Redis Cache Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is Redis Cache?
Redis is an open-source, advance key value data store and cache. It is also referred as a data structure server which keys not only contains strings, but also hashes, sets, lists, and sorted sets.
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Ques 2. What is the meaning of Redis?
Redis stands for REmote DIctionary Server.
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Ques 3. How is Redis different from other databases?
Redis is a NoSQL, Opensource, in-memory data-structure store. It follows the principle of key-value store.
It is extremely fast, persistent, portable and supports many languages such as C, C++, C#, Clojure, Common Lisp, D, Dart, Erlang, Go, Haskell, Haxe, Io, Java, JavaScript (Node.js), Julia, Lua, Objective-C, Perl, PHP, Pure Data, Python, R, Racket, Ruby, Rust, Scala, Smalltalk and Tcl.
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Ques 4. In which language Redis is written?
Redis is written in ANSI C and mostly used for cache solution and session management. It creates unique keys for store values.
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Ques 5. What is the usage of Redis?
Redis is a special key-value store database that can function as a NoSQL database or as a memory-cache store to improve performance when serving data that is stored in system memory.
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Ques 6. How to interect with Redis?
After the installation of the server you can run the Redis Client provided by redis installation or you can open the command prompt and use the following command:
redis-cli
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Ques 7. Which are the most popular companies using Redis?
Twitter, Github, Stackoverflow etc. are the most popular companies using Redis.
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Ques 8. What are the main features of Redis?
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 is very fast because it loads the whole dataset in primary memory.
- 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.
- It supports various types of data structure such as strings, hashes, sets, lists, sorted sets etc.
- 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.
- Redis is portable.
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Intermediate / 1 to 5 years experienced level questions & answers
Ques 9. Explain the Replication Features of Redis?
Replication is important in order to archive high level of availability in big data systems. The data needs to be replicated at n number of places. This follows the master-slave approach where the master copy is maintained by master-slave and replicated to n other nodes.
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Ques 10. What are the advantages of using Redis?
Following is a list of some important advantages of Redis:
- Redis is very fast.
- It supports a server-side locking.
- It has a rich client side library.
- It is a good counter.
- It supports Atomic Operation.
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Ques 11. What are the disadvantages/limitations of using Redis?
Following are the disadvantages/ limitations of Redis:
- It is 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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Ques 12. What is the difference between Redis and RDBMS?
There are a lot of differences between Redis and RDBMS:
- Redis is a NoSQL database while RDBMS is an SQL database.
- Redis follows the key-value structure while RDBMS follows the table structure.
- Redis extremely fast while RDBMS is comparatively slow.
- Redis stores all the dataset in primary memory while RDBMS stores its dataset in secondary memory.
- Redis is generally used to store small and frequently used files while RDBMS is used to store big files.
- Redis provides only official support for Linux, BSD, Mac OS X, Solaris. It doesn?t provide official support for Windows currently while RDBMS provides support for both.
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Experienced / Expert level questions & answers
Ques 13. Differentiate Memcached and Redis?
| Memcached | Redis |
|---|---|
| Memcached only does caching information. | It provides some more functionalities like replication and persistence along with caching information. |
| Memcached supports the functionality of LRU (Least Recently Used) eviction of values. | LRU is not supported by Redis. |
| In Memcached, when they overflow memory, the one you have not used recently (LRU- Least Recently Used) will get deleted. | In Redis, there is a time set for each function, Three keys are maintained, the one, which is closest to expiry, will get deleted. |
| CAS (Check and Set) is supported by Memcached. | CAS is not supported by Redis. |
| Array objects are needed to be serialized in order to get saved. We need to unserialize them for their retrieval. | Redis has got stronger data structures; it can handle strings, binary safe strings, list of binary safe strings, sorted lists, etc. |
| Memcached has at most 250 bytes length. | Redis has at most 2 GB key length. |
| It is Multi-threaded | It is single threaded. |
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Ques 14. Mention Operation Keys of Redis.
- TYPE key
- TTL key
- EXPIRE key seconds
- EXPIREAT key timestamp
- EXISTS key
- DEL key
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Ques 15. Which are the different data types used in Redis?
