Memcached Interview Questions and Answers for 5 years experience

1. What is Memcached?

   • Answer: Memcached is a high-performance, distributed memory object caching system. It's used to speed up dynamic database-driven websites by alleviating database load. It stores data in RAM, providing very fast access times.

2. Explain the architecture of Memcached.

   • Answer: Memcached is a client-server architecture. Clients connect to one or more Memcached servers to store and retrieve data. The servers distribute the data across their available memory using a hashing algorithm. No central coordination is required, making it highly scalable and fault-tolerant (to a degree).

3. How does Memcached handle data expiry?

   • Answer: Memcached uses an LRU (Least Recently Used) algorithm combined with an expiry time set by the user. Items are evicted based on both their age and how recently they were accessed. The expiry time is a suggestion, not a guarantee; the server may evict items before their expiry time due to memory pressure.

4. What data types does Memcached support?

   • Answer: Memcached primarily supports strings. While you can store other data types, they are ultimately serialized into strings before being stored and deserialized upon retrieval by the client.

5. Explain the concept of "slabs" in Memcached.

   • Answer: Memcached uses "slabs" to manage memory efficiently. Slabs are chunks of memory of different sizes. Items are stored in the smallest slab that can accommodate them, minimizing memory waste.

6. How does Memcached handle data distribution across multiple servers?

   • Answer: Memcached uses consistent hashing to distribute data across multiple servers. This ensures that data remains on the same server even if servers are added or removed, minimizing data migration.

7. What are the benefits of using Memcached?

   • Answer: Improved website performance, reduced database load, increased scalability, faster response times, and simplified application development.

8. What are the limitations of Memcached?

   • Answer: Limited data types (primarily strings), volatile storage (data is lost on server restart), limited persistence options (requires external mechanisms for persistence), and potential for data eviction under memory pressure.

9. How do you monitor Memcached performance?

   • Answer: Using the `telnet` command to interact directly with the server, using monitoring tools like `memcached-tool`, and integrating with system monitoring solutions like Nagios or Zabbix. Key metrics include cache hits/misses, memory usage, and connection counts.

10. Describe a situation where you used Memcached to improve application performance.

    • Answer: [This requires a personalized answer based on your experience. Example: "In a previous project, we used Memcached to cache frequently accessed user profile data. This reduced database queries by 80%, resulting in a significant improvement in page load times and overall application responsiveness."]

11. Explain the difference between Memcached and Redis.

    • Answer: While both are in-memory data stores, Redis offers more data structures (lists, sets, sorted sets, hashes) and persistence options than Memcached, making it more versatile but potentially less performant for simple key-value storage.

12. How would you troubleshoot a Memcached server that is running slowly?

    • Answer: I would check server resource utilization (CPU, memory, network), examine Memcached logs for errors, monitor cache hit/miss ratios, and investigate potential memory leaks. Tools like `top` and `memcached-tool` are invaluable here.

13. What are some best practices for using Memcached effectively?

    • Answer: Choose appropriate data to cache (frequently accessed, relatively static data), properly size your Memcached servers, manage cache invalidation strategies, monitor performance closely, and use a robust client library.

14. Explain the concept of cache invalidation in Memcached.

    • Answer: Cache invalidation refers to removing outdated data from the cache. Memcached doesn't automatically invalidate; it relies on explicit removal (using `delete` commands) or implicit removal through expiry or LRU eviction. Proper cache invalidation is crucial for data consistency.

15. How can you implement caching strategies with Memcached in a distributed environment?

    • Answer: Using consistent hashing for data distribution, employing a client-side caching library that handles connections and distribution transparently, and potentially implementing a cache invalidation mechanism across multiple servers are key.

16. Discuss your experience with different Memcached client libraries.

    • Answer: [This requires a personalized answer, mentioning specific libraries and their strengths and weaknesses based on your experience. Examples include python's `python-memcached`, PHP's `memcached`, and others.]

17. How do you handle failures in a Memcached cluster?

    • Answer: Memcached's architecture provides some inherent fault tolerance. Client libraries often have retry mechanisms. For more robust handling, consider using a load balancer and monitoring tools to detect and address failures promptly. Data loss is a possibility, depending on the caching strategy and persistence options.

18. What are some common performance bottlenecks you've encountered while using Memcached?

    • Answer: [This should be a personalized answer based on your experience. Examples include network latency, insufficient server resources, poorly chosen data to cache, and inefficient cache invalidation strategies.]

19. How do you ensure data consistency when using Memcached?

    • Answer: This is a challenging aspect of Memcached. Strategies include careful cache invalidation, using techniques like cache-aside pattern (checking the database if the cache misses), and understanding the implications of eventual consistency.

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