Redis Interview Questions & Answers

  1. Question 1. What Is Redis?

    Answer :

    Redis is an open source released under BSD licensed, in-memory data structure store, it can be used as a database, cache and message broker.

  2. Question 2. What Is Main Feature Of Redis?

    Answer :

    • Data is being persistent in-memory as well as stored on disk that’s make Redis fast.
    • Redis supports simple master to slave replication.
    • Redis can execute 1, 00,000 queries per second.
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  4. Question 3. What Is Advantages Of Using Redis?

    Answer :

    • High speed
    • Server-side locking
    • Rich Client Side Library
    • Support Atomic Operation
  5. Question 4. List The Data Structures Supported By Redis?

    Answer :

    Redis supports following Data Structures:

    • Strings
    • Hashes
    • Lists
    • Sets
    • Sorted sets with range queries
    • Bitmaps
    • Hyperloglogs
    • Geospatial indexes with radius queries
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  7. Question 5. What Is Limitations Of Redis?

    Answer :

    • Single threaded
    • It has got limited client support for consistent hashing
    • It has significant overhead for persistence
    • It is not deployed widely
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  9. Question 6. List Out The Operation Keys Of Redis?

    Answer :

    Operation keys of Redis includes:

    • TYPE key
    • TTL key
    • KEYS pattern
    • EXPIRE key seconds
    • EXPIREAT key timestamp
    • EXISTS key
    • DEL key
  10. Question 7. How We Can Use Redis With .net Application?

    Answer :

    For use Redis in .net application need to follow given steps:

    • Download Redis Server
    • Install Redis Server
    • Download Redis Client
    • Set Configuration into Web.config File
    • Use Redis Client Class
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  12. Question 8. What Is Redis-cli?

    Answer :

    Redis-cli is the Redis command line interface, a simple program that allows sending commands to Redis, and read the replies sent by the server, directly from the terminal.

  13. Question 9. Explain Repl?

    Answer :

    REPL stands for Read Eval Print Loop. It an interactive mode where the user types commands and get replies.

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  15. Question 10. What The Basic Features Are Of Redis Which Makes It Awesome Compared To Memcache?

    Answer :

    • Powerful data types and powerful commands to leverage them. Hashes, Sorted Sets, Lists, and more.
    • Persistence to disk, by default.
    • Transactions with optimistic locking (WATCH/MULTI/EXEC)
    • Pub/sub. Extremely fast.
    • Values up to 512MB in size (memcached limited to 1MB per key)
    • Lua scripting (as of 2.6)
    • Extremely fast at everything. Benchmarks are often conflicting, but this much is clear: when used like memcached Redis falls somewhere between nearly as fast or maybe even a little faster. Like memcached it is often bound by available network or even memory bandwidth instead of CPU or other bottlenecks and will rarely be the culprit when your app is slowing down.
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  17. Question 11. How Can You Use Redis With .net Application?

    Answer :

    To use Redis in .Net applications, follow these steps:

    • First, Download Redis Server.
    • Install Redis Server.
    • Download Redis Client.
    • Set Configuration into Web.config File.
    • Use Redis Client Class.
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  19. Question 12. We All Know That Reds Is Fast, But Is It Also Durable?

    Answer :

    No. Redis compromises with durability to enhance the speed. In Redis, in the case of system failure or crash, it writes to disk but may fall behind and lose the data which is not stored.

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  21. Question 13. What Are The Main Features Of Redis?

    Answer :

    Following are the main features of Redis:

    • Redis is very simple to install setup and manage.
    • Redis is very fast. It can execute 100000 queries per second.
    • Redis is fast because data is being persistent in memory as well as stored on the disk.
    • Redis operations working on different data types are atomic so these operations can be accomplished safely i.e. to set or increase
    • a key, add or remove elements from a set or increase a counter.
    • Redis supports a variety of languages i.e. C, C++, C#, Ruby, Python, Twisted Python, PHP, Erlang, Tcl, Perl, Lua, Java, Scala etc.
    • If your favorite language is not supported yet, you can write your own client library, as the Protocol is pretty simple.
    • Redis supports simple master to slave replication.
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  23. Question 14. In Which Language Redis Is Written?

    Answer :

    Redis is written in ANSI C and mostly used for cache solution and session management. It creates unique keys for store values.

