Why does redis create 16 databases by default?

In actual development work, we usually use the redis database for caching, distributed locks/message queues, etc. But we usually have this question, why are 16 databases created by default after setting up and configuring the redis server?

Let’s introduce this question to you.

1. The origin of 16 databases

redis is a dictionary-structured storage server. A redis instance provides multiple dictionaries for storing data. , the client can specify in which dictionary the data is stored. This is similar to how multiple databases can be created in a relational database instance (as shown in the figure below), so each dictionary can be understood as an independent database.

Redis supports 16 databases by default. You can modify this value by adjusting the databases in the redis configuration file redis/redis.conf. After the setting is completed, restart redis and it will be completed. configuration.

#After the client establishes a link with redis, database No. 0 will be selected by default, but you can use the select command to change the database at any time.

# 切换数据库操作：切换到1
127.0.0.1:6379> SELECT 1
OK
127.0.0.1:6379[1]>
127.0.0.1:6379[1]>
# 切换到0
127.0.0.1:6379[1]> SELECT 0
OK
127.0.0.1:6379>
# 从1号库中获取username
127.0.0.1:6379[1]> get username 。

(Learning video sharing: redis video tutorial)

In actual projects, you can specify the database in the form of a redis configuration file, as shown in the figure below Instructions

2. Correctly understand the "database" concept of redis

Since redis does not support custom database names, all databases Named by number. Developers need to record the correspondence between the stored data and the database themselves. In addition, redis does not support setting different access passwords for each database. All clients can either access all databases or have no permission to access all databases. To correctly understand the "database" concept of redis, we have to mention a command:
Clear the data in all databases in the redis instance
127.0.0.1:6379> FLUSH ALL
Clear the data in a certain redis database The data of other libraries will not be cleared
127.0.0.1:6379> FLUSH db0
This command can clear all database data under the instance, which is different from the relational database we are familiar with. Multiple libraries of relational databases are often used to store data for different applications, and there is no way to clear all library data under the instance at the same time. For redis, these dbs are more like namespaces and are not suitable for storing data from different applications. For example, you can use database No. 0 to store data in the development environment, and use database No. 1 to store data in the test environment. However, it is not suitable to use database No. 0 to store the data of application A and use database No. 1 to store the data of application B. Different environments Different redis instances should be used to store data. Redis is very lightweight. An empty redis instance only occupies about 1M of memory, so there is no need to worry about multiple redis instances taking up a lot of additional memory.

3. Does one instance support multiple DBs in a cluster?

The above are all based on the situation of single redis. In the case of a cluster, the use of the select command to switch db is not supported, because there is only one db0 in redis cluster mode

Recommended learning:redis database tutorial

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