Top 10 Redis Interview Questions and Answers for 2023

Redis has become one of the most popular in-memory data stores used as a database, cache, and message broker With large companies like Twitter, Pinterest, and Snapchat relying on Redis, it’s no surprise that Redis experience is a sought-after skill for developers and engineers.

If you have an upcoming job interview that involves Redis you need to be prepared for the types of Redis interview questions you’ll face. In this article we provide the top 10 Redis interview questions and example answers to help you ace your next interview.

1. List the Main Operation Keys of Redis

Some of the most common Redis operation keys include

• KEYS – Returns all keys matching a specified pattern
• TYPE – Returns the data type of a key
• EXISTS – Checks if a key exists
• DEL – Deletes one or more keys
• EXPIRE – Sets a timeout on a key
• TTL – Gets the remaining time to live for a key
• GET – Returns the value of a key
• SET – Sets the value for a key
• INCR – Increments the number stored at a key
• LPUSH/RPUSH – Pushes data to the head/tail of a list

Knowing the basic Redis keys shows your understanding of how to perform fundamental operations like getting, setting, deleting, and expiring cache data.

2. Differentiate Between Redis and MongoDB

While Redis and MongoDB are both popular NoSQL databases, there are some key differences:

• Data Types – Redis supports simple data structures like strings, hashes, lists, sets, etc. MongoDB supports more complex data objects and rich queries.

• Use Cases – Redis is often used for caching, message brokering, and transient data. MongoDB is better suited for permanent storage of complex application data.

• Data Persistence – Redis provides optional persistence – data is lost if Redis server restarts. MongoDB persists data to disk by default.

• Query Support – Redis only supports basic GET/SET type operations. MongoDB offers advanced query functionality like filtering, sorting, aggregations, etc.

• Scaling – Redis scales via sharding. MongoDB scales via sharding and replication.

Highlighting your grasp of when to use Redis vs MongoDB demonstrates you understand the strengths and differences of each database.

3. Mention and Describe Some Commands of Redis

Some important Redis commands include:

• SET – Sets a key-value pair. For example, SET username john creates a key called “username” with value “john”.

• GET – Retrieves the value of a key. For example, GET username would return “john”.

• INCR – Increments a numeric key value. Can be used to implement counters.

• EXPIRE – Sets a timeout on a key after which it will automatically be deleted. Useful for cache management.

• LPUSH/RPUSH – Adds a value to the head/tail of a list stored in a key. Useful for queues.

• SADD – Adds a member to a set stored in a key. Helpful for managing collections with uniqueness.

Discussing a few common commands shows you have practical experience in leveraging Redis beyond a basic cache.

4. Explain the SET Operations of Redis

Redis Sets are an unordered collection of unique strings stored under a single key. Some key operations include:

• SADD – Adds new members to a Set key. For example, SADD colors red green blue adds those colors to a “colors” set.

• SMEMBERS – Returns all members of a Set.

• SISMEMBER – Checks if a value is contained in a Set. Returns 1 if member exists, 0 otherwise.

• SDIFF – Returns the difference between two or more Sets. Helpful for finding elements only present in one Set.

• SUNION – Returns the union of one or more Sets.

Understanding Redis Sets shows you can utilize appropriate data structures to model your data efficiently.

5. What is Meant by Data Modeling in Redis?

Data modeling refers to strategically organizing data across different Redis data structures to optimize for your application’s use cases.

For example, a social media feed might store posts in a Sorted Set keyed by timestamps to maintain time ordering. Comments on each post could be stored in a Hash where the key is the post ID. User profiles could be stored in individual Hashes with fields for name, bio, etc.

The goal is to pick data structures that can support your app’s queries and operations in the most effective way. This requires understanding your access patterns and Redis capabilities like sorted sets for ordered data or Hashes for object storage.

6. Differentiate Between Sharding and Replication in Redis

Sharding and replication both help scale out Redis but in different ways:

• Sharding splits the dataset across multiple Redis instances such that each instance contains only a subset of the keys. This allows for horizontal scaling of writes and storage capacity.

• Replication provides redundant copies of the dataset for high availability. Write operations are performed on a single master instance that replicates changes to one or more slave replicas. Replication improves read scalability and resilience.

Sharding is useful when you need to scale writes and storage for large datasets. Replication helps scale out reads and provide failover for high availability. Using both sharding and replication is a common strategy for large production Redis deployments.

Understanding the fundamentals of sharding and replication shows you grasp Redis scaling capabilities at an advanced level.

7. How Does Redis Provide Data Persistence?

Redis offers two main options for data persistence:

• RDB snapshots – Redis periodically dumps all data to disk as an RDB snapshot. This allows for point-in-time recovery in case of failure.

• AOF log – Every write operation is logged to an AOF (append-only file) that Redis replays on restart to rebuild state. AOF can be configured for fsync policies ranging from always fsyncing every write to never fsyncing for better performance.

By default, Redis is ephemeral – all data is lost on restart. Enabling either RDB or AOF provides durability, with AOF offering better data loss minimization due to more frequent fsyncing. Managing persistence properly is critical for production robustness according to data sensitivity.

8. When Would You Use Redis Pub/Sub Messaging?

Redis Pub/Sub is useful for building real-time, scalable messaging pipelines between processes:

• Chat apps – Chat rooms with many recipients for each message

• Notifications – Fanning out updates to relevant users

• Event streaming – Propagating state changes to interested services

Pub/Sub allows broadcasting messages to many interested subscribers without point-to-point queues. Subscribers can listen on channels selectively. Workloads with many producers and consumers are well-suited for Pub/Sub.

Describing Pub/Sub use cases demonstrates understanding of applying Redis beyond a simple cache to power asynchronous communication at scale.

9. How Does Redis Lua Scripting Work?

Redis Lua scripting allows atomic execution of multiple Redis commands as a transaction via a Lua script:

• Script contains code to execute multiple operations like GET, INCR, etc.

• Entire script runs atomically – no other clients can intervene

• Great for ensuring atomic updates, e.g. incrementing a counter

• Can also implement client-side logic like input validation

• Executed on Redis server so avoids network round trips

Lua support makes Redis more than a simple key-value store, enabling atomic transactions and app logic within Redis itself. This improves performance through reduced network messaging.

10. How Can You Troubleshoot Redis Performance Issues?

Some best practices for diagnosing Redis performance include:

• Profile load using Redis INFO command stats like total operations, connected clients, requests per second

• Check for slow commands with SLOWLOG – can help identify hot keys/expensive operations

• Monitor memory fragmentation with INFO memory and MEMORY STATS

• Leverage client-side monitoring tools like Redis Insight to identify bottlenecks

• Simulate load with Redis benchmarking tools to reproduce issues

• Tune configuration like maxmemory policy, eviction thresholds, timeout values

Discussing performance tuning demonstrates hands-on experience running Redis at scale. Key is having a methodology to measure, benchmark, and isolate areas for optimization.

Summary

This covers the top Redis interview questions you should be prepared for. Focus on highlighting your hands-on experience with Redis in production, understanding of Redis data structures and capabilities, and ability to solve problems like scaling and troubleshooting. With the right preparation, you can confidently ace your next Redis interview!