Technical Interview Q&A: TCP, Redis, Kafka, CAP Theorem, Singleton, C++ STL, and Algorithms

The article presents a collection of technical interview questions and answers covering backend development topics.

1. TCP TIME_WAIT – explains why the active closer enters TIME_WAIT to prevent old packets from affecting new connections and to ensure the passive side receives the final ACK.

2. Listening on multiple ports – TCP can listen on several ports by creating separate sockets (e.g., Nginx on 80 and 443).

3. Full page loading process – URL parsing, cache checks, DNS resolution, MAC address acquisition, TCP handshake, optional TLS handshake, HTTP request, server response.

4. Redis data structures – core types: String, Hash, List, Set, Zset; newer types: BitMap, HyperLogLog, GEO, Stream, with typical use cases.

5. Kafka consumer count – consumer number ≤ partition number. Example formula:

consumerCount = min(partitionCount, expectedThroughput / singleConsumerCapacity)

Illustrated with a 12‑partition topic requiring 8 consumers for 8000 msg/s throughput.

6. Kafka “at most once” semantics – achieve by disabling auto‑commit and committing offsets only after successful processing.

7. CAP theorem – consistency, availability, partition tolerance cannot be simultaneously guaranteed; brief definitions provided.

8. Singleton pattern – ensures a single instance; typical scenarios: global configuration, logger, database connection pool.

9. C++ std::map complexity – implemented with a red‑black tree, insertion O(log n).

10. Doubly linked list reversal – iterative algorithm with O(n) time and O(1) space. Sample code:

public ListNode reverseDoublyLinkedList(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode prev = null;
    ListNode current = head;
    while (current != null) {
        ListNode nextNode = current.next; // save next
        current.next = prev;              // reverse next
        current.prev = nextNode;          // reverse prev
        prev = current;
        current = nextNode;
    }
    return prev; // new head
}

The content is primarily educational and useful for backend engineers preparing for interviews.