30 COMMON REDIS INTERVIEW QUESTIONS

A. Beginner-Level Redis Questions:

1. What is Redis?

   • Redis (Remote Dictionary Server) is an open-source, in-memory key-value data store that can be used as a database, cache, and message broker.

2. What are the major features of Redis?

   • In-memory data storage, persistence, data replication, sharding, support for different data types, atomic operations, pub/sub messaging, and Lua scripting.

3. How is Redis different from traditional relational databases?

   • Redis is an in-memory key-value store with extremely low latency, while traditional relational databases are typically disk-based and use structured query language (SQL) for data retrieval. Redis also supports non-relational data structures like lists, sets, and hashes.

4. What are some common use cases of Redis?

   • Caching, session management, real-time analytics, leaderboards, message queues, distributed locking, and pub/sub messaging.

5. What data structures does Redis support?

   • Strings, Lists, Sets, Sorted Sets, Hashes, Bitmaps, HyperLogLogs, Streams, and Geospatial indexes.

6. What is a Redis key, and what are the naming conventions for Redis keys?

   • A key in Redis is a unique identifier for a piece of data. Redis keys are binary-safe and can be up to 512MB in length. Common naming conventions include using colons (:) as separators to form a namespace (e.g., user:1001:info).

7. What is Redis caching?

   • Redis caching involves using Redis as a high-performance, in-memory store for frequently accessed data to improve application response times by reducing the need to query a slower, persistent database.

8. How do you set a value in Redis?

   • The command SET key value is used to store a value in Redis under a given key.

9. How do you retrieve a value from Redis?

   • The command GET key retrieves the value associated with the specified key.

10. What is the EXPIRE command in Redis?

    • The EXPIRE key seconds command sets a timeout on a key, after which the key will be automatically deleted. The expiration time is specified in seconds.

11. What is the difference between SETEX and SET with EXPIRE?

    • SETEX sets a key with a value and an expiration time in a single command (SETEX key seconds value), whereas SET followed by EXPIRE involves two commands to achieve the same effect.

12. What is a Redis eviction policy?

    • Redis eviction policy determines how Redis handles memory when it reaches the maximum memory limit. Common policies include noeviction, allkeys-lru, volatile-lru, allkeys-random, and volatile-ttl.

13. What is the Least Recently Used (LRU) eviction policy in Redis?

    • The LRU policy evicts the least recently used keys when Redis reaches its memory limit. This ensures that frequently accessed data stays in memory.

14. What is the purpose of the FLUSHALL command in Redis?

    • FLUSHALL is used to remove all keys from all databases in Redis, effectively clearing the entire Redis instance.

15. How does Redis ensure data persistence?

    • Redis provides two methods for persistence: RDB snapshots (point-in-time snapshots of the dataset) and AOF (Append-Only File), which logs every write operation for more durable persistence.

B. Intermediate-Level Redis Questions:

1. What is a Redis cluster?

   • Redis Cluster is a distributed implementation that automatically shards data across multiple Redis nodes, providing high availability and scalability without a single point of failure.

2. What are the differences between Redis replication and Redis clustering?

   • Replication involves copying data from a master node to one or more slave nodes to provide redundancy and read scalability. Clustering partitions data across multiple master nodes, providing horizontal scalability.

3. What is the TTL command in Redis, and what does it do?

   • TTL key returns the remaining time to live (in seconds) for a key that has a set expiration.

4. What is Redis Pub/Sub?

   • Redis Pub/Sub (Publish/Subscribe) is a messaging pattern that allows one or more publishers to send messages to multiple subscribers through channels, enabling real-time communication between services.

5. What is the difference between Redis Lists and Sets?

   • Lists in Redis are ordered collections of strings that allow duplicate elements, whereas Sets are unordered collections of unique strings.

6. What is a Redis Sorted Set?

   • A Sorted Set is a collection of unique elements, where each element is associated with a score. The elements are ordered by their score, allowing for efficient range queries based on scores.

7. How do you implement caching with Redis in a web application?

   • A common pattern is to first check if the data exists in Redis (cache). If the data is not present, the application queries the database, stores the result in Redis, and returns the data. This reduces the load on the database for subsequent requests.

8. What is a Redis transaction, and how is it executed?

   • A Redis transaction groups a series of commands together and executes them sequentially. It is started with the MULTI command, followed by the commands to execute, and then EXEC to run the transaction.

9. What are Redis pipelines?

   • Pipelining allows multiple commands to be sent to Redis at once without waiting for a response to each command. This reduces the round-trip time and increases throughput.

10. What is Redis Sentinel?

    • Redis Sentinel is a system for managing and monitoring Redis instances, ensuring high availability by automatically performing failover if the master node goes down, and notifying the application of changes in the Redis topology.

C. Advanced-Level Redis Questions:

1. How does Redis handle concurrency?

   • Redis is single-threaded for executing commands, which avoids concurrency issues at the application level. However, Redis uses I/O multiplexing to handle multiple connections concurrently.

2. What is a Redis Lua script, and when would you use it?

   • Redis supports Lua scripting to execute a sequence of commands atomically. Scripts are useful for performing multiple operations that should be treated as a single atomic operation, such as checking for a value before updating it.

3. What is the difference between Redis mget() and get()?

   • get() retrieves the value of a single key, while mget() retrieves the values of multiple keys at once, which reduces the overhead of multiple round-trips to Redis.

4. How do Redis Streams work?

   • Redis Streams is a data structure for managing log-like data, supporting consumer groups, message acknowledgment, and persistence. It is useful for event sourcing, message queues, and real-time data processing.

5. How do you scale Redis for high availability and fault tolerance?

   • To scale Redis, you can use replication to distribute read traffic, clustering for horizontal scaling and data partitioning, and Redis Sentinel for monitoring and automatic failover to ensure high availability and fault tolerance.