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Kafka Is FunProgress

Intermediate5 min

Min ISR Block (NOT_ENOUGH_REPLICAS)

Demonstrate how min.insync.replicas protects against unsafe writes under broker failures.

min.insync.replicas=2 means Kafka refuses acks=all writes when ISR drops below 2. This is intentional — it prevents acknowledging writes that can't survive a subsequent broker failure. It's Kafka's durability guarantee in action.

The steps below reference the interactive simulator, which is being migrated into /simulate next — until it lands, treat this as the written runbook for the exercise.

Steps

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Verification

The key lesson: acks=all + minISR=2 rejects writes rather than risk data loss. Prefer partition unavailability over silent data loss.

The interview angle

Interviewers love this: 'What's the difference between RF=3 and minISR=2?' Answer: RF controls how many copies exist; minISR controls the minimum guarantees for acks=all writes.