How to develop Pulsar connectors

This guide describes how to develop Pulsar connectors to move data between Pulsar and other systems.

Pulsar connectors are special Pulsar Functions, so creating a Pulsar connector is similar to creating a Pulsar function.

Pulsar connectors come in two types:

Type	Description	Example
Source	Import data from another system to Pulsar.	RabbitMQ source connector imports the messages of a RabbitMQ queue to a Pulsar topic.
Sink	Export data from Pulsar to another system.	Kinesis sink connector exports the messages of a Pulsar topic to a Kinesis stream.

Develop

You can develop Pulsar source connectors and sink connectors.

Source

Developing a source connector is to implement the Source interface, which means you need to implement the open method and the read method.

1. Implement the open method.

   
   /**
   * Open connector with configuration
   *
   * @param config initialization config
   * @param sourceContext
   * @throws Exception IO type exceptions when opening a connector
   */
   void open(final Map<String, Object> config, SourceContext sourceContext) throws Exception;
   

   This method is called when the source connector is initialized.

   In this method, you can retrieve all connector specific settings through the passed-in config parameter and initialize all necessary resources.

   For example, a Kafka connector can create a Kafka client in this open method.

   Besides, Pulsar runtime also provides a SourceContext for the connector to access runtime resources for tasks like collecting metrics. The implementation can save the SourceContext for future use.

2. Implement the read method.

   
       /**
       * Reads the next message from source.
       * If source does not have any new messages, this call should block.
       * @return next message from source.  The return result should never be null
       * @throws Exception
       */
       Record<T> read() throws Exception;
   

   If nothing to return, the implementation should be blocking rather than returning null.

   The returned Record should encapsulate the following information, which is needed by Pulsar IO runtime.

   • Record should provide the following variables:

     Variable	Required	Description
     TopicName	No	Pulsar topic name from which the record is originated from.
     Key	No	Messages can optionally be tagged with keys. For more information, see Routing modes.
     Value	Yes	Actual data of the record.
     EventTime	No	Event time of the record from the source.
     PartitionId	No	If the record is originated from a partitioned source, it returns its PartitionId. PartitionId is used as a part of the unique identifier by Pulsar IO runtime to deduplicate messages and achieve exactly-once processing guarantee.
     RecordSequence	No	If the record is originated from a sequential source, it returns its RecordSequence. RecordSequence is used as a part of the unique identifier by Pulsar IO runtime to deduplicate messages and achieve exactly-once processing guarantee.
     Properties	No	If the record carries user-defined properties, it returns those properties.
     DestinationTopic	No	Topic to which message should be written.
     Message	No	A class which carries data sent by users. For more information, see Message.java.

   • Record should provide the following methods:

     Method	Description
     ack	Acknowledge that the record is fully processed.
     fail	Indicate that the record fails to be processed.

tip

For more information about how to create a source connector, see KafkaSource.

Sink

Developing a sink connector is similar to developing a source connector, that is, you need to implement the Sink interface, which means implementing the open method and the write method.

1. Implement the open method.

   
       /**
       * Open connector with configuration
       *
       * @param config initialization config
       * @param sinkContext
       * @throws Exception IO type exceptions when opening a connector
       */
       void open(final Map<String, Object> config, SinkContext sinkContext) throws Exception;
   

2. Implement the write method.

   
       /**
       * Write a message to Sink
       * @param record record to write to sink
       * @throws Exception
       */
       void write(Record<T> record) throws Exception;
   

   During the implementation, you can decide how to write the Value and the Key to the actual source, and leverage all the provided information such as PartitionId and RecordSequence to achieve different processing guarantees.

   You also need to ack records (if messages are sent successfully) or fail records (if messages fail to send).

Test

Testing connectors can be challenging because Pulsar IO connectors interact with two systems that may be difficult to mock—Pulsar and the system to which the connector is connecting.

It is recommended writing special tests to test the connector functionalities as below while mocking the external service.

Unit test

You can create unit tests for your connector.

Integration test

Once you have written sufficient unit tests, you can add separate integration tests to verify end-to-end functionality.

Pulsar uses testcontainers for all integration tests.

tip

For more information about how to create integration tests for Pulsar connectors, see IntegrationTests.

Package

Once you've developed and tested your connector, you need to package it so that it can be submitted to a Pulsar Functions cluster.

There are two methods to work with Pulsar Functions' runtime, that is, NAR and uber JAR.

note

If you plan to package and distribute your connector 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 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 connector.

NAR

NAR stands for NiFi Archive, which is a custom packaging mechanism used by Apache NiFi, to provide a bit of Java ClassLoader isolation.

tip

For more information about how NAR works, see here.

Pulsar uses the same mechanism for packaging all built-in connectors.

The easiest approach to package a Pulsar connector is to create a NAR package using nifi-nar-maven-plugin.

Include this nifi-nar-maven-plugin in your maven project for your connector as below.

<plugins>
  <plugin>
    <groupId>org.apache.nifi</groupId>
    <artifactId>nifi-nar-maven-plugin</artifactId>
    <version>1.2.0</version>
  </plugin>
</plugins>

You must also create a resources/META-INF/services/pulsar-io.yaml file with the following contents:

name: connector name
description: connector description
sourceClass: fully qualified class name (only if source connector)
sinkClass: fully qualified class name (only if sink connector)

For Gradle users, there is a Gradle Nar plugin available on the Gradle Plugin Portal.

tip

For more information about an how to use NAR for Pulsar connectors, see TwitterFirehose.

Uber JAR

An alternative approach is to create an uber JAR that contains all of the connector's JAR files and other resource files. No directory internal structure is necessary.

You can use maven-shade-plugin to create a uber JAR as below:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-shade-plugin</artifactId>
  <version>3.1.1</version>
  <executions>
    <execution>
      <phase>package</phase>
      <goals>
        <goal>shade</goal>
      </goals>
      <configuration>
        <filters>
          <filter>
            <artifact>*:*</artifact>
          </filter>
        </filters>
      </configuration>
    </execution>
  </executions>
</plugin>