description : multiply two non-nullable INTs.

Use the MULTIPLY function in a query. If you follow the steps in Writing Streaming Queries Against Apache Kafka® Using KSQL (Local), you can multiply the two BIGINT fields in the pageviews_original stream:

SELECT MULTIPLY(rowtime, viewtime) FROM pageviews_original;

Your output should resemble:

^CQuery terminated

Press Ctrl+C to terminate the query.

Custom Aggregation Function (UDAF)

Implementing a user-defined aggregation function (UDAF) is similar to the way that you implement a UDF. You use the UdafDescription and UdafFactory annotations in your Java code, and you deploy a JAR to the KSQL extensions directory. For more information, see UDAFs.