One 、 First time to know Redis

1. Redis Brief introduction

• Redis Born in 2009 The full name of the year is Remote Dictionary Server Remote Dictionary Service device , It's a Memory based Of Key value type NoSQL database .

features ：

• Key value （key-value） type ,value Support a variety of different data structures , Rich in functions
• Single thread , Every command is atomic （Redis 6 Multithreading is only for network request processing ）
• Low latency , Fast （ Memory based 、IO Multiplexing 、 Good coding （C Language writing ））.
• Support data persistence
• Support the master-slave cluster 、 Fragmentation cluster
• Support for multilingual clients

Key value type , Refer to Redis The data stored in is in key、value Form storage of , and value There are many kinds of forms , Can be a string 、 The number 、 even to the extent that json etc. .

2. SQL And NoSql contrast

• NoSql Can translate Not Only Sql（ not only SQL）, Or is it No Sql（ Not Sql Of ） database . Compared with traditional relational database , A special database with great differences , So it's also called Non relational database .

2.1 Structured vs. unstructured

• SQL Traditional relational databases are structured data , Every table has strict constraint information ： Field name 、 Field data type 、 Information such as field constraints , The inserted data must comply with these constraints
• NoSQL There are no strict restrictions on the database format , Often the form is loose , free . It can be key value type （Redis）、 Document type （MongoDB）、 Column valued type （HBase）、Graph Pattern （Neo4j）
  

2.2 Related and non related

• There are often associations between tables in traditional databases , For example, foreign key ：
  
• There is no association relationship in non relational database
• But it can go through json Document nesting , for example ： The relationship between users and orders , But there will be redundant data （ Zhang San has this order , Li Si may also have duplicate orders ）
  

2.3 SQL Query and non SQL Inquire about

• Traditional relational databases will be based on Sql Statement to query , The grammar is fixed , You can use the same statement to query
  
• The query syntax of non relational databases varies greatly
  

2.4 Business

• Traditional relational database can meet the transaction ACID Principles , High safety
  
• Non relational databases often do not support transactions , Or it cannot be strictly guaranteed ACID Characteristics of , Only basic consistency can be achieved （BASE theory ）

3. Summary

• storage
  • Relational databases are stored on disk , There will be a lot of disks IO, Has a certain impact on Performance
  • Non relational database , It mainly depends on memory to operate , Memory is very fast in reading and writing , So the performance is good

• Extensibility
  • The relational database cluster mode is generally master-slave , Master slave data is consistent , Play the role of data backup , Called vertical expansion .
  • Non relational databases can split data , Stored on different machines , It can save huge amounts of data , Solve the problem of limited memory size . Called horizontal expansion .
  • Relational database because of the association between tables , Horizontal expansion will bring a lot of trouble to data query .