What are the usage scenarios of Redis?

Summary of common Redis commands: including time complexity summary and data structures used internally by specific data types;

Advanced functions of Redis: including persistence, replication, sentinel, and cluster introduction;

Understanding Redis: Understand memory and blocking; this part is very important. All the previously introduced techniques can be used as techniques, and this should be part of the Tao;

Development skills: mainly a summary of some practical development, Including cache design and common pitfalls.

Let’s start the first part and take another look at Redis.

The content of this series is based on: redis-3.2.12

Redis is not a panacea

During interviews, I am often asked to compare the advantages and disadvantages of Redis and Memcache. Personally, I think The two are not suitable for comparison. One is a non-relational database that can not only cache but also do other things, and the other is only used for caching. Redis is often used as a cache, which is the main reason why we often compare it to other technologies. So what can Redis do? What can't you do?

Redis can do anything

Caching, there is no doubt that this is the most well-known usage scenario of Redis today. It is very effective in improving server performance;

Ranking list. If you use a traditional relational database to do this, it will be very troublesome, but using the SortSet data structure of Redis can be very convenient;

Calculator/speed limiter, using the atomic auto-increment operation in Redis, we can count the number of user likes, user visits, etc. If MySQL is used for this type of operation, frequent reading and writing will bring considerable inconvenience. Pressure; A typical usage scenario of the speed limiter is to limit the frequency of a user's access to a certain API. Commonly used ones include rush buying to prevent users from unnecessary pressure caused by crazy clicking;

Friend relationships, using collections Some commands, such as intersection, union, difference, etc. It can easily handle functions such as mutual friends and common hobbies;

Simple message queue, in addition to Redis's own publish/subscribe mode, we can also use List to implement a queue mechanism, such as: arrival notification, Requirements such as email sending do not require high reliability, but will bring a lot of DB pressure. List can be used to complete asynchronous decoupling;

Session sharing, taking PHP as an example, the default Session is saved In the server file, if it is a cluster service, the same user may land on different machines, which will cause users to log in frequently; after using Redis to save the Session, the user can obtain the corresponding information no matter which machine he lands on. Session information.

What can't Redis do

Although Redis is rich in functions, it is not omnipotent. It is suitable for its specific fields and can get twice the result with half the effort. If abused, it may lead to system instability, increased costs and other problems.

For example, Redis is used to save basic user information. Although it can support persistence, its persistence solution cannot guarantee the absolute landing of data, and may also cause Redis performance to decrease because of persistence. Too frequently will increase the pressure on the Redis service.

A simple summary is that businesses with too large amounts of data and very low data access frequency are not suitable for using Redis. If the data is too large, it will increase costs, and the access frequency is too low. Storing it in memory is a waste of resources.

You always need to find a reason for your choice

The above mentioned some usage scenarios of Redis, so there are many other options for solutions to these scenarios, such as caching Using Memcache, session sharing can also be implemented with MySql, and message queue can be implemented with RabbitMQ. Why must we use Redis?

Fast speed, completely based on memory, implemented in C language, the network layer uses epoll to solve high concurrency problems, the single-threaded model avoids unnecessary context switches and race conditions; Note: single-thread only refers to the network The request module uses a request to process the client's request. Like persistence, it will reopen a thread/process to process

Rich data types. Redis has 8 data types. Of course, the main ones are commonly used There are five types: String, Hash, List, Set, and SortSet. They all organize data based on key values. Each data type provides a very rich set of operation commands, which can meet most needs. If you have special needs, you can also create new commands yourself through Lua scripts (with atomicity);

In addition to the rich data types provided, Redis also provides personalized functions such as slow query analysis, performance testing, Pipeline, transactions, Lua custom commands, Bitmaps, HyperLogLog, publish/subscribe, Geo, etc.

The code of Redis is open source on GitHub. The code is very simple and elegant, and anyone can understand its source code; its compilation and installation is also very simple, without any system dependencies; there is a very active community, various The client's language support is also very complete. In addition, it also has transaction support (not tried yet), persistence, master-slave replication and other functions, realizing the feasibility of high availability and distributed processing.

As a developer, the things we use cannot be made into a black box. We should go deep into it to understand and become more familiar with it.

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