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Integration / Apache Kafka Interview questions

How does Kafka handle partition leader election?

When a partition's current leader broker fails or becomes unreachable, the KRaft active controller detects this (via missed heartbeats/session timeout) and selects a new leader from among that partition's in-sync replicas (ISR), then propagates the new leader assignment through the metadata log so every broker and client learns about it.

By default, Kafka only promotes an in-sync replica — one that was fully caught up with the old leader — which guarantees no committed data is lost in the handover. If every replica in the ISR is also unavailable at the same time (a broader outage), the partition simply becomes unavailable for writes rather than silently promoting an out-of-sync replica and risking data loss, unless unclean.leader.election.enable is explicitly turned on to allow that riskier trade-off. Clients (producers and consumers) discover the new leader automatically on their next metadata refresh or when a request to the old leader fails, without needing any manual reconfiguration.

Which replicas are eligible to become the new leader by default when the current leader fails?
What happens if every in-sync replica is unavailable at the same time as the leader failure?

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What is a Kafka topic? Mention some of the Apache Kafka terminologies. What is the Global Unique identifier of a Kafka Message? What is a partition in Kafka? What is a Kafka producer? What is Apache Kafka? Explain the role of the ZooKeeper in Kafka. What is a Kafka consumer? What is a consumer group in Kafka? What is Apache ZooKeeper? What is a Kafka broker? Difference between Apache Kafka and Confluent Kafka. Kafka's zero-copy principle. Define replication in Kafka? What is KRaft mode in Kafka? Explain sequential I/O principle in Kafka. What are the types of message delivery semantics in Kafka? What is a Kafka offset used for? List the core APIs provided by Kafka? What is the purpose of a Kafka topic's retention policy? Describe the role of a partition leader in Kafka? What is a Kafka Connect connector? What are Kafka Streams used for? How do you create a topic using the Kafka CLI? What is a serializer in Kafka producer configuration? How do you list existing topics in a Kafka cluster? Why is Kafka better suited than a traditional message queue for high-throughput event streaming? How does Kafka differ from RabbitMQ? What is the difference between at-least-once, at-most-once, and exactly-once delivery semantics? How does Kafka's KRaft controller quorum manage cluster metadata? Why should you configure min.insync.replicas alongside acks=all? How does Kafka handle partition leader election? When should you increase the number of partitions for a topic? What happens when a consumer in a group fails to send a heartbeat in time? Explain the execution flow of a Kafka producer sending a message to a broker? How can you optimize Kafka producer throughput? How do you troubleshoot consumer lag in a Kafka application? Why is the in-sync replica (ISR) set important for durability? Explain the lifecycle of a Kafka consumer group rebalance? How does Kafka achieve exactly-once semantics with idempotent producers and transactions? What is the difference between log compaction and log deletion cleanup policies? How do you implement a custom partitioner in Kafka? How does Kafka Streams manage local state using state stores? Which is better and why: the classic consumer rebalance protocol or the new KIP-848 protocol? How do you integrate a schema registry with Kafka producers and consumers? Explain the internal working of Kafka's replication protocol between leader and follower brokers? How do you configure tiered storage for a Kafka topic? What is the difference between Kafka Connect source and sink connectors? How does Kafka support message compression? When would you choose the cooperative sticky partition assignment strategy? How do you secure a Kafka cluster with SASL and ACLs? Why should unclean leader election be disabled in most production clusters? How do you configure MirrorMaker for cross-cluster replication? Explain the execution flow of a Kafka Streams topology processing a record? How does Kafka report and expose broker and consumer metrics for monitoring? Why doesn't increasing partition count always improve throughput? How do you migrate a Kafka cluster from ZooKeeper mode to KRaft mode? What is the difference between Kafka's Queues feature (KIP-932) and traditional partitioned consumption?
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