Redis from entry to proficiency [Advanced] Lua script detailed explanation

0. Preface

Redis has a built-in scripting language Lua, which allows users to perform a series of operations through Lua scripts. This blog will discuss in depth the execution mechanism of Redis Lua scripts, including the process of loading scripts, compiling scripts and executing scripts, as well as the atomicity and transactionality of scripts. We will demonstrate how to use Lua scripts in Redis and the application scenarios of Lua scripts in Redis through example demonstrations. Let's understand the power of Redis Lua script together!

1. Introduction to Redis Lua script

1.1 Lua script introduction

Lua language overview:

Lua is a lightweight scripting language that is concise, flexible and efficient. The Lua language was originally designed for embedded systems, but now it has been widely used in game development, Web development, embedded device control and other fields.

Features of Lua script:

1. Easy to learn: Lua syntax is simple and easy to learn, so you can get started quickly.
2. Lightweight: The running environment of Lua scripts is very lightweight and occupies less resources.
3. Efficient performance: Lua scripts execute very fast and are particularly good at processing large amounts of data.
4. Extensibility: Lua scripts can be extended by calling functions written in C/C++.

1.2 Why choose Lua in Redis

Advantages of combining Lua and Redis

Redis is a high-performance Key-Value storage system that provides rich data structures and powerful caching capabilities. In Redis, complex operations can be implemented using Lua scripts, which have the following advantages:

1. Atomic operation: The execution of Lua scripts in Redis is atomic, which can ensure the atomic execution of multiple commands and avoid problems caused by concurrency.
2. Reduce network overhead: By encapsulating multiple commands in a Lua script for execution, network overhead can be reduced and performance improved.
3. Complex calculation logic: The Lua scripting language is highly flexible and can write complex calculation logic to reduce the load on the Redis server.

Application scenario of Lua script in Redis

The application scenarios of Lua script in Redis are very extensive, including but not limited to the following aspects:

1. Atomic operations: By using Lua scripts, atomic operations such as transaction processing, optimistic locks, and exclusive locks can be realized.
2. Complex calculations: Lua scripts can perform complex calculations, such as calculating statistics, sorting, filtering, etc.
3. Batch operations: Batch operations can be realized through Lua scripts, such as batch insertion, batch deletion, etc.
4. Distributed locks: Lua scripts can be used to implement distributed locks to prevent multiple clients from accessing shared resources at the same time.

2. Execution process of Redis Lua script

For the execution process of Redis Lua script, it can be divided into three stages: loading script, compiling script and executing script.

1. Load the script:

1.1 Script cache mechanism:

• The purpose of Redis script caching is to improve the execution efficiency of scripts and avoid the need to reload and compile scripts every time they are executed.
• The script cache is implemented by saving the SHA1 hash value of the script and the script content in the script cache of the Redis server.

1.2 Relationship between script loading and cache:

• The script loading process is to transfer the script to the Redis server, and judge whether the script already exists in the cache through the SHA1 hash value.
• If the script already exists in the cache, the SHA1 hash value of the script is returned directly.
• If the script does not exist in the cache, the script is cached and the SHA1 hash value of the script is returned.
• When using a script, the script can be referenced by the SHA1 hash value of the script, without the need to transmit the content of the script every time.

2. Compile the script:

2.1 Lua script syntax:

• Lua script is a scripting language based on Lua language and has its own grammar rules.
• Commonly used Lua syntax elements include variables, expressions, control structures, functions, etc.

2.2 Script compilation process:

• The principle of script compilation is to parse Lua scripts into an intermediate representation (bytecode).
• During compilation, the script is checked for syntax errors and converted into executable bytecode.

3. Execute the script:

3.1 Atomicity of script execution:

• Redis Lua scripts are atomic, that is, all operations in the script are either executed successfully or not executed at all.
• This is because Redis will execute the script as a whole when executing the script and will not be interrupted by other operations.

3.2 Transactionality of script execution:

• Redis transaction is a collection of atomic operations that can encapsulate multiple operations in one transaction for execution.
• In Lua scripts, you can use Redis transaction commands (such as MULTI, EXEC, WATCH, etc.) to implement transactional operations.

The execution process of Redis Lua script includes three stages: loading script, compiling script and executing script. Script caching will be performed when the script is loaded, and the script will be converted into executable bytecode when compiling the script. Executing the script has the characteristics of atomicity and transaction.

