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Fixing Python Kafka Headers: Causes and Solutions

Complete guide to fixing Python Kafka headers: Learn how to reproduce the issue, check version compatibility, find workarounds, and explore alternative libraries.

#confluent-kafka#kafka-headers#librdkafka#message-headers#python-kafka

Python Kafka not seeing headers?

Python Kafka is not receiving headers from Apache Kafka. How can I reproduce this issue?

I’m trying to read headers through Kafka Consumer:

python
consumer = KafkaConsumer(
    "mz-core-document-manager-responses",
    auto_offset_reset="latest",
    group_id=None,
    api_version=(0, 10, 1),
    bootstrap_servers=[
         ....
    ],
)
for message in consumer:
    # message value and key are raw bytes -- decode if necessary!
    # e.g., for unicode: `message.value.decode('utf-8')`
    print(
        "%s:%d:%d: key=%s headers=%s"
        % (
            message.topic,
            message.partition,
            message.offset,
            message.key,
            "".join(message.headers),
        )
    )

Kafka consistently receives only the payload without headers.

After investigation, I found that BrokerConnection sends a request like:
FetchRequest_v3(replica_id=-1, max_wait_time=500, min_bytes=1, max_bytes=52428800, topics=[(topic=‘my-topic’, partitions=[(partition=0, offset=19857795, max_bytes=1048576), (partition=3, offset=18443170, max_bytes=1048576)])]) for the BrokerConnection that accepts it. But the response already doesn’t contain headers.

Example response:
b’\x00\x00\x00F\x00\x00\x00\x01\x00\x00\x00\x01\x00"***x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\x00\xfaOc’

There’s a reference to an open source bug in Kafka.

Could you suggest where to look for a solution to this problem?

NeuroAgent

Python Kafka doesn’t see headers due to issues with message header support in the confluent-kafka library, especially in versions before 1.0.0. This is a known issue that can be reproduced by sending messages with headers and attempting to read them through KafkaConsumer.

Table of Contents

Main causes of the problem

The issue with Python Kafka headers is related to several factors:

  1. confluent-kafka library limitations: According to Confluent documentation, “Currently message headers are not supported on the message returned to the callback. The msg.headers() will return None even if the original message had headers set.” source

  2. Version compatibility: Requires librdkafka >= v0.11.4 and broker version >= 0.11.0.0 for proper header support source

  3. Implementation issues: Issue #574 reports that msg.headers() always returns None even when headers are set in the producer source

  4. Message format: As you’ve noticed, FetchRequest_v3 requests are sent correctly, but headers are missing in the response, indicating a problem at the protocol or deserialization level.

How to reproduce the problem

Here’s a complete code example to reproduce the issue:

python
from confluent_kafka import Producer, Consumer, KafkaException
import json

# Producer configuration
producer_conf = {
    'bootstrap.servers': 'localhost:9092',
    'api.version.request': True,
    'api.version.fallback.ms': 0
}

# Consumer configuration
consumer_conf = {
    'bootstrap.servers': 'localhost:9092',
    'group.id': 'test-group',
    'auto.offset_reset': 'earliest',
    'enable.auto.commit': False,
    'api.version.request': True,
    'api.version.fallback.ms': 0
}

try:
    # Create producer
    producer = Producer(producer_conf)
    
    # Create consumer
    consumer = Consumer(consumer_conf)
    consumer.subscribe(['test-headers-topic'])
    
    # Send message with headers
    topic = 'test-headers-topic'
    headers = [('header-key', 'header-value'), ('source', 'python-test')]
    
    print("Sending message with headers...")
    producer.produce(topic, value='test message', headers=headers)
    producer.flush()
    
    print("Attempting to receive message...")
    msg = consumer.poll(timeout=10.0)
    
    if msg is None:
        print("No messages received")
    elif msg.error():
        print(f"Error receiving message: {msg.error()}")
    else:
        print(f"Topic: {msg.topic()}")
        print(f"Partition: {msg.partition()}")
        print(f"Offset: {msg.offset()}")
        print(f"Key: {msg.key()}")
        print(f"Value: {msg.value().decode('utf-8')}")
        print(f"Headers: {msg.headers()}")  # Should show None or empty
        
finally:
    consumer.close()
    producer.flush()

Expected result: msg.headers() will return None or an empty list despite headers being sent.

Version compatibility checking

Check Kafka version compatibility:

Component Minimum Version Recommended Version
Kafka Broker 0.11.0.0 2.8+
librdkafka 0.11.4 1.9+
confluent-kafka 1.0.0 2.0+

To check versions:

python
from confluent_kafka import KafkaException, TopicPartition
import confluent_kafka as ck

# Check librdkafka version
print(f"librdkafka version: {ck.libversion()}")

# Check API version
try:
    admin_client = ck.AdminClient({'bootstrap.servers': 'localhost:9092'})
    cluster_metadata = admin_client.list_topics(timeout=10)
    print(f"Cluster available, API version: {cluster_metadata.orig_broker_api_versions}")
except KafkaException as e:
    print(f"Connection error: {e}")

Solutions and workarounds

1. Update confluent-kafka

bash
pip install --upgrade confluent-kafka

2. Manual header serialization

If updating doesn’t help, you can use a workaround by serializing headers into the message value:

python
import json
import base64

# Sending with headers encoded in value
headers_data = {'my-header': 'header-value'}
encoded_headers = base64.b64encode(json.dumps(headers_data).encode()).decode()

producer.produce(
    topic,
    value=json.dumps({'data': 'message', 'headers': encoded_headers}).encode(),
    key='test-key'
)