There are mainly 5 types of data types supported by Redis:
- Strings
- Hashes
- Lists
- Sets
- Sorted Sets
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Ques 16. Which are the most popular commands of the Redis database?
| Command | Description |
|---|---|
| APPEND key value | Append a value to a key |
| AUTH password | Authenticate to the server |
| BGREWRITEAOF | Asynchronously rewrite the append-only file |
| BGSAVE | Asynchronously save the dataset to disk |
| BITCOUNT key [start end] | Count set bits in a string |
| BITFIELD key [GET type offset] [SET type offset value] [INCRBY type offset increment] [OVERFLOW WRAP|SAT|FAIL] | Perform arbitrary bitfield integer operations on strings |
| BITOP operation destkey key [key ...] | Perform bitwise operations between strings |
| BITPOS key bit [start] [end] | Find first bit set or clear in a string |
| BLPOP key [key ...] timeout | Remove and get the first element in a list, or block until one is available |
| BRPOP key [key ...] timeout | Remove and get the last element in a list, or block until one is available |
| BRPOPLPUSH source destination timeout | Pop a value from a list, push it to another list and return it; or block until one is available |
| CLIENT KILL [ip:port] [ID client-id] [TYPE normal|master|slave|pubsub] [ADDR ip:port] [SKIPME yes/no] | Kill the connection of a client |
| CLIENT LIST | Get the list of client connections |
| CLIENT GETNAME | Get the current connection name |
| CLIENT PAUSE timeout | Stop processing commands from clients for some time |
| CLIENT REPLY ON|OFF|SKIP | Instruct the server whether to reply to commands |
| CLIENT SETNAME connection-name | Set the current connection name |
| CLUSTER ADDSLOTS slot [slot ...] | Assign new hash slots to receiving node |
| CLUSTER COUNT-FAILURE-REPORTS node-id | Return the number of failure reports active for a given node |
| CLUSTER COUNTKEYSINSLOT slot | Return the number of local keys in the specified hash slot |
| CLUSTER DELSLOTS slot [slot ...] | Set hash slots as unbound in receiving node |
| CLUSTER FAILOVER [FORCE|TAKEOVER] | Forces a slave to perform a manual failover of its master. |
| CLUSTER FORGET node-id | Remove a node from the nodes table |
| CLUSTER GETKEYSINSLOT slot count | Return local key names in the specified hash slot |
| CLUSTER INFO | Provides info about Redis Cluster node state |
| CLUSTER KEYSLOT key | Returns the hash slot of the specified key |
| CLUSTER MEET ip port | Force a node cluster to handshake with another node |
| CLUSTER NODES | Get Cluster config for the node |
| CLUSTER REPLICATE node-id | Reconfigure a node as a slave of the specified master node |
| CLUSTER RESET [HARD|SOFT] | Reset a Redis Cluster node |
| CLUSTER SAVECONFIG | Forces the node to save cluster state on disk |
| CLUSTER SET-CONFIG-EPOCH config-epoch | Set the configuration epoch in a new node |
| CLUSTER SETSLOT slot IMPORTING|MIGRATING|STABLE|NODE [node-id] | Bind a hash slot to a specific node |
| CLUSTER SLAVES node-id | List slave nodes of the specified master node |
| CLUSTER SLOTS | Get array of Cluster slot to node mappings |
| COMMAND | Get array of Redis command details |
| COMMAND COUNT | Get total number of Redis commands |
| COMMAND GETKEYS | Extract keys given a full Redis command |
| COMMAND INFO command-name [command-name ...] | Get array of specific Redis command details |
| CONFIG GET parameter | Get the value of a configuration parameter |
| CONFIG REWRITE | Rewrite the configuration file with the in memory configuration |
| CONFIG SET parameter value | Set a configuration parameter to the given value |
| CONFIG RESETSTAT | Reset the stats returned by INFO |
| DBSIZE | Return the number of keys in the selected database |
| DEBUG OBJECT key | Get debugging information about a key |
| DEBUG SEGFAULT | Make the server crash |
| DECR key | Decrement the integer value of a key by one |
| DECRBY key decrement | Decrement the integer value of a key by the given number |
| DEL key [key ...] | Delete a key |
| DISCARD | Discard all commands issued after MULTI |
| DUMP key | Return a serialized version of the value stored at the specified key. |
| ECHO message | Echo the given string |
| EVAL script numkeys key [key ...] arg [arg ...] | Execute a Lua script server side |
| EVALSHA sha1 numkeys key [key ...] arg [arg ...] | Execute a Lua script server side |
| EXEC | Execute all commands issued after MULTI |
| EXISTS key [key ...] | Determine if a key exists |
| EXPIRE key seconds | Set a key\\\'s time to live in seconds |
| EXPIREAT key timestamp | Set the expiration for a key as a UNIX timestamp |
| FLUSHALL [ASYNC] | Remove all keys from all databases |
| FLUSHDB [ASYNC] | Remove all keys from the current database |
| GEOADD key longitude latitude member [longitude latitude member ...] | Add one or more geospatial items in the geospatial index represented using a sorted set |
| GEOHASH key member [member ...] | Returns members of a geospatial index as standard geohash strings |
| GEOPOS key member [member ...] | Returns longitude and latitude of members of a geospatial index |
| GEODIST key member1 member2 [unit] | Returns the distance between two members of a geospatial index |