  24. Question 15. What Are The Things You Must Have To Take Care While Using Redis?

    Answer :

    While using Redis, you must have to take care of following instructions:

    • Select a consistent method to name and prefix your keys. Manage your namespace.
    • Create a “Registry” of key prefixes that maps each of your internal documents for those applications which “own” them.
    • For every class you put through into your Redis infrastructure: design, implement and test the mechanisms for garbage collection or data migration to archival storage.
    • Design, implement and test a sharding library before you have invested much into your application deployment and make sure that you keep a registry of “shards” replicated on each server.
    • Separate all your K/V store and related operations into your own library/API or service.
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  26. Question 16. What Is The Difference Between Overriding A Value By Using Set Vs Append?

    Answer :

    A value set by using SET command will set the value for the key.Doing it over again will override the value. However , using append has the effect of set only when the key has no value earlier, but if there is a value already assigned to the key , then doing append on the key would lead to an appended value to the existing value of the key.

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  28. Question 17. What Do You Mean By Data Modeling In Redis?

    Answer :

    Just like any other database, data modeling represents the storage pattern of how and which data structures are used to store the data to achieve a domain requirement. For example in relational database world, we use primary key to establish a relationship between two entities. Data stored in relational databases are in table format, where as in Redis there are set of data structures available, which are used to represent the domain data. There is certainly different design mindset needed to convert the relational data to a Redis dataset.

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  30. Question 18. Why Redis Is Different As Compared To Other Key-value Stores?

    Answer :

    • Redis values can contain more complex data types, with atomic operations.
    • Redis is an in-memory but persistent on disk database.
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  32. Question 19. How Do I Move A Redis Database From One Server To Another?

    Answer :

    • use BGSAVE command to Save a spanshot of the database into a dump.rdb
    • Copy this dump.rdb file into another server.
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  34. Question 20. How To Re-start Redis?

    Answer :

    You can restart Redis by using following path:

    /etc/init.d/redis-server restart

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  36. Question 21. How To Start Redis?

    Answer :

    You can start Redis by using following path:

    /etc/init.d/redis-server start

  37. Question 22. How To Stop Redis?

    Answer :

    You can stop Redis by using following path:
    /etc/init.d/redis-server stop

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  39. Question 23. How To Get All Keys From Redis?

    Answer :

    Use the following commands:

    $keyList = $redis->keys(“*”);


    /*Array ( [0] => tutorials [1] => name ) */

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  41. Question 24. How To Get Array Data From Redis?

    Answer :

    To get the array data from Redis use the following command :

    $list = $redis->lrange(“tutorials”, 0 ,5);   
    /*Array ( [0] => Mysql [1] => Mongodb [2] => Redis [3] => MySQL [4] => PHP [5] => Mysql ) */

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  43. Question 25. How To Set Multiple Values (array) In Redis?

    Answer :

    To set multiple values in Redis use the following commands:

    $redis->lpush(“tutorials”, “PHP”);
    $redis->lpush(“tutorials”, “MySQL”);
    $redis->lpush(“tutorials”, “Redis”);
    $redis->lpush(“tutorials”, “Mongodb”);
    $redis->lpush(“tutorials”, “Mysql”);

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  45. Question 26. How To Get Value From Redis?

    Answer :

    To get value from Redis :

    echo $redis->get(‘name’); //Set string in redis

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  47. Question 27. How To Set Value In Redis?

    Answer :

    To set the value in Redis use the following command:

    $redis->set(“name”, “Set string in redis”);

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  49. Question 28. How To Check Redis Is Running?

    Answer :

    To check Redis is running try out the following code :

        $redis = new Redis();
        $redis->connect(‘’, 6379);
        echo “Redis is running.”;
        echo “Server is running: ” . $redis->ping();
    catch (Exception $e)
        echo $e->getMessage();

  50. Question 29. How To Remove All Database?

    Answer :

    To remove all database use the following code:

    redis-cli flushall

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  52. Question 30. How To Delete Current Database?

    Answer :

    To delete current database use the following code:

    redis-cli flushdb

  53. Question 31. What Do You Mean By “redis Is Binary Safe”?

    Answer :

    Binary safe means that it has a known length but not limited by any special character. You can store any value upto the given size. A string value can be 512 MB in length.

  54. Question 32. How Does Redis Differ From Mongodb? Is There A Use Case When For Redis If Using Mongodb?

    Answer :

    MongoDB is an unstructured (at least, in definition) distributed document storage. For clearing purpose you can think about it as a high scalable and not so high performance JSON storage (though actually it uses BSON). It has a simple but effective multitype index system, replication and sharding, and a lot of more nice features.

    Redis is a simplest key/value store, with transient data caching. It can works as an efficient queue system.

    Looking for more specific information you will find a lot of scenarios where both them are used, Mongo as a true persistent canonical storage, and Redis as an transient/cache storage.

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