Create a stream reactor source connector from Apache Cassandra® to Apache Kafka®
================================================================================

**The Apache Cassandra® stream reactor source connector** enables you to move data from **a Apache Cassandra® database** to **an Aiven for Apache Kafka® cluster**. The Lenses.io implementation enables you to write `KCQL transformations <https://docs.lenses.io/5.0/integrations/connectors/stream-reactor/sources/cassandrasourceconnector/>`_ on the topic data before sending it to the Apache Kafka cluster.

.. note::

    You can check the full set of available parameters and configuration options in the `connector's documentation <https://docs.lenses.io/connectors/source/cassandra.html>`_.


.. _connect_cassandra_lenses_source_prereq:

Prerequisites
-------------

To setup a Apache Cassandra source connector, you need an Aiven for Apache Kafka service :doc:`with Kafka Connect enabled <enable-connect>` or a :ref:`dedicated Aiven for Apache Kafka Connect cluster <apache_kafka_connect_dedicated_cluster>`.

Furthermore you need to collect the following information about the target Cassandra database upfront:

* ``CASSANDRA_HOSTNAME``: The Cassandra hostname
* ``CASSANDRA_PORT``: The Cassandra port
* ``CASSANDRA_USERNAME``: The Cassandra username
* ``CASSANDRA_PASSWORD``: The Cassandra password
* ``CASSANDRA_SSL``: The Cassandra SSL setting, can be ``true``, ``false`` or ``default``
* ``CASSANDRA_KEYSTORE``: The path to the Keystore containing the CA certificate, used when connection is secured via SSL
* ``CASSANDRA_KEYSTORE_PASSWORD``: The Keystore password, used when connection is secured via SSL 

.. Note::

    If you're using Aiven for Apache Cassandra, you can use the following keystore values
    
    * ``CASSANDRA_TRUSTSTORE``: ``/run/aiven/keys/public.truststore.jks``
    * ``CASSANDRA_TRUSTSTORE_PASSWORD``: ``password``

* ``CASSANDRA_KEYSPACE``: The Cassandra keyspace to use to source the data from
* ``KCQL_TRANSFORMATION``: The KCQL syntax to parse the topic data, should be in the format:

  .. code::

     INSERT INTO APACHE_KAFKA_TOPIC
     SELECT LIST_OF_FIELDS 
     FROM CASSANDRA_TABLE
     [PK CASSANDRA_TABLE_COLUMN]
     [INCREMENTAL_MODE=MODE]

  .. Warning::

     By default the connector acts in **bulk mode**, extracting all the rows from the Cassandra table on a polling interval and pushing them to the Apache Kafka topic. You can however define **incremental options** by defining the `incremental mode and primary key <https://docs.lenses.io/5.0/integrations/connectors/stream-reactor/sources/cassandrasourceconnector/>`_.

* ``APACHE_KAFKA_HOST``: The hostname of the Apache Kafka service, only needed when using Avro as data format
* ``SCHEMA_REGISTRY_PORT``: The Apache Kafka's schema registry port, only needed when using Avro as data format
* ``SCHEMA_REGISTRY_USER``: The Apache Kafka's schema registry username, only needed when using Avro as data format
* ``SCHEMA_REGISTRY_PASSWORD``: The Apache Kafka's schema registry user password, only needed when using Avro as data format


.. Note::

    If you're using Aiven for Cassandra and Aiven for Apache Kafka, the above details are available in the `Aiven console <https://console.aiven.io/>`_ service *Overview tab* or via the dedicated ``avn service get`` command with the :ref:`Aiven CLI <avn_service_get>`.

    The ``SCHEMA_REGISTRY`` related parameters are available in the Aiven for Apache Kafka® service page, *Overview* tab, and *Schema Registry* subtab

    As of version 3.0, Aiven for Apache Kafka no longer supports Confluent Schema Registry. For more information, read `the article describing the replacement, Karapace <https://help.aiven.io/en/articles/5651983>`_

Setup an Apache Cassandra source connector with Aiven Console
-------------------------------------------------------------

The following example demonstrates how to setup an Apache Cassandra source connector for Apache Kafka using the `Aiven Console <https://console.aiven.io/>`_.

