Redis interview questions | Freshers & Experienced

An open-source in-memory data structure store that is can be used as a Database. It stores data in key-value format. Redis supports a wide range of data structures like strings, lists, maps, sets, sorted sets, HyperLogLogs, bitmaps, streams, and spatial indexes

Various applications of Redis.

• Used as Application Cache.
• Used as a database.
• Used message broker.
• For creating Leaderboards.
• Storing Multi-Tenant Metrics

Below is the list of some popular companies who are using Redis:

• Twitter
• GitHub
• Weibo
• Pinterest
• Snapchat
• Craigslist
• StackOverflow
• Flickr

redis-cli is command-line interface for Redis that allows sending commands to the Redis server directly from the terminal.

Redis is mainly used by the system admins, and some of the major commands in Redis are,

• Redis-CLI – to connect to Redis.
• INFO – to get server statistics
• CONFIG GET * - to change runtime configuration
• SELECT “database number” - to select the database by its number
• FLUSHDB – to drop the currently selected database and its keys
• FLUSHALL – to drop all the database and its keys
• BGSAVE – to do a database backups
• CLIENT LIST – to list all the connections with the database
• MONITOR – to track all the network traffic

Some of the significant advantages of using Redis are the following,

• Redis uses its hashing algorithm called Redis hashing to store the user details as key-value pairs.
• Insertion of the mass volume of data into the Redis cache is easy.
• Data replication in Redis is asynchronous (i.e.), when a master node updates, all the slave node gets updated asynchronously.
• Since Redis offers data replication, it can withstand failures more effectively and offer its service.
• Redis commands can be executed as a bunch of queues instead of one at a time .
• The memory required to install Redis us small, so it can be easily installed on arm devices or a raspberry pi.
• You can store huge data of key-value pairs in Redis as big as 512 MB of data.
• Redis offers API in programming languages like c, python, java, and Javascript. So, development is made easy in Redis.
• You can develop a messaging system easily using Redis as it offers a pub/sub messaging mechanism.
• You can develop a messaging system easily using Redis as it offers pub/sub messaging mechanism.
• Redis offers a rich set of data types for key-value data stores.
• Some other advantages of using Redis is its exceptionally fast, atomic operations, and persistent database memory.

MGET is a command in Redis that returns the value of the key. You pass the name of the key along with the MGET command, and the value gets returned as an array. MGET returns nil for keys that don’t exist or keys that don’t have any value.

//syntax 
> MGET keyname 
//example 
> SET key “keyValue” 
> MGET key 
1) “keyValue” 

KEYS command returns all the available keys in the database. The time complexity for this command is 0(n) where n is the number of available keys. It is fairly high so it shouldn’t be used only in the development time.

KEYS * // this command returns all the keys

KEYS *pattern* //returns all the keys that has the related pattern as the word “pattern”

Example

>MSET keyName keyValue  
> MSET Var Val 
> KEYS * 
1) keyValue 
2) Val 
> KEYS *key?* 
1) keyValue 

To get a list of all the database connection (16 maximum database connection can be done by default) in redis, do the following command.

$ redis-cli INFO | grep db 

The above command returns the output like the following,

db0:keys=50000,expires=3 
db1:keys=3000,expires=15 
db2:keys=4005,expires=1

Note: the command only works on redis command line interface.

In order to use PHP with Redis, you need to install PHP Redis client or module. The PHP module is “Phpredis”. To download the module, use the pecl repository. After downloading the module, you can use Redis in PHP in the following way.

$redis = new Redis(); //creates a new Redis object to be used in PHP 
//Connecting to Redis 
$redis->connect('hostname', port);  
$redis->auth('password'); 

You can create a Redis database connection in PHP and do your database operation in PHP using the Phpredis module.

To delete all the database keys at once, do the following command.

$redis-cli FLUSHALL 

It deletes all the caches, database, and its keys. The time complexity is 0(n) where n is the number of keys.

$redis-cli –n  FLUSHDB

It deletes all the keys of the database that is mentioned in the command. It also has a time complexity of 0(n).

This command is used to remove the fields from the hash that is specified and stored at the key. If the key is not there, then the command returns null (0) or 1 if it is present. If the field is not there then the command is just ignored. The time complexity is 0(n) where n is the number of fields to be removed.

HDEL KEY_NAME FIELD1.. FIELDN // command syntax

>HDEL hashName field1 //command example 
> 1 //returns 1 so, the field1 is removed 
> HDEL hashName field2 
>0 //returns 0 so, the field2 is not there

Redis is one of the powerful PHP Redis clients that enables the user to work with Redis in PHP. To delete all the cache using pRedis in PHP, use the following command.

$this->_redis->flushAll();

This command is the Redis equivalent of FLUSHALL that deletes all the cache of the database connected to the Redis.

A single instance of a Redis supports 16 databases indexed from 0 to 15. By default, the Redis used database numbered 0 to work but you can change it at any time. To support more database, you configure it at redis.conf file.

To select a database, use the following command.

>select 2 //select database indexed 2 
>ok 

After selecting the database, you can do the database operation like storing, retrieving, deleting, etc. Each database has a unique key and it can be moved between the database.

DBSIZE command in Redis is used to return the number of keys in the currently selected database. It returns the number of keys as an integer value.

INFO command returns all the statistics about the database that is connected to the Redis. Here the information like the number of keys is displayed.