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You wrote a test using TopologyTestDriver but your output topic always returns null. What is a likely cause?

medium📝 Debug Q6 of 15
Kafka - Advanced Stream Processing
You wrote a test using TopologyTestDriver but your output topic always returns null. What is a likely cause?
AThe topology was not closed after processing
BThe Properties object is missing bootstrap servers
CThe input topic was never piped any records
DThe output topic name is misspelled in the test
Step-by-Step Solution
Solution:
  1. Step 1: Analyze why output is always null

    If no input records are piped, the topology has no data to process, so output is empty.
  2. Step 2: Evaluate other options

    Closing topology or bootstrap servers do not affect in-memory test driver; misspelled output topic would cause errors, not null.
  3. Final Answer:

    The input topic was never piped any records -> Option C
  4. Quick Check:

    No input records = no output [OK]
Quick Trick: Always pipe input records before reading output [OK]
Common Mistakes:
MISTAKES
  • Forgetting to pipe input records
  • Confusing test driver with real Kafka cluster
  • Ignoring topic name typos causing silent failures

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