How do you solve the problem of event duplication and inconsistency between the database and Kafka?

To address the issue of event duplication and inconsistency between the database and Kafka, the "Idempotency" pattern and transactional processing are often used.

Main approaches:

1. Idempotency — processing repeated messages without changing the result. For example, storing a unique event identifier and ignoring repeated processing.

2. Outbox Pattern Transactions — changes in the database and recording the event in a separate table (outbox) occur within a single transaction. Then, a separate process reads from the outbox and publishes events to Kafka, ensuring consistency.

3. Using Kafka Transactions — if the client and broker support transactions, events can be written atomically.

Example of Outbox Pattern in Python (simplified):

import psycopg2

def save_data_and_event(data, event):
    conn = psycopg2.connect(...)
    try:
        with conn:
            with conn.cursor() as cur:
                cur.execute("INSERT INTO main_table (data) VALUES (%s)", (data,))
                cur.execute("INSERT INTO outbox (event_data) VALUES (%s)", (event,))
    finally:
        conn.close()

Then, a separate service reads from the outbox and publishes to Kafka, after which it marks the event as sent.