Lua script writing and application for Redis

Redis is an open source key-value pair storage database, and Lua script is a very powerful programming tool in Redis. Lua is a lightweight, efficient scripting language that can be used as an extension language for the Redis server. This article will introduce the writing, calling methods and practical applications of Lua scripts in Redis.

1. Lua scripting in Redis

1.1 Introduction to Lua scripting language

As a lightweight language, Lua script has a very small set of specific syntax and dynamic types Language and good performance and other features, but these features make Lua script more suitable for solving some specific problems.

1.2 Advantages and disadvantages of Lua scripting language in Redis

Advantages:

• Atomicity: Lua scripts are atomic and will not be interrupted by other clients , is executed separately.
• Reusability: The code of the Lua script can be reused on multiple REDIS nodes.
• Excellent performance: The Lua script interpreter and Redis Server achieve very efficient performance by sharing processes.

Disadvantages:

• Difficult to debug: The language features of Lua scripts are different from the Redis language features, and it is difficult to conduct a complete investigation of Lua scripts;
• Link between Redis and Lua script: Lua script needs to pass parameters and return data through the client, adding additional overhead.

1.3 Lua scripting rules in Redis

• The Lua environment used by Redis is 5.1 and is compatible with some extended syntax of version 5.2;
• All Redis commands must be called through Redis.call;
• All Redis.key value references need to be passed using KEYS or ARGV;
• All error handling requires the use of the error function.

1.4 Lua script code example in Redis

The following is a Lua script example of a counter:

local count = tonumber(redis.call("get ", KEYS[1])) or 0
if count > tonumber(ARGV[1]) then

redis.call("set", KEYS[1], ARGV[1])
return 0

else

count = redis.call("incr", KEYS[1])
return count 

end

2. Redis How to call Lua scripts

There are two ways to call Lua scripts in Redis:

2.1 Use the EVAL command

Redis provides the EVAL command, which can be used to run and write Good Lua script.

Syntax:
EVAL script numkeys key [key ...] arg [arg ...]

Example:

redis.eval('return redis. call("GET", KEYS[1])', 1, "mykey")

2.2 Use the SCRIPT LOAD command

In Redis, you can also use the SCRIPT LOAD command to load Lua in advance script and then call the SHA1 hash to execute the script.

Syntax:
SCRIPT LOAD script

Example:

local script = [[

local key = KEYS[1]
local max_count = tonumber(ARGV[1])
local current_count = tonumber(redis.call("get", key))
if current_count and current_count >= max_count then
    redis.call("del", key)
end
redis.call("incr", key)
return true

]]

local key = 'limiter:xxx'
local max_count = 10
local script_sha = redis.call('SCRIPT', 'LOAD', script)
redis.call('EVALSHA', script_sha, 1, key, max_count)

3. Lua script application examples in Redis

3.1 Distributed lock

Distributed lock requires the same script code on all Redis nodes. This kind of Design can improve the efficiency of application operation.

Example of Lua script implementing distributed lock:

local lock_key = KEYS[1]
local lock_timeout = tonumber(ARGV[1])
local lock_value = KEYS[2 ]
local lock_valid_time = tonumber(ARGV[2])

if redis.call("set", lock_key, lock_value, "NX", "EX", lock_timeout) then

redis.call("expire", lock_key, lock_valid_time)
return lock_value

else

return nil

end

3.2 Ordered collection paging query

Redis ordered collection provides the function of paging query, which can be based on the score range in the ordered collection Perform paging queries.

Example of Lua script to implement ordered set paging query:

local page_no = tonumber(ARGV[1])
local page_size = tonumber(ARGV[2])
local start = (page_no - 1) * page_size
local stop = page_no * page_size - 1
local opts = {score_cast_func = tonumber}

local result = {}

local data = redis.call("ZRANGE", KEYS[1], start, stop, "WITHSCORES")
for idx = 1, #data, 2 do

local k = data[idx]
local v = tonumber(data[idx + 1])
table.insert(result, {k, v})

end

return result

Conclusion:

Lua script is a very powerful tool in Redis, so the writing and calling method of Lua script is very important. In practical applications, we can write corresponding Lua scripts for specific application scenarios to improve the performance and running speed of Redis.

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