Define a Kafka Connect configuration file
'''''''''''''''''''''''''''''''''''''''''

Define the connector configurations in a file (we'll refer to it with the name ``cassandra_source.json``) with the following content, creating a file is not strictly necessary but allows to have all the information in one place before copy/pasting them in the `Aiven Console <https://console.aiven.io/>`_:

.. code-block:: json

    {
        "name":"CONNECTOR_NAME",
        "connector.class": "com.datamountaineer.streamreactor.connect.cassandra.source.CassandraSourceConnector",
        "connect.cassandra.host": "CASSANDRA_HOSTNAME",
        "connect.cassandra.port": "CASSANDRA_PORT",
        "connect.cassandra.username": "CASSANDRA_USERNAME",
        "connect.cassandra.password": "CASSANDRA_PASSWORD",
        "connect.cassandra.ssl.enabled": "CASSANDRA_SSL",
        "connect.cassandra.trust.store.path": "CASSANDRA_TRUSTSTORE",
        "connect.cassandra.trust.store.password": "CASSANDRA_TRUSTSTORE_PASSWORD",
        "connect.cassandra.key.space": "CASSANDRA_KEYSPACE",
        "connect.cassandra.kcql": "KCQL_TRANSFORMATION",
        "key.converter": "io.confluent.connect.avro.AvroConverter",
        "key.converter.schema.registry.url": "https://APACHE_KAFKA_HOST:SCHEMA_REGISTRY_PORT",
        "key.converter.basic.auth.credentials.source": "USER_INFO",
        "key.converter.schema.registry.basic.auth.user.info": "SCHEMA_REGISTRY_USER:SCHEMA_REGISTRY_PASSWORD",
        "value.converter": "io.confluent.connect.avro.AvroConverter",
        "value.converter.schema.registry.url": "https://APACHE_KAFKA_HOST:SCHEMA_REGISTRY_PORT",
        "value.converter.basic.auth.credentials.source": "USER_INFO",
        "value.converter.schema.registry.basic.auth.user.info": "SCHEMA_REGISTRY_USER:SCHEMA_REGISTRY_PASSWORD"
    }

The configuration file contains the following entries:

* ``name``: the connector name, replace ``CONNECTOR_NAME`` with the name you want to use for the connector.
* ``connect.cassandra.*``: source parameters collected in the :ref:`prerequisite <connect_cassandra_lenses_source_prereq>` phase. 

* ``key.converter`` and ``value.converter``:  defines the messages data format in the Apache Kafka topic. The ``io.confluent.connect.avro.AvroConverter`` converter translates messages from the Avro format. To retrieve the messages schema we use Aiven's `Karapace schema registry <https://github.com/aiven/karapace>`_ as specified by the ``schema.registry.url`` parameter and related credentials.

.. Note::

    The ``key.converter`` and ``value.converter`` sections define how the topic messages will be parsed and needs to be included in the connector configuration. 

    When using Avro as source data format, you need to set following parameters

    * ``value.converter.schema.registry.url``: pointing to the Aiven for Apache Kafka schema registry URL in the form of ``https://APACHE_KAFKA_HOST:SCHEMA_REGISTRY_PORT`` with the ``APACHE_KAFKA_HOST`` and ``SCHEMA_REGISTRY_PORT`` parameters :ref:`retrieved in the previous step <connect_cassandra_lenses_source_prereq>`.
    * ``value.converter.basic.auth.credentials.source``: to the value ``USER_INFO``, since you're going to login to the schema registry using username and password.
    * ``value.converter.schema.registry.basic.auth.user.info``: passing the required schema registry credentials in the form of ``SCHEMA_REGISTRY_USER:SCHEMA_REGISTRY_PASSWORD`` with the ``SCHEMA_REGISTRY_USER`` and ``SCHEMA_REGISTRY_PASSWORD`` parameters :ref:`retrieved in the previous step <connect_cassandra_lenses_source_prereq>`. 


