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

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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:

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.
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.
Final Answer:
The input topic was never piped any records -> Option C
Quick Check:
No input records = no output [OK]

Quick Trick: Always pipe input records before reading output [OK]

Common Mistakes:

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