Share common Redis interview questions

Introduction: Redis is an open source log-type, Key-Value database written in ANSI C language, complying with the BSD protocol, supporting the network, and can be based on memory or persistence, and provides Non-relational database with APIs in multiple languages.

Special recommendation: 2020 redis interview questions collection (latest)

Traditional databases follow ACID rules. Nosql (abbreviation for Not Only SQL, a collective name for database management systems that are different from traditional relational databases) is generally distributed and distributed generally follows the CAP theorem.

Github source code: https://github.com/antirez/redis

Redis official website: https://redis.io/

Recommended: "redis Tutorial》

#What data types does Redis support?

String string:

Format: set key value

The string type is binary safe. This means that the redis string can contain any data. For example, jpg images or serialized objects.

The string type is the most basic data type of Redis, and a key can store up to 512MB.

Hash(Hash)

Format: hmset name key1 value1 key2 value2

Redis hash is a set of key-value (key=>value) pairs.

Redis hash is a mapping table of string type fields and values. Hash is particularly suitable for storing objects.

List (List)

Redis list is a simple list of strings, sorted in insertion order. You can add an element to the head (left) or tail (right) of the list

Format: lpush name value

Add a string element to the head of the list corresponding to the key

Format: rpush name value

Add string elements at the end of the key corresponding list

Format: lrem name index

Delete count items from the key corresponding list that are the same as value Element

Format: llen name

Returns the length of the key corresponding to the list

Set (set)

Format: sadd name value

Redis's Set is an unordered collection of string type.

Sets are implemented through hash tables, so the complexity of adding, deleting, and searching is O(1).

zset(sorted set: ordered set)

Format: zadd name score value

Redis zset, like set, is also a collection of string type elements, and duplicates are not allowed member.

The difference is that each element is associated with a double type score. Redis uses scores to sort the members of the collection from small to large.

The members of zset are unique, but the scores can be repeated.

What is Redis persistence? What persistence methods does Redis have? What are the pros and cons?

Persistence is to write the memory data to the disk to prevent the memory data from being lost if the service goes down.

Redis provides two persistence methods: RDB (default) and AOF

RDB:

rdb is the abbreviation of Redis DataBase

Function core functions rdbSave (generate RDB file) and rdbLoad (load memory from file) two functions

AOF:

Aof is the abbreviation of Append-only file

##The flushAppendOnlyFile function will be called whenever a server (scheduled) task or function is executed. This function performs the following two tasks

aof write and save:

WRITE: According to the conditions, write the cache in aof_buf to the AOF file

SAVE: According to the conditions, call the fsync or fdatasync function to save the AOF file to disk.

Storage structure:

The content is command text storage in redis communication protocol (RESP) format.

Comparison:

1. Aof files are updated more frequently than rdb, so use aof to restore data first.

2. AOF is safer and larger than rdb

3. RDB performance is better than aof

4. If both are configured, AOF is loaded first

You just mentioned the redis communication protocol (RESP) above. Can you explain what RESP is? What are the characteristics? (You can see that many interviews are actually a series of questions. The interviewer is actually waiting for you to answer this point. If you answer the question, it will add another point to your evaluation)

RESP It is a communication protocol previously used by the redis client and server;

Characteristics of RESP: simple implementation, fast parsing, and good readability

For Simple Strings the first byte of the reply is " " Reply

For Errors the first byte of the reply is "-" Error

For Integers the first byte of the reply is ":" Integer

For Bulk Strings the first byte of the reply is "$" String

For Arrays the first byte of the reply is "*" Array

What are the architectural patterns of Redis? Talk about their respective characteristics

stand-alone version

Features: Simple

Problems:

1. Limited memory capacity 2. Limited processing power 3. Unable to be highly available.

Master-slave replication

##The replication function of Redis allows users to replicate data based on a Redis server. Create any number of replicas of the server, where the replicated server is the master and the server replicas created through replication are slaves. As long as the network connection between the master and slave servers is normal, the master and slave servers will have the same data, and the master server will always synchronize the data updates that occur on itself to the slave servers, thus ensuring that the data of the master and slave servers are always the same.

Features:

1. Master/slave roles

2. Master/slave data are the same

3. Reduce the read pressure of the master before transferring it to the slave library

Problem:

High availability cannot be guaranteed

Failed to solve the pressure of master writing

Sentinel

Redis sentinel is a distributed system that monitors redis master and slave servers and automatically performs failover when the master server goes offline. Three of the features:

Monitoring: Sentinel will constantly check whether your master server and slave server are operating normally.

Notification: When a problem occurs on a monitored Redis server, Sentinel can send notifications to the administrator or other applications through the API.

Automatic failover: When a main server cannot work properly, Sentinel will start an automatic failover operation.

Features:

1. Ensure high availability

2. Monitor each node

3. Automatic fault migration

Disadvantages: Main From mode, switching takes time to lose data

Does not solve the master write pressure

Cluster (proxy type):

Twemproxy is a Twitter open source redis and memcache fast/lightweight proxy server; Twemproxy is a fast single-threaded proxy program that supports the Memcached ASCII protocol and the redis protocol.

