Redis Interview Questions: Preparing You for Your First Job

Ajay Kumar Reddy 02 Feb, 2023 • 8 min read

Introduction

Year after year, the intake for either freshers or experienced in the fields dealing with Data Science, AI/ML, and Data Engineering has been increasing rapidly. Besides different programming language skills and AI/ML skills, recruiters are always on the hunt for more knowledgeable workers with an idea of as many databases as possible. And one such database is Redis, a popular NoSQL database.

Redis is a widely used in-memory database that acts like a cache or database and message broker. It is used in areas requiring low latencies, like reading real-time data or performing analysis on real-time data. Redis allows users to store data in different formats through its different data types like strings, hashes, geospatial indexes, and much more. Redis allows you to increase its Data Structures by loading different Redis Modules.

Knowing Redis interview questions is essential when applying for jobs in the fields of Data Science, Data Engineering, and Database. Redis is employed in many companies all over the world. Popular companies like Netflix, Pinterest, Uber, Airbnb, Swiggy, etc., use Redis as their secondary database or cache. If you’re new to Redis, this article can help you prepare for your following job interview by listing common Redis Interview Questions and answers.

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Learning Objectives

  1. To learn Redis interview questions for the first interview.
  2. To gain an overall understanding of what Redis is and its use cases.
  3. To become familiar with the strengths as well as limitations of Redis.
  4. To understand the common Data Structures that Redis support.
  5. To know how data persistence takes place in Redis.
  6. Identify different Redis Modules.

This article was published as a part of the Data Science Blogathon.

Table of Contents

  1. What is Redis, and why is it used?
  2. How is Redis different from other NoSQL databases?
  3. What all Data Structures Does Redis Support?
  4. What are Redis Streams, and Where can you Use Them?
  5. What is the Difference Between DEL and UNLINK in Redis?
  6. Explain Pipelining in Redis.
  7. What Are the Different Modules Available in Redis?
  8. How is Pub/Sub Different From Streams in Redis?
  9. What are the Range Commands Used for Sorted Sets(SS)?
  10. What is an Eviction in Redis, and Name the Different Eviction Policies?
  11. How does Persistence Work in Redis?
  12. Explain Sharding in Redis.
  13. Redis Developer Certification – A Step Ahead
  14. Conclusion
Redis Interview Questions

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Q1. What is Redis, and Why is it Used?

Answer: Redis is a free, open-source in-memory database server that can act like a cache, database, or message broker. It is quick, efficient, and capable of handling massive amounts of data, making it a popular choice for several applications like session management, real-time analytics, leaderboards, and message queuing.

Q2. How is Redis Different From Other NoSQL Databases?

Answer: Redis is a NoSQL database that differs from many of them. Firstly, Redis is an in-memory database that stores data in memory instead of other databases that store data on disk. Also, Redis supports several data types for different use cases. Unlike other NoSQL databases, Redis is designed for high performance due to its in-memory nature. Redis supports master-slave replication, allowing for easy horizontal scaling and handling increasing data volumes and traffic volumes.

Q3. What all Data Structures Does Redis Support?

Answer: Redis supports a wide variety of data types. They include hashes, lists, strings, sets, streams, geospatial indexes, bitfields, sorted sets, bitmaps, and hyperloglogs. These different Data Structures in Redis are used in different scenarios, from queuing to caching to event processing. These Data Structures store the data and retrieve it in unique ways.

Q4. What are Redis Streams, and Where can you Use Them?

Answer: Redis Streams is a newly introduced Data Structure in Redis. Streams behave like an append-only list. Redis Streams are implemented based on radix trees, thus making most of its commands, like accessing a value from a Redis Stream at a random place and appending data to a Redis Stream, have a time complexity of O(1). Entries to the Redis Stream are added in the form of hashes. Each entry to a Redis Stream is associated with a unique ID. Stream entries are immutable, i.e., once appended cannot be changed. However, you can delete an entry from a Redis Stream. Streams can be used for saving real-time data, like collecting sensor data from a faraway location, and can act as a record to store notifications of a particular user or when you want to process continuous data.

Q5. What is the Difference Between DEL and UNLINK in Redis?

Answer: DEL and UNLINK are two commands to delete data in Redis. DEL command, when used, deletes the key from the server. Deleting N keys using the DEL command takes O(N) time. So when you use DEL to delete many keys at a single time, it will block the server until all keys are deleted. At the same time, the UNLINK command unlinks the key from the keyspace. The actual removal of data takes place at a later point in time, thus making it asynchronous. It only takes O(1) time to UNLINK many keys simultaneously because it doesn’t remove them at a single go and thus has a non-blocking nature. So it’s better to go with UNLINK rather than DEL for better performance.

Q6. Explain Pipelining in Redis.

Answer: Redis pipelining is a way to build up performance by sending in more than one command, that is, multiple commands simultaneously, without waiting for each command’s response. After all the commands that are sent are executed, the responses of all the commands are sent to the client. This way of sending in multiple commands at a single time and receiving all the responses in one go greatly reduces the round trip time (RTT), i.e., that is, the time taken by the command to go from the client to the Redis server and come back again to the client. It is a good practice to send multiple messages in the form of batches because queueing these replies that are to be sent to the client from the server will end up taking a lot of memory.

Q7. What Are the Different Modules Available in Redis?

