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You wrote code to send messages with a partition key, but all messages are going to partition 0. What is the most likely cause?

medium📝 Debug Q6 of 15
Kafka - Topics and Partitions
You wrote code to send messages with a partition key, but all messages are going to partition 0. What is the most likely cause?
AThe producer is not connected to Kafka
BThe topic has only one partition
CThe partition key is always null or empty
DThe message value is too large
Step-by-Step Solution
Solution:

  1. Step 1: Analyze why all messages go to partition 0

    If the topic has only one partition, all messages will go to partition 0 regardless of key.

  2. Step 2: Exclude other causes

    Null or empty keys may cause random partition assignment, but with one partition, all go to 0. Connection or size issues cause errors, not partition routing.

  3. Final Answer:

    The topic has only one partition -> Option B

  4. Quick Check:

    Single partition topic causes all messages to go to partition 0 [OK]
Quick Trick: Single partition topic sends all messages to partition 0 [OK]
Common Mistakes:
  • Assuming topic has multiple partitions without checking
  • Thinking producer connection causes partition choice
  • Blaming message size for partition routing

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