Question 1

What is Redis and what problem does it solve?

Accepted Answer

Redis is an open-source, in-memory data structure store used as a database, cache, and message broker. It solves performance bottlenecks by providing sub-millisecond latency for real-time applications that need fast data access.

Question 2

What is the difference between Redis and traditional databases?

Accepted Answer

Redis stores data in RAM using key-value pairs with rich data structures, while traditional databases store data on disk in tables with SQL queries. Redis prioritizes speed (sub-millisecond) over complex queries and ACID guarantees.

Question 3

What are the main Redis data types?

Accepted Answer

Redis supports Strings (text/numbers), Lists (ordered collections), Sets (unique values), Sorted Sets (ranked data), Hashes (field-value maps), Streams (event logs), Bitmaps, HyperLogLogs, and Geospatial indexes.

Question 4

How does Redis persistence work?

Accepted Answer

Redis offers two persistence options: RDB (point-in-time snapshots saved to disk periodically) and AOF (append-only file logging every write operation). Both can be used together for maximum durability.

Question 5

What is Redis Sentinel and when should I use it?

Accepted Answer

Redis Sentinel provides high availability through automatic failover, monitoring, and notification. Use it when you need automatic master-replica failover for production systems without data sharding requirements.

Question 6

What is Redis Cluster and how does it differ from Sentinel?

Accepted Answer

Redis Cluster provides horizontal scaling by distributing data across multiple nodes using hash slots. Unlike Sentinel (HA only), Cluster handles both sharding and failover, but has limitations like no multi-key operations across slots.

Question 7

How does Redis handle memory management and eviction?

Accepted Answer

Redis uses configurable eviction policies (LRU, LFU, TTL-based, random) when memory limits are reached. You set maxmemory and choose a policy like allkeys-lru to automatically remove least recently used keys.

Question 8

What are Redis transactions and how do they work?

Accepted Answer

Redis transactions use MULTI/EXEC to queue commands and execute them atomically. Unlike SQL transactions, Redis transactions don't support rollback—if a command fails, others still execute. Use WATCH for optimistic locking.

Question 9

How can I implement a distributed lock with Redis?

Accepted Answer

Use SET with NX and EX options: SET lock:resource unique_id NX EX 30. This sets a key only if it doesn't exist with automatic expiration. For production, use Redlock algorithm across multiple Redis instances.

Question 10

What is Redis pub/sub and when should I use it?

Accepted Answer

Redis Pub/Sub enables real-time messaging where publishers send messages to channels and subscribers receive them instantly. Use it for real-time notifications, chat systems, or event broadcasting. Note: messages aren't persisted.

Question 11

What are Redis Streams and how do they differ from pub/sub?

Accepted Answer

Redis Streams provide persistent, append-only message logs with consumer groups for distributed processing. Unlike pub/sub, Streams store messages durably, support acknowledgments, and allow multiple consumers to process messages reliably.

Question 12

How do I implement caching with Redis?

Accepted Answer

Store frequently accessed data with TTL: SET key value EX 3600. Use cache-aside pattern (check cache, fetch from DB on miss, store in Redis) or write-through (update cache on DB writes). Monitor hit rates with INFO stats.

Question 13

What are hash tags in Redis Cluster?

Accepted Answer

Hash tags force related keys to the same slot by using curly braces: {user:1000}:profile and {user:1000}:orders both hash based on 'user:1000'. This enables multi-key operations and transactions in Redis Cluster.

Question 14

How do I secure a Redis instance?

Accepted Answer

Secure Redis by: binding to localhost or private IPs, enabling password authentication (requirepass), using TLS encryption, renaming dangerous commands, running with minimal privileges, and keeping Redis updated.

Question 15

What are common Redis performance optimization techniques?

Accepted Answer

Optimize Redis by: using pipelining for bulk operations, choosing appropriate data structures, avoiding blocking commands (KEYS, SMEMBERS on large sets), using SCAN for iteration, and enabling memory optimization features.

Redis Fundamentals(5 questions)

What is Redis and what are its primary use cases?

How does Redis differ from traditional relational databases?

Explain Redis's single-threaded architecture and how it achieves high performance.

What are the advantages and disadvantages of Redis being an in-memory database?

What is Redis persistence and why is it important?

Data Types and Structures(7 questions)

What are the basic data types in Redis?

How do Strings work in Redis and what operations can you perform on them?

What are Redis Lists and when would you use them?

What are Sorted Sets (ZSets) in Redis and how do they differ from regular Sets?

How do Redis Hashes work and when should you use them?

What are Redis Streams and when would you use them?

How do you choose the right data structure for your use case?

Commands and Operations(4 questions)

What are the basic CRUD operations in Redis?

What is the difference between EXPIRE, EXPIREAT, and PEXPIRE?

What is the SCAN command and when would you use it over KEYS?

What is the difference between DEL and UNLINK?

Persistence(5 questions)

What is RDB (Redis Database) persistence?

What is AOF (Append-Only File) persistence?

What are the differences between RDB and AOF persistence?

When should you use RDB vs AOF vs both?

What are the trade-offs between durability and performance in Redis persistence?

Replication and High Availability(4 questions)

What is Redis replication and how does it work?

What is Redis Sentinel and what problems does it solve?

How does automatic failover work with Redis Sentinel?

What is the difference between Sentinel and Redis Cluster for high availability?

Redis Cluster(5 questions)

What is Redis Cluster and when would you use it?

How does Redis Cluster handle data distribution (hash slots)?

What is the difference between Redis Cluster and Redis Sentinel?

What are hash tags and how do they ensure keys are on the same node?

What are the limitations of Redis Cluster?

Transactions and Atomicity(4 questions)

What are Redis transactions and how do MULTI, EXEC, DISCARD work?

What is the WATCH command and how does it enable optimistic locking?

What are Lua scripts in Redis and why would you use them?

What is the difference between transactions and Lua scripts for atomicity?

Pub/Sub and Messaging(4 questions)

What is Redis Pub/Sub and how does it work?

What are the limitations of Redis Pub/Sub?

What is the difference between Pub/Sub and Redis Streams?

What are consumer groups in Redis Streams?

Caching Strategies(6 questions)

What are the common caching strategies (cache-aside, read-through, write-through, write-behind)?

How do you implement cache-aside pattern with Redis?

What is cache invalidation and what strategies exist?

How do you handle cache stampede (thundering herd) problems?

What are the eviction policies in Redis and how do you choose one?

How do you handle cache consistency with a database?

Performance and Optimization(5 questions)

How do you identify performance bottlenecks in Redis?

What is the SLOWLOG command and how do you use it?

What are the best practices for Redis key naming conventions?

What is pipelining in Redis and how does it improve performance?

How do you handle hot keys in Redis?

Security(4 questions)

How do you enable authentication in Redis?

What is the ACL (Access Control List) system in Redis 6.0+?

What are the security best practices for Redis deployments?

What dangerous commands should be disabled or renamed in production?

Monitoring and Administration(4 questions)

What are the important Redis metrics to monitor?

How do you use the INFO command for monitoring?

How do you perform Redis backup and restore operations?

How do you analyze Redis memory usage?

Use Cases and Patterns(5 questions)

How do you implement session storage with Redis?

How do you build a rate limiter using Redis?

How do you implement a leaderboard with Sorted Sets?

How do you implement distributed locks with Redis (Redlock)?

What is the SET NX pattern for distributed locking?

Troubleshooting(4 questions)

How do you diagnose Redis memory issues?

What causes Redis to become unresponsive?

How do you handle "OOM command not allowed" errors?

How do you handle Redis latency spikes?