3. Application scenario of Redis Lua script

3.1 Atomic operations

Cases of atomic operations using Lua scripts:

Lua scripts can implement atomic operations in Redis, including transaction processing, optimistic locks, exclusive locks, etc. The following is a case of implementing an exclusive lock using a Lua script:

-- 加锁脚本
local key = KEYS[1]
local value = ARGV[1]
local ttl = tonumber(ARGV[2])

local lock = redis.call('set', key, value, 'NX', 'PX', ttl)

if lock then
    return true
else
    return false
end

We use the Redis setcommand to set a key-value pair, and use NXparameters to ensure that it is set only when the key does not exist, achieving the effect of an exclusive lock. Use the script by passing the key name, value, and expiration time.

3.2 Complex data processing

An example of using Lua scripts to process complex data structures:

Lua scripts can handle complex data structures in Redis, such as statistics, sorting, filtering and other operations. Here is an example of calculating the sum of all elements in a list using a Lua script:

-- 计算列表中所有元素的总和
local key = KEYS[1]
local sum = 0

local values = redis.call('lrange', key, 0, -1)
for i, value in ipairs(values) do
    sum = sum + tonumber(value)
end

return sum

Redis lrangecommands to get all the elements in the list, then use the Lua script to sum these elements, and finally return the calculation result.

3.3 Batch operation - an example of batch operation using Lua script:

Lua scripts can implement batch operations in Redis, such as batch insertion, batch deletion, etc. The following is a case of batch deletion using Lua script:

-- 批量删除指定前缀的键
local prefix = ARGV[1]

local keys = redis.call('keys', prefix .. '*')
for i, key in ipairs(keys) do
    redis.call('del', key)
end

return #keys

The keys 命令获取指定前缀的键名列表，然后使用Lua脚本循环遍历这些键名，使用del` command performs a batch delete operation and returns the number of keys deleted.

Through the above simple examples, we can understand how to use Lua scripts to implement atomic operations, process complex data structures, and perform batch operations in Redis. But the scenarios in real projects are more complicated than these. But the principle is similar.

4. Advantages and precautions of Redis Lua script

4.1 Improve execution efficiency

The execution efficiency advantages of Lua scripts in Redis are mainly reflected in the following aspects:

• Reduce network overhead: Encapsulate multiple operations in one script and execute them through one network transmission, reducing multiple network overheads.
• Atomic operation: Redis treats the entire Lua script as an atomic operation, ensuring that the execution of the script is thread-safe.
• Reduce parsing time: When the same Lua script is executed multiple times, Redis will cache it, reducing parsing time.

4.2 Script Security

Ensuring the security of Lua scripts is very important, here are some considerations:

• Input verification: Before executing the Lua script, input verification is required to ensure that the incoming parameters meet expectations and avoid security holes.
• Parameterized scripts: Avoid splicing user input directly into Lua scripts, instead use parameterized scripts, passing user input as parameters to the script.
• Restricting script permissions: Use Redis SCRIPT LOADcommands to load scripts into Redis, and then EVALSHAexecute them through commands, avoiding script content transmission over the network and reducing potential attack risks.

4.3 Debugging and maintenance of scripts

The debugging and maintenance of Lua scripts can use the following techniques and tools:

• Log output: Add a log output statement in the script to view the execution of the script during execution.
• Single-step debugging: By adding breakpoints in the script, the script can be executed step by step, and the value of the variable and the execution flow of the script can be observed.
• The MONITOR command of Redis: You can use the MONITOR command of Redis to view the command execution status of the Redis server in real time, so as to debug and troubleshoot problems.
• Lua debugger: You can use Lua debugger tools, such as LuaInspect, ZeroBrane Studio, etc., to debug and analyze Lua scripts to help find problems and optimize performance.

Redis Lua scripts have the advantages of improving execution efficiency, ensuring atomic operations, and reducing parsing time. To ensure the security of scripts, input validation, parameterized scripts, and restricted script permissions are required. In terms of debugging and maintenance, tools and techniques such as log output, single-step debugging, Redis MONITOR command, and Lua debugger can be used to help analyze and solve problems.

5. Summary

5.1 Execution mechanism of Redis Lua script

Through the above study and summary, we can probably know that the execution mechanism of Redis Lua script can be summarized as the following steps:

• Load: Use SCRIPT LOADthe command to load the Lua script into Redis to obtain a SHA1 checksum.
• Compile: Redis compiles the loaded Lua script to generate executable bytecode.
• Execution: EVALSHAPass the SHA1 checksum and parameters through the command, and Redis will find and execute the corresponding Lua script based on the SHA1 checksum.

