lua-scripts.md

Lua scripts are programs written in Lua, which is a lightweight, high-level, and embeddable scripting language. It was designed to be fast, simple, and flexible, making it popular for extending applications and embedding into larger systems.

Here’s a breakdown:

🔹 Key Features of Lua

1. Lightweight & Fast – Very small footprint, ideal for embedding in applications like games or databases.
2. Embeddable – Often used as a scripting engine inside larger systems (e.g., games, Redis, Nginx).
3. Simple Syntax – Easy to learn, similar to Python and JavaScript in readability.
4. Extensible – Can be extended with C, C++, or Java libraries.
5. Dynamic Typing – No need to declare variable types.
6. Garbage Collection – Automatic memory management.

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🔹 Where Lua Scripts Are Used

1. Game Development – Popular in game engines (e.g., Roblox, World of Warcraft addons, CryEngine).
2. Databases – Redis supports Lua scripts for transactions and complex operations.
3. Web Servers – Nginx (via OpenResty) uses Lua for request handling and customization.
4. Embedded