Custom Connectors

Custom Connectors Overview

Custom Connectors is a building block you will reach for often in Apache Kafka. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn custom connectors properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real Apache Kafka projects.

// consume, transform, and re-produce (a simple stream stage)
await consumer.run({
  eachMessage: async ({ message }) => {
    const event = JSON.parse(message.value.toString());
    const enriched = { ...event, receivedAt: Date.now() };
    await producer.send({
      topic: 'orders-enriched',
      messages: [{ key: event.id, value: JSON.stringify(enriched) }],
    });
  },
});

Stream processing consumes from one topic, transforms events, and produces to another.

Custom Connectors Example

import { Kafka } from 'kafkajs';

const kafka = new Kafka({ clientId: 'app', brokers: ['localhost:9092'] });
const producer = kafka.producer();
const consumer = kafka.consumer({ groupId: 'group' });

• Start from a minimal Custom Connectors example and grow it only as needed.
• Keep configuration explicit so Custom Connectors behaves the same in every environment.
• Name things clearly so teammates understand your Custom Connectors at a glance.
• Add tests around Custom Connectors early to lock in expected behaviour.

Apache Kafka Cheatsheet

Handy KafkaJS reference related to custom connectors.

How Custom Connectors Works in Apache Kafka

Custom Connectors builds on Kafka's log-based design, where producers append events to partitioned topics and consumer groups read them independently, tracking their own offsets.

Stream processing consumes from one topic, transforms events, and produces to another.

• Topics are split into partitions for parallelism and ordering per key.
• Producers choose a partition, usually by message key.
• Consumer groups share partitions so work scales horizontally.
• Offsets record how far each group has read.

Practical Guidance for Custom Connectors

In production, custom connectors needs attention to delivery guarantees, retries, and observability. Make handlers idempotent and monitor consumer lag closely.

Common Mistakes

• Skipping error handling and edge cases when wiring up custom connectors.
• Leaving custom connectors untested, so regressions slip into production.
• Over-engineering custom connectors before you actually need the extra flexibility.
• Ignoring documentation, which makes custom connectors hard for the next developer to change.

Key Takeaways

• Custom Connectors is a core part of working effectively with Apache Kafka.
• Start small and keep custom connectors focused on a single responsibility.
• Apply consistent patterns so custom connectors scales across your project.
• Test and document custom connectors to keep it maintainable over time.