Introduction:
Change Data Capture (CDC) is a technique used to track changes in a database, such as inserts, updates, and deletes. In this blog post, we will show you how to implement a custom CDC in PostgreSQL to track changes in your database. By using a custom CDC, you can keep a record of changes in your database and use that information in your applications, such as to provide a history of changes, track auditing information, or trigger updates in other systems
Implementing a Custom CDC in PostgreSQL:
To implement a custom CDC in PostgreSQL, you will need to create a new table to store the change information, create a trigger function that will be executed whenever a change is made in the target table, and create a trigger that will call the trigger function. The trigger function will insert a new row into the change table with the relevant information, such as the old and new values of the record, the time of the change, and any other relevant information.
To demonstrate this, we will show you an example of a custom CDC for a table called “employee”. The change table will be called “employee_cdc” and will contain columns for the timestamp, employee ID, old values, and new values of the employee record. The trigger function will be executed after an update on the “employee” table and will insert a new row into the “employee_cdc” table with the relevant information. Finally, we will show you how to query the “employee_cdc” table to retrieve a list of all changes that have occurred in the “employee” table since a certain timestamp.
- Create the Employee and CDC table
To store the CDC information, you need to create a new table in your PostgreSQL database. In this example, we will create a table called “employee”, “employee_cdc”, “employee_audit” with the following columns:
CREATE TABLE employee (
id SERIAL PRIMARY KEY,
name VARCHAR(100) NOT NULL,
department VARCHAR(50) NOT NULL,
salary NUMERIC(10,2) NOT NULL,
hire_date DATE NOT NULL
);
CREATE TABLE employee_cdc (
timestamp TIMESTAMP DEFAULT now(),
employee_id INTEGER,
old_values JSONB,
new_values JSONB
);
In this table, “id” is an auto-incremented primary key, “timestamp” is a timestamp with time zone to store the time of the change, “employee_id” is the primary key of the employee record that was changed, and “old_values” and “new_values” are text columns to store the old and new values of the employee record, respectively.
2. Create the Audit table
CREATE TABLE employee_audit (
audit_timestamp TIMESTAMP DEFAULT now(),
employee_id INTEGER,
old_values JSONB,
new_values JSONB
);
3. Create the trigger function
To capture the changes in the employee table, you will need to create a trigger function that will be executed whenever a record is inserted, updated, or deleted in the table. The trigger function will insert a new row into the “employee_cdc” table with the relevant information. Here is an example trigger function:
CREATE OR REPLACE FUNCTION employee_cdc() RETURNS TRIGGER AS $$
BEGIN
IF (TG_OP = 'UPDATE') THEN
INSERT INTO employee_cdc (timestamp, employee_id, old_values, new_values)
VALUES (now(), NEW.id, row_to_json(OLD), row_to_json(NEW));
INSERT INTO employee_audit (employee_id, old_values, new_values)
VALUES (NEW.id, row_to_json(OLD), row_to_json(NEW));
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;
This trigger function uses the “row_to_json” function to convert the old and new values of the employee record into JSON strings, which are then stored in the “old_values” and “new_values” columns of the “employee_cdc” table. The “NOW()” function is used to get the current timestamp.
4. Create the trigger
Now that the trigger function has been created, you need to create the trigger on the “employee” table that will call the function whenever a record is updated. You can create the trigger with the following command:
CREATE TRIGGER employee_cdc_trigger
AFTER UPDATE ON employee
FOR EACH ROW
EXECUTE FUNCTION employee_cdc();
4. Query the CDC table
In your application code, you can query the “employee_cdc” table to get a list of all changes that have occurred since a certain timestamp. For example, to get all changes since January 1st, 2023, you can use the following SQL query:
SELECT * FROM employee_cdc
WHERE timestamp >= '2023-01-01 00:00:00';
You can then process these changes as needed in your application code.
Conclusion:
In this blog post, we have shown you how to implement a custom Change Data Capture (CDC) in PostgreSQL to track changes in your database. By using a custom CDC, you can keep a record of changes in your database and use that information in your applications. Whether you are tracking changes for auditing purposes, providing a history of changes, or triggering updates in other systems, a custom CDC is a useful tool to have in your PostgreSQL toolkit.
AWS CLOUD DBA 