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You have a KStream-KTable join but the output values are always null. What is a common cause of this issue?

medium📝 Debug Q7 of Q15
Kafka - Streams
You have a KStream-KTable join but the output values are always null. What is a common cause of this issue?
AThe join window is too large.
BThe KTable is empty or has no matching keys for the KStream records.
CThe KStream keys are not serialized properly.
DThe joiner function returns null explicitly.
Step-by-Step Solution
Solution:

  1. Step 1: Understand null output in KStream-KTable join

    If the KTable has no matching keys, the join outputs null for the table side.

  2. Step 2: Check KTable data presence

    An empty or missing KTable data causes null join values.

  3. Final Answer:

    The KTable is empty or has no matching keys for the KStream records. -> Option B

  4. Quick Check:

    Null join output = missing KTable keys [OK]
Quick Trick: Check KTable data if join outputs null values [OK]
Common Mistakes:
MISTAKES
  • Assuming joiner function causes null without checking data
  • Ignoring key serialization issues
  • Misconfiguring join window size

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