| GEORADIUS key longitude latitude radius m|km|ft|mi [WITHCOORD] [WITHDIST] [WITHHASH] [COUNT count] [ASC|DESC] [STORE key] [STOREDIST key] | Query a sorted set representing a geospatial index to fetch members matching a given maximum distance from a point |
| GEORADIUSBYMEMBER key member radius m|km|ft|mi [WITHCOORD] [WITHDIST] [WITHHASH] [COUNT count] [ASC|DESC] [STORE key] [STOREDIST key] | Query a sorted set representing a geospatial index to fetch members matching a given maximum distance from a member |
| GET key | Get the value of a key |
| GETBIT key offset | Returns the bit value at offset in the string value stored at key |
| GETRANGE key start end | Get a substring of the string stored at a key |
| GETSET key value | Set the string value of a key and return its old value |
| HDEL key field [field ...] | Delete one or more hash fields |
| HEXISTS key field | Determine if a hash field exists |
| HGET key field | Get the value of a hash field |
| HGETALL key | Get all the fields and values in a hash |
| HINCRBY key field increment | Increment the integer value of a hash field by the given number |
| HINCRBYFLOAT key field increment | Increment the float value of a hash field by the given amount |
| HKEYS key | Get all the fields in a hash |
| HLEN key | Get the number of fields in a hash |
| HMGET key field [field ...] | Get the values of all the given hash fields |
| HMSET key field value [field value ...] | Set multiple hash fields to multiple values |
| HSET key field value | Set the string value of a hash field |
| HSETNX key field value | Set the value of a hash field, only if the field does not exist |
| HSTRLEN key field | Get the length of the value of a hash field |
| HVALS key | Get all the values in a hash |
| INCR key | Increment the integer value of a key by one |
| INCRBY key increment | Increment the integer value of a key by the given amount |
| INCRBYFLOAT key increment | Increment the float value of a key by the given amount |
| INFO [section] | Get information and statistics about the server |
| KEYS pattern | Find all keys matching the given pattern |
| LASTSAVE | Get the UNIX time stamp of the last successful save to disk |
| LINDEX key index | Get an element from a list by its index |
| LINSERT key BEFORE|AFTER pivot value | Insert an element before or after another element in a list |
| LLEN key | Get the length of a list |
| LPOP key | Remove and get the first element in a list |
| LPUSH key value [value ...] | Prepend one or multiple values to a list |
| LPUSHX key value | Prepend a value to a list, only if the list exists |
| LRANGE key start stop | Get a range of elements from a list |
| LREM key count value | Remove elements from a list |
| LSET key index value | Set the value of an element in a list by its index |
| LTRIM key start stop | Trim a list to the specified range |
| MGET key [key ...] | Get the values of all the given keys |
| MIGRATE host port key|\\\"\\\" destination-db timeout [COPY] [REPLACE] [KEYS key [key ...]] | Atomically transfer a key from a Redis instance to another one. |
| MONITOR | Listen for all requests received by the server in real time |
| MOVE key db | Move a key to another database |
| MSET key value [key value ...] | Set multiple keys to multiple values |
| MSETNX key value [key value ...] | Set multiple keys to multiple values, only if none of the keys exist |
| MULTI | Mark the start of a transaction block |
| OBJECT subcommand [arguments [arguments ...]] | Inspect the internals of Redis objects |
| PERSIST key | Remove the expiration from a key |
| PEXPIRE key milliseconds | Set a key\\\'s time to live in milliseconds |
| PEXPIREAT key milliseconds-timestamp | Set the expiration for a key as a UNIX timestamp specified in milliseconds |
| PFADD key element [element ...] | Adds the specified elements to the specified HyperLogLog. |
| PFCOUNT key [key ...] | Return the approximated cardinality of the set(s) observed by the HyperLogLog at key(s). |
| PFMERGE destkey sourcekey [sourcekey ...] | Merge N different HyperLogLogs into a single one. |
| PING [message] | Ping the server |
| PSETEX key milliseconds value | Set the value and expiration in milliseconds of a key |
| PSUBSCRIBE pattern [pattern ...] | Listen for messages published to channels matching the given patterns |
| PUBSUB subcommand [argument [argument ...]] | Inspect the state of the Pub/Sub subsystem |
| PTTL key | Get the time to live for a key in milliseconds |
| PUBLISH channel message | Post a message to a channel |
| PUNSUBSCRIBE [pattern [pattern ...]] | Stop listening for messages posted to channels matching the given patterns |
| QUIT | Close the connection |
| RANDOMKEY | Return a random key from the keyspace |
| READONLY | Enables read queries for a connection to a cluster slave node |
| READWRITE | Disables read queries for a connection to a cluster slave node |
| RENAME key newkey | Rename a key |
| RENAMENX key newkey | Rename a key, only if the new key does not exist |
| RESTORE key ttl serialized-value [REPLACE] | Rename a key, only if the new key does not exist |
| RESTORE key ttl serialized-value [REPLACE] | Create a key using the provided serialized value, previously obtained using DUMP. |
