Redis Cache Interview Questions and Answers
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Experienced / Expert level questions & answers
Ques 1. 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 2. Mention Operation Keys of Redis.
- TYPE key
- TTL key
- EXPIRE key seconds
- EXPIREAT key timestamp
- EXISTS key
- DEL key
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Ques 3. 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 4. 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 5. 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 6. 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 7. 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 8. 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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