Features: 1. Multiple hash algorithms: MD5, CRC16, CRC32, CRC32a, hsieh, murmur, Jenkins

2. Support automatic deletion of failed nodes

3. The end-side Sharding sharding logic is transparent to the business, and the business side's reading and writing methods are consistent with operating a single Redis

Disadvantages: A new proxy is added, and its high availability needs to be maintained.

failover logic needs to be implemented by yourself. It cannot support automatic fault transfer and has poor scalability. Manual intervention is required for expansion and contraction.

Cluster (direct Connection type):

Redis-cluster cluster is supported from redis 3.0 and later versions. Redis-Cluster adopts a centerless structure, each node Save data and the entire cluster state, each node is connected to all other nodes.

Features:

1. No central architecture (no node affects the performance bottleneck), without the proxy layer.

2. Data is stored and distributed in multiple nodes according to slots. Data is shared between nodes and data distribution can be dynamically adjusted.

3. Scalability, can be linearly expanded to 1000 nodes, and nodes can be dynamically added or deleted.

4. High availability. When some nodes are unavailable, the cluster is still available. By adding a Slave to make a backup data copy

5, automatic failover is realized. The nodes exchange status information through the gossip protocol, and use the voting mechanism to complete the role promotion from Slave to Master.

Disadvantages:

1. Resource isolation is poor and it is easy for mutual influence to occur.

2. Data is replicated asynchronously, and strong consistency of data is not guaranteed

What is a consistent hash algorithm? What is a hash slot?

Commonly used commands in Redis?

Keys pattern

* means allocating all

starting with bit

Check whether the Exists key exists

Set

Set the value corresponding to the key to a string type value.

setnx

Set the value corresponding to key to the value of string type. If key already exists, return 0, nx means not exist.

Delete a key

Returns 1 for the first time and returns 0 for the second time after deleting it

Expire Set expiration time (in seconds)

TTL view How much time is left

If a negative number is returned, the key will be invalid and the key does not exist

Setex

Set the value corresponding to the key to a string type value, and specify the corresponding value of this key validity period.

Mset

Set the values ​​of multiple keys at one time. Returning ok on success means that all values ​​are set, and returning 0 on failure means that no value is set.

Getset

Set the value of key and return the old value of key.

Mget

Get the values ​​of multiple keys at one time. If the corresponding key does not exist, nil will be returned accordingly.

Incr

Add the value of key and return the new value. Note that if incr is a value that is not int, an error will be returned. If incr is a key that does not exist, set the key to 1

incrby

is similar to incr, add the specified value, and it will be set when the key does not exist key, and thinks that the original value is 0

Decr

The value of the key is subtracted. If the decr key does not exist, set the key to -1

Decrby

Same as decr, minus the specified value.

Append

Append value to the string value of the specified key and return the length of the new string value.

Strlen

Get the length of the value of the specified key.

persist xxx (cancel expiration time)

Select database (0-15 database)

Select 0 //Select database

move age 1// Move age to 1 library

Randomkey randomly returns a key

Rename rename

Type returns the data type

08

Have you ever used Redis distributed lock? How is it implemented?

First use setnx to grab the lock. After grabbing it, use expire to add an expiration time to the lock to prevent the lock from forgetting to release.

What happens if the process crashes unexpectedly or needs to be restarted for maintenance before expire is executed after setnx?

The set instruction has very complex parameters. This should be able to combine setnx and expire into one instruction at the same time!

09

Have you ever used Redis as an asynchronous queue? How did you use it? What are the disadvantages?

Generally use the list structure as the queue, rpush produces messages, and lpop consumes messages. When there is no message from lpop, sleep for a while and try again.

Disadvantages:

When the consumer goes offline, the produced messages will be lost, so you have to use a professional message queue such as rabbitmq, etc.

Can I produce it once and consume it multiple times?

Using the pub/sub topic subscriber mode, a 1:N message queue can be achieved.

10

What is cache penetration? How to avoid it? What is cache avalanche? How to avoid it?

Cache Penetration

General cache systems cache queries based on key. If the corresponding value does not exist, it should be searched in the back-end system (such as DB). Some malicious requests will deliberately query non-existent keys. If the request volume is large, it will put a lot of pressure on the back-end system. This is called cache penetration.

How to avoid?

1: The query result is also cached when it is empty. The cache time is set shorter, or the cache is cleared after the data corresponding to the key is inserted.

2: Filter keys that must not exist. You can put all possible keys into a large Bitmap and filter through the bitmap when querying.

Cache Avalanche

When the cache server is restarted or a large number of caches fail in a certain period of time, this will put a lot of pressure on the back-end system when it fails. causing the system to crash.

How to avoid?

1: After the cache expires, control the number of threads that read the database and write the cache through locking or queuing. For example, only one thread is allowed to query data and write cache for a certain key, while other threads wait.

2: Make a second-level cache, A1 is the original cache, A2 is the copy cache, when A1 fails, you can access A2, A1 cache expiration time is set to short-term, A2 is set to long-term

3 : Different keys, set different expiration times, so that the cache invalidation time is as even as possible.

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