# Receiving and decoding
msg = consumer.poll()
if msg:
    try:
        data = json.loads(msg.value().decode())
        headers = json.loads(base64.b64decode(data['headers']).decode())
        print(f"Headers: {headers}")
    except (json.JSONDecodeError, KeyError) as e:
        print(f"Decoding error: {e}")

3. Use Kafka Admin API for diagnostics

python
from confluent_kafka import AdminClient, KafkaException

def check_kafka_features(bootstrap_servers):
    admin_client = AdminClient({'bootstrap.servers': bootstrap_servers})
    
    try:
        cluster_metadata = admin_client.list_topics(timeout=10)
        print("Available topics:", list(cluster_metadata.topics.keys()))
        
        # Check API support
        if hasattr(cluster_metadata, 'orig_broker_api_versions'):
            print("Broker API versions:", cluster_metadata.orig_broker_api_versions)
        
    except KafkaException as e:
        print(f"Check error: {e}")

check_kafka_features('localhost:9092')

4. Check consumer configuration

python
consumer_conf = {
    'bootstrap.servers': 'localhost:9092',
    'group.id': 'test-group',
    'auto.offset_reset': 'earliest',
    'enable.auto.commit': False,
    'api.version.request': True,
    'api.version.fallback.ms': 0,
    'fetch.max.bytes': 1048576,  # Increased size for receiving headers
    'fetch.max.wait.ms': 500,
    'fetch.min.bytes': 1,
    'check.crcs': False,  # May help with performance
    'message.max.bytes': 10485760  # Maximum message size
}

Alternative libraries

If the problem persists, consider using alternative libraries:

1. kafka-python

bash
pip install kafka-python
python
from kafka import KafkaConsumer

consumer = KafkaConsumer(
    'test-headers-topic',
    bootstrap_servers='localhost:9092',
    auto_offset_reset='earliest',
    enable_auto_commit=False,
    consumer_timeout_ms=1000
)

for message in consumer:
    print(f"Headers: {message.headers}")
    print(f"Value: {message.value.decode('utf-8')}")

2. aiokafka (asynchronous operation)

bash
pip install aiokafka
python
import asyncio
from aiokafka import AIOKafkaConsumer

async def consume_messages():
    consumer = AIOKafkaConsumer(
        'test-headers-topic',
        bootstrap_servers='localhost:9092',
        group_id='async-group',
        auto_offset_reset='earliest'
    )
    await consumer.start()
    try:
        async for msg in consumer:
            print(f"Headers: {msg.headers}")
            print(f"Value: {msg.value.decode('utf-8')}")
    finally:
        await consumer.stop()

asyncio.run(consume_messages())

Additional recommendations

1. Check Kafka broker

Ensure your Kafka broker supports headers:

bash
# Check Kafka version
kafka-broker-api-versions.sh --bootstrap-server localhost:9092

# Create topic with header support
kafka-topics.sh --create --topic test-headers-topic --bootstrap-server localhost:9092 --partitions 3 --replication-factor 1 --config message.format.version=2.8-IV2

2. Monitor network traffic

Use Wireshark to analyze network traffic between your client and Kafka broker to see if headers are actually being sent in requests.

3. Test with console utilities

bash
# Send message with headers via console
kafka-console-producer.sh --broker-list localhost:9092 --topic test-headers-topic --property parse.headers=true --property headers.separator=,

# Receive messages
kafka-console-consumer.sh --bootstrap-server localhost:9092 --topic test-headers-topic --from-beginning --property print.headers=true

4. Report the issue

If the issue is confirmed, create a report in the confluent-kafka repository:

  1. Collect version information:

    • confluent-kafka: pip show confluent-kafka
    • librdkafka: ck.libversion()
    • Kafka broker: kafka-broker-api-versions.sh

  2. Minimal reproduction example

  3. Error logs

  4. Configuration information

This issue is actively discussed in the community, and regular library updates gradually improve header support. Monitor new releases of confluent-kafka on GitHub.

Sources

  1. Confluent Kafka Python Documentation - Overview
  2. GitHub Issue #574 - msg.headers none in the produce callback
  3. GitHub Issue #287 - Support for Message headers
  4. Stack Overflow - Kafka python consumer not reading the message from topics
  5. Confluent Documentation - Supported Versions and Interoperability
  6. PyPI - confluent-kafka vs kafka-python comparison

Conclusion

The issue with headers in Python Kafka is a known limitation of the confluent-kafka library, especially in older versions. The main recommendations are:

  1. Update confluent-kafka to the latest version (minimum 1.0.0, preferably 2.0+)
  2. Check compatibility of Kafka broker and librdkafka versions
  3. As a workaround, encode headers in the message value
  4. Consider alternative libraries (kafka-python, aiokafka) if the issue isn’t resolved
  5. Create a bug report in the confluent-kafka repository with a complete reproduction example

Regularly check for library updates, as header support gradually improves in new releases.

How to update confluent-kafka to the latest version for header support?How to check compatibility between Kafka broker and Python client versions?How to use kafka-python to read message headers?What alternative Python libraries for Kafka support headers?How to manually serialize headers into message values in Kafka?How to check header support in Kafka broker through console utilities?
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