Create a Kafka Connect connector with the Aiven Console
'''''''''''''''''''''''''''''''''''''''''''''''''''''''

To create a Kafka Connect connector, follow these steps: 

1. Log in to the `Aiven Console <https://console.aiven.io/>`_ and select the Aiven for Apache Kafka® or Aiven for Apache Kafka Connect® service where the connector needs to be defined. 
2. Select **Connectors** from the left sidebar. 
3. Select **Create New Connector**, the button is enabled only for services :doc:`with Kafka Connect enabled <enable-connect>`.
4. Select **Stream Reactor Cassandra Source**. 
5. In the **Common** tab, locate the **Connector configuration** text box and select on **Edit**.
6. Paste the connector configuration (stored in the ``cassandra_source.json`` file) in the form.
7. Select **Apply**.

   .. Note::

      The Aiven Console parses the configuration file and fills the relevant UI fields. You can review the UI fields across the various tab and change them if necessary. The changes will be reflected in JSON format in the **Connector configuration** text box.

8. After all the settings are correctly configured, select **Create new connector**. 
9. Verify the connector status under the **Connectors** screen. 
10. Verify the presence of the data in the target Cassandra service. 


You can also create connectors using the :ref:`Aiven CLI command <avn_service_connector_create>`.

Example: Create a Cassandra source connector
-------------------------------------------------------

If you have a Cassandra table named ``students`` in the ``students_keyspace`` keyspace, with four columns (``id``, ``name``, ``age`` and ``timestamp_added``) and you want to load incrementally an Apache Kafka topic called ``students_topic``, you can use the following connector configuration, after replacing the placeholders for ``CASSANDRA_HOST``, ``CASSANDRA_PORT``, ``CASSANDRA_USERNAME``, ``CASSANDRA_PASSWORD``, ``CASSANDRA_KEYSTORE``, ``CASSANDRA_KEYSTORE_PASSWORD``, ``CASSANDRA_TRUSTSTORE``, ``CASSANDRA_TRUSTSTORE_PASSWORD``, ``CASSANDRA_KEYSPACE``:

.. code-block:: json

    {
        "name": "my-cassandra-source",
        "connector.class": "com.datamountaineer.streamreactor.connect.cassandra.source.CassandraSinkConnector",
        "topics": "TOPIC_LIST",
        "connect.cassandra.host": "CASSANDRA_HOSTNAME",
        "connect.cassandra.port": "CASSANDRA_PORT",
        "connect.cassandra.username": "CASSANDRA_USERNAME",
        "connect.cassandra.password": "CASSANDRA_PASSWORD",
        "connect.cassandra.ssl.enabled": "CASSANDRA_SSL",
        "connect.cassandra.trust.store.path": "CASSANDRA_TRUSTSTORE",
        "connect.cassandra.trust.store.password": "CASSANDRA_TRUSTSTORE_PASSWORD",
        "connect.cassandra.key.space": "students_keyspace",
        "value.converter": "org.apache.kafka.connect.json.JsonConverter",
        "connect.cassandra.kcql": "INSERT INTO students_topic SELECT id, name, age, timestamp_added FROM students PK timestamp_added INCREMENTALMODE=TIMESTAMP"    
    }

The configuration file contains the following peculiarities:

* ``"topics": "students"``: setting the topic to source
* ``"connect.cassandra"``: the connection parameters placeholders
* ``"value.converter": "org.apache.kafka.connect.json.JsonConverter"``: the topic will be populated in JSON format
* ``"connect.cassandra.kcql": "INSERT INTO students_topic SELECT id, name, age, timestamp_added FROM students PK timestamp_added INCREMENTALMODE=TIMESTAMP"``: the connector logic is to insert every row in a new topic message and use the column ``timestamp_added`` to check for new rows compared to the previous poll.

Once the connector is created successfully, you should see one message per row in the Cassandra table appearing in the target Apache Kafka ``students_topic`` topic.

.. Tip::

    If your Aiven for Apache Kafka instance doesn't have the :doc:`automatic creation of topic enabled </docs/products/kafka/howto/create-topics-automatically>`, you might need to create the ``students_topic`` topic upfront before starting the connector