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Change Data Capture (CDC) of PostgreSQL table using Apache Flink®

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Change Data Capture (CDC) is a technique you can use to track row-level changes in database tables in response to create, update, and delete operations. In this article, we use CDC Connectors for Apache Flink®, which offer a set of source connectors for Apache Flink. The connectors integrate Debezium® as the engine to capture the data changes.

Flink supports to interpret Debezium JSON and Avro messages as INSERT/UPDATE/DELETE messages into Apache Flink SQL system.

This support is useful in many cases to:

  • Synchronize incremental data from databases to other systems
  • Audit logs
  • Build real-time materialized views on databases
  • View temporal join changing history of a database table

Now, let's learn how to monitor changes on PostgreSQL table using Flink-SQL CDC. The PostgreSQL CDC connector allows for reading snapshot data and incremental data from PostgreSQL database.

Prerequisites

Prepare PostgreSQL table & Client

  • Using a Linux virtual machine, install PostgreSQL client using below commands

    sudo apt-get update
sudo apt-get install postgresql-client

  • Install the certificate to connect to PostgreSQL server using SSL

    wget --no-check-certificate https://dl.cacerts.digicert.com/DigiCertGlobalRootCA.crt.pem

  • Connect to the server (replace host, username and database name accordingly)

    psql --host=flinkpostgres.postgres.database.azure.com --port=5432 --username=admin --dbname=postgres --set=sslmode=require --set=sslrootcert=DigiCertGlobalRootCA.crt.pem

  • After connecting to the database successfully, create a sample table

    CREATE TABLE shipments (
    shipment_id SERIAL NOT NULL PRIMARY KEY,
    order_id SERIAL NOT NULL,
    origin VARCHAR(255) NOT NULL,
    destination VARCHAR(255) NOT NULL,
    is_arrived BOOLEAN NOT NULL
  );
  ALTER SEQUENCE public.shipments_shipment_id_seq RESTART WITH 1001;
  ALTER TABLE public.shipments REPLICA IDENTITY FULL;
  INSERT INTO shipments
  VALUES (default,10001,'Beijing','Shanghai',false),
     (default,10002,'Hangzhou','Shanghai',false),
     (default,10003,'Shanghai','Hangzhou',false);

  • To enable CDC on PostgreSQL database, you're required to make the following changes.

    • WAL level must be changed to logical. This value can be changed in server parameters section on Azure portal.

      Screenshot showing how to enable-cdc-on-postgres-database.

    • User accessing the table must have 'REPLICATION' role added

      ALTER USER <username> WITH REPLICATION;

  • To create Flink PostgreSQL CDC table, download all the dependent jars. Use the pom.xml file with the following contents.

    <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>com.dep.download</groupId>
    <artifactId>dep-download</artifactId>
    <version>1.0-SNAPSHOT</version>
        <!-- https://mvnrepository.com/artifact/com.ververica/flink-sql-connector-sqlserver-cdc -->
    <dependencies>
    <dependency>
        <groupId>com.ververica</groupId>
        <artifactId>flink-sql-connector-postgres-cdc</artifactId>
        <version>2.4.2</version>
    </dependency>
    </dependencies>
</project>

  • Use maven command to download all the dependent jars

       mvn -DoutputDirectory=target -f pom.xml dependency:copy-dependencies -X

    Note

  • Once the dependent jars are downloaded start the Flink SQL client, with these jars to be imported into the session. Complete command as follows,

    /opt/flink-webssh/bin/sql-client.sh -j
/opt/flink-webssh/target/flink-sql-connector-postgres-cdc-2.4.2.jar -j
/opt/flink-webssh/target/slf4j-api-1.7.15.jar -j
/opt/flink-webssh/target/hamcrest-2.1.jar -j
/opt/flink-webssh/target/flink-shaded-guava-31.1-jre-17.0.jar-j
/opt/flink-webssh/target/awaitility-4.0.1.jar -j
/opt/flink-webssh/target/jsr308-all-1.1.2.jar

    These commands start the sql client with the dependencies as,

    user@sshnode-0 [ ~ ]$ bin/sql-client.sh -j flink-sql-connector-postgres-cdc-2.4.2.jar -j slf4j-api-1.7.15.jar -j hamcrest-2.1.jar -j flink-shaded-guava-31.1-jre-17.0.jar -j awaitility-4.0.1.jar -j jsr308-all-1.1.2.jar 

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   ______ _ _       _      _____  ____  _        _____ _ _            _  BETA   
  | ____| (_)     | |     / ____|/ __ \| |       / ____| (_)          | | 
  | |__ | |_ _ __ | | __ | (___ | |  | | |      | |    | |_ ___ _ __ | |_ 
  |  __| | | | '_ \| |/ /  \___ \| |  | | |     | |    | | |/ _ \ '_ \| __|
  | |   | | | | | |   <   ____) | |__| | |____  | |____| | | __/ | | | |_ 
  |_|   |_|_|_| |_|_|\_\ |_____/ \___\_\______| \_____|_|_|\___|_| |_|\__|

       Welcome! Enter 'HELP;' to list all available commands. 'QUIT;' to exit.

Command history file path: /home/xcao/.flink-sql-history

Flink SQL>

  • Create a Flink PostgreSQL CDC table using CDC connector

    CREATE TABLE shipments (
  shipment_id INT,
  order_id INT,
  origin STRING,
  destination STRING,
  is_arrived BOOLEAN,
  PRIMARY KEY (shipment_id) NOT ENFORCED
) WITH (
  'connector' = 'postgres-cdc',
  'hostname' = 'flinkpostgres.postgres.database.azure.com',
  'port' = '5432',
  'username' = 'username',
  'password' = 'password',
  'database-name' = 'postgres',
  'schema-name' = 'public',
  'table-name' = 'shipments',
  'decoding.plugin.name' = 'pgoutput',
  'slot.name' = 'flink'
);

Validation

  • Run 'select *' command to monitor the changes.

    select * from shipments;

    Screenshot showing how to run-select-command.

Reference