| ROLE | Return the role of the instance in the context of replication |
| RPOP key | Remove and get the last element in a list |
| RPOPLPUSH source destination | Remove the last element in a list, prepend it to another list and return it |
| RPUSH key value [value ...] | Append one or multiple values to a list |
| RPUSHX key value | Append a value to a list, only if the list exists |
| SADD key member [member ...] | Add one or more members to a set |
| SAVE | Synchronously save the dataset to disk |
| SCARD key | Get the number of members in a set |
| SCRIPT DEBUG YES|SYNC|NO | Set the debug mode for executed scripts. |
| SCRIPT EXISTS sha1 [sha1 ...] | Check existence of scripts in the script cache. |
| SCRIPT FLUSH | Remove all the scripts from the script cache. |
| SCRIPT KILL | Kill the script currently in execution. |
| SCRIPT LOAD script | Load the specified Lua script into the script cache. |
| SDIFF key [key ...] | Subtract multiple sets |
| SDIFFSTORE destination key [key ...] | Subtract multiple sets and store the resulting set in a key |
| SELECT index | Change the selected database for the current connection |
| SET key value [EX seconds] [PX milliseconds] [NX|XX] | Set the string value of a key |
| SETBIT key offset value | Sets or clears the bit at offset in the string value stored at key |
| SETEX key seconds value | Set the value and expiration of a key |
| SETNX key value | Set the value of a key, only if the key does not exist |
| SETRANGE key offset value | Overwrite part of a string at key starting at the specified offset |
| SHUTDOWN [NOSAVE|SAVE] | Synchronously save the dataset to disk and then shut down the server |
| SINTER key [key ...] | Intersect multiple sets |
| SINTERSTORE destination key [key ...] | Intersect multiple sets and store the resulting set in a key |
| SISMEMBER key member | Determine if a given value is a member of a set |
| SLAVEOF host port | Make the server a slave of another instance, or promote it as master |
| SLOWLOG subcommand [argument] | Manages the Redis slow queries log |
| SMEMBERS key | Get all the members in a set |
| SMOVE source destination member | Move a member from one set to another |
| SORT key [BY pattern] [LIMIT offset count] [GET pattern [GET pattern ...]] [ASC|DESC] [ALPHA] [STORE destination] | Sort the elements in a list, set or sorted set |
| SPOP key [count] | Remove and return one or multiple random members from a set |
| SRANDMEMBER key [count] | Get one or multiple random members from a set |
| SREM key member [member ...] | Remove one or more members from a set |
| STRLEN key | Get the length of the value stored in a key |
| SUBSCRIBE channel [channel ...] | Listen for messages published to the given channels |
| SUNION key [key ...] | Add multiple sets |
| SUNIONSTORE destination key [key ...] | Add multiple sets and store the resulting set in a key |
| SWAPDB index index | Swaps two Redis databases |
| SYNC | Internal command used for replication |
| TIME | Return the current server time |
| TOUCH key [key ...] | Alters the last access time of a key(s). Returns the number of existing keys specified. |
| TTL key | Get the time to live for a key |
| TYPE key | Determine the type stored at key |
| UNSUBSCRIBE [channel [channel ...]] | Stop listening for messages posted to the given channels |
| UNLINK key [key ...] | Delete a key asynchronously in another thread. Otherwise it is just as DEL, but non blocking. |
| UNWATCH | Forget about all watched keys |
| WAIT numslaves timeout | Wait for the synchronous replication of all the write commands sent in the context of the current connection |
| WATCH key [key ...] | Watch the given keys to determine execution of the MULTI/EXEC block |
| ZADD key [NX|XX] [CH] [INCR] score member [score member ...] | Add one or more members to a sorted set, or update its score if it already exists |
| ZCARD key | Get the number of members in a sorted set |
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Ques 17. We all know that Reds is fast, but is it also durable?
In Redis, there is always a trade-of between durability and speed. In the case of system failure, it may lose data which is not stored.
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Ques 18. How can you enhance the durability of Redis?
- Whenever a new command is added to the append log file, call Fsysnc() each time.
- Keep calling Fsysnc() in every second. Despite of the 1 second data lose in the case of system fails.
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Ques 19. What are the concerned things while using Redis?
- Consistent method selection in order to name and prefix the keys. Namespace management.
- Make key prefixes registry which can map every documents to their owner applications.
- Designing, implementing and testing the Garbage collection mechanism for each class we keep into the redis architecture.
- Maintain a sharding library before investing so much into the application.
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Ques 20. How can you use Redis with .Net application?
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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