Maxwell Format

    Maxwell is a CDC (Changelog Data Capture) tool that can stream changes in real-time from MySQL into Kafka, Kinesis and other streaming connectors. Maxwell provides a unified format schema for changelog and supports to serialize messages using JSON.

    Flink supports to interpret Maxwell JSON messages as INSERT/UPDATE/DELETE messages into Flink SQL system. This is useful in many cases to leverage this feature, such as

    • synchronizing incremental data from databases to other systems
    • auditing logs
    • temporal join changing history of a database table and so on.

    Flink also supports to encode the INSERT/UPDATE/DELETE messages in Flink SQL as Maxwell JSON messages, and emit to external systems like Kafka. However, currently Flink can’t combine UPDATE_BEFORE and UPDATE_AFTER into a single UPDATE message. Therefore, Flink encodes UPDATE_BEFORE and UDPATE_AFTER as DELETE and INSERT Maxwell messages.

    In order to use the Maxwell format the following dependencies are required for both projects using a build automation tool (such as Maven or SBT) and SQL Client with SQL JAR bundles.

    How to use Maxwell format

    Maxwell provides a unified format for changelog, here is a simple example for an update operation captured from a MySQL table in JSON format:

    Note: please refer to Maxwell documentation about the meaning of each fields.

    The MySQL products table has 4 columns (id, name, description and weight). The above JSON message is an update change event on the products table where the weight value of the row with id = 111 is changed from 5.18 to 5.15. Assuming this messages is synchronized to Kafka topic products_binlog, then we can use the following DDL to consume this topic and interpret the change events.

    After registering the topic as a Flink table, then you can consume the Maxwell messages as a changelog source.

    The following example shows how to access Maxwell metadata fields in Kafka:

    Format Options

    The Maxwell application allows to deliver every change event exactly-once. Flink works pretty well when consuming Maxwell produced events in this situation. If Maxwell application works in at-least-once delivery, it may deliver duplicate change events to Kafka and Flink will get the duplicate events. This may cause Flink query to get wrong results or unexpected exceptions. Thus, it is recommended to set job configuration table.exec.source.cdc-events-duplicate to true and define PRIMARY KEY on the source in this situation. Framework will generate an additional stateful operator, and use the primary key to deduplicate the change events and produce a normalized changelog stream.

    Data Type Mapping

    Currently, the Maxwell format uses JSON for serialization and deserialization. Please refer to JSON Format documentation for more details about the data type mapping.