5.2 Benefits of using Lua scripts

Using Lua scripts in Redis has the following advantages:

• Improve execution efficiency: reduce network overhead, atomic operations, and reduce parsing time.
• Simplify complex operations: By encapsulating multiple operations into one script, complex operation logic is simplified.
• Atomicity Guarantee: The execution of the script is an atomic operation, which avoids race conditions in a multi-threaded environment.
• Security control: Ensure the security of scripts by parameterizing scripts and restricting script permissions.

5.3 Suggestions for learning Lua script

Learning Lua scripting can follow the following routes and resources:

• Official documentation: Read the official Lua documentation to understand the basic syntax and features of Lua.
• Online tutorials: Refer to online tutorials, such as w3cschool, official Lua tutorials, etc., to learn the basic usage and programming skills of Lua.
• Practical projects: By writing practical projects, such as writing Lua scripts to operate Redis, deepen the understanding and application of Lua.

The execution mechanism of Redis Lua script includes the process of loading, compiling and executing. Using Lua scripts in Redis can improve execution efficiency, simplify complex operations, and ensure atomicity and security control. You can learn Lua script by reading official documents and online tutorials.

6. Redis from entry to proficiency series of articles

"Redis from entry to proficiency [advanced chapter] detailed explanation of the high-availability sentinel mechanism (Redis Sentinel)" "
Redis from entry to mastery [advanced chapter] detailed explanation of redis master-slave replication" "
Redis from entry to proficiency [advanced chapter] 】Detailed Explanation of Redis Transactions"
"Detailed Explanation of the Object Mechanism of Redis from Entry to Master [Advanced]" "Detailed
Explanation of the Messaging Publishing and Subscription Mode of Redis from Entry to Master [Advanced]" "
Redis From Entry to Master [Advanced] "Detailed Explanation of Persistence AOF"
"Detailed Explanation of Persistent RDB of Redis from Beginner to Master [Advanced Chapter]" "
Detailed Explanation of the Underlying Data Structure Dictionary (Dictionary) of Redis from Beginner to Master [Advanced Chapter]"
"Redis from Beginner To be proficient in [Advanced Chapter], the detailed explanation of the underlying data structure QuickList" "
Redis from entry to proficiency [Advanced Chapter]: the detailed explanation of the underlying data structure Simple Dynamic String (SDS)" "
Redis from entry to proficiency [Advanced Chapter] ] Detailed explanation of the underlying data structure compression list (ZipList) "
"Redis from entry to proficiency [advanced] detailed explanation and usage examples of the data type Stream"

Redis Lua script common interview questions

1. What is the execution mechanism of Redis Lua script?

• When executing a Lua script, Redis will send the script to the server for compilation and execution.
• If the script has already been compiled, Redis will use the SHA1 hash value of the script to execute, reducing network transmission.
• Execution of scripts is atomic and will not be interrupted by requests from other clients.
• Scripts can access Redis data structures and commands, and can call Redis commands through the redis.call and redis.pcall functions.

2. How to use Redis Lua script in Spring Boot?

• First, you need to add Redis dependencies, such as Spring Data Redis.
• Then, configure Redis connection information, including host, port, password, etc.
• Create a RedisTemplate Bean to execute Redis commands, including executing Lua scripts.
• Use LettuceConnectionFactory to configure the Redis connection factory to support the execution of Lua scripts.

3. How about the performance of Redis Lua script?

• The performance of Redis Lua scripts is usually better than executing multiple Redis commands individually because it reduces network overhead.
• In some scenarios, using Lua scripts can atomize multiple operations, reducing the overhead of multiple requests.
• However, if the script is too complex or computationally intensive, it may negatively impact performance.

4. How to write efficient Redis Lua scripts?

• Minimize network transfers and avoid executing a large number of Redis commands in scripts.
• Use Redis data structures and commands to optimize script logic and performance.
• To avoid a large number of calculation operations in the script, you can consider using data structures such as Redis's Sorted Set to achieve.

5. How about the error handling of Redis Lua script?

• When a Lua script fails to execute, Redis will return an error message. You can check whether the script executed successfully by checking the return value.
• You can use the pcall function to call Redis commands and handle error conditions by judging the return value.

Hello everyone, I am Freezing Point, and today's detailed explanation of Redis Lua scripts is all about these. If you have questions or opinions, you can leave a message in the comment area.