Answer: Redis modules are packages that can be accessed using the MODULE LOAD command or loaded into Redis at startup. Redis Modules allow rapid implementation of new Redis commands. They do this by extending Redis functionality through the use of external modules. The Redis modules include:
  1. RedisJSON: This module is used with Redis when JSON data type needs to be stored in Redis. It even allows you to update JSON data in Redis

  2. RediSearch: By default, Redis doesn’t support indexing like other Relational Databases. The RediSearch module with Redis allows you to perform querying and secondary indexing and even provides a full-text search.

  3. RedisTimeSeries: This module extends Redis to allow saving time series in Redis. It supports saving multiple time series. It has features like aggregating data, taking a maximum, and many more.

  4. RedisGraph: It is a graph database that is built on top of Redis.

  5. RedisBloom: It consists of a set of probabilistic Data Structures. Four of them include the cuckoo filter, bloom filter, top-k, and count-min sketch.

  6. RedisGears: It is an engine that can be programmed by different programming languages like Python, and JS, to perform data processing in Redis itself.

  7. RedisAI: is a module useful for storing features in machine learning or deep learning development.

Q8. How is Pub/Sub Different From Streams in Redis?

Answer: In Publish and Subscribe, if a message/entry is sent to an offline client, then that client never receives that note and is forever lost. Whereas in Streams, the client will receive them when the client goes online. In Streams, the data will be stored, so they can be later consumed by the consumer groups. But coming to Pub/Sub, it never saves published entries. Pub/Sub is used where there is a need for at-most-once delivery semantics. Streams are used when there is a need for at-most-once or at-least-once delivery semantics.

Q9. What are the Range Commands Used for Sorted Sets(SS)?

Answer: Range commands available in SS are:
  1. ZRANGE: It is used to get a range of members in a SS with low to high scores.

  2. ZREVRANGE: Similar to ZRANGE but returns the members in a reverse SS order, with high to low scores.

  3. ZRANGEBYSCORE: Gets all the members in a SS by order of their scores.

  4. ZREVRANGEBYSCORE: Similar to the ZRANGEBYSCORE, but returns the members in a reverse SS order.

  5. ZRANGEBYLEX: Gets all the members in a SS by lexicographical range.

  6. ZREVRANGEBYLEX: Similar to the ZRANGEBYLEX, but returns the members in a reverse SS order.

Q10. What is an Eviction in Redis, and Name the Different Eviction Policies?

Answer: Eviction is a process of removing data. Redis, which widely acts like a cache, it’s necessary for the Redis Server to remove the old data stored in the cache, so the new data can get in. There are different eviction policies in Redis to select and remove the data from the server. These include allkeys-lru, volatile-lru, noeviction, allkeys-lfu, volatile-lfu, volatile-random, allkeys-random, and volatile-ttl. These policies are based on the Least Frequently Used (LFU), Least Recently Used (LRU), and whether the expiry is set to a Redis Key.

Q11. How does Persistence Work in Redis?

Answer: Persistence is a way of saving data on a disk. Redis, an in-memory database, needs to save data to the disk, so the data is not lost on startup. There are different options to persist data in Redis. These include:
  1. No Persistence: This option is selected when you do not want to persist data. This option is used when Redis acts like a cache instead of a database.

  2. RDB (Redis Database): At predetermined intervals, RDB persistence creates point-in-time snapshots of your dataset. These are stored as .rdb files. It maximizes the Redis performance but has low durability compared to AOF.

  3. AOF (Append Only File): AOF persistence option is used when you want to log every write operation that the client sends to the server. Every time the system starts up, it goes through this log, thus recreating the entire dataset. It is highly durable compared to RDB but affects the performance of Redis.

Q12. Explain Sharding in Redis.

Answer: Sharding is the process of breaking a large dataset into small chunks. These chunks are known as shards, and the processing is known as sharding. Sharding is done when you want to scale Redis horizontally. Sharding is performed based on the hash functions. Each key in Redis is assigned a hash, and this hash function is responsible for mapping these keys to a particular shard. This function ensures that all the keys are equally distributed among the shards.

Redis Developer Certification –  A Step Ahead

Certificates are a great way to showcase your talent. These might also be really helpful when coming to the interview and preparing for Redis interview questions. Having a Redis Developer Certification boosts your confidence in Redis and gives you an advantage over others during your job interviews.

Redis Interview Questions

Redis Certified Developer Badge

 

Redis provides free courses on their Redis University website, which you can enroll in and then learn and clear accordingly at your own pace. After clearing these courses, you will be eligible for the Redis Developer Certification exam from Redis, which you can attempt free of cost. Upon clearing the exam, you will be awarded the certification. Clearing this certification exam ensures you have a solid foundation on the basics of Redis. This certification will greatly boost your knowledge of Redis and your confidence in the interviews and will put you a step ahead of your peers.

Conclusion

It is good practice to prepare for an interview to showcase your knowledge, be it a pro or a beginner in Redis. Going through these common Redis interview questions, you’ll be ready to show your understanding of Redis and its use cases. One must be good with the basics of Redis, like the Redis commands. Additionally, by being familiar with Redis Interview Questions, you’ll be able to show that you can handle a variety of data management and storage responsibilities, making you an invaluable asset. Having knowledge of at least one of the Redis clients can be handy.

The following are the key takeaways from this article:

  • Redis interview questions help in acing your interview and bagging the job of your dreams.
  • Redis is an in-memory database that stores data as key-value pairs.
  • Redis Server supports a wide range of Data Structures.
  • Redis is commonly employed when there is a need for real-time data collection/processing, session management, game leaderboards, etc.
  • External modules can be loaded into Redis to create new Data Structures.
  • Having a Redis Developer Certification will greatly boost your knowledge of Redis and confidence in your interviews.

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Ajay Kumar Reddy 02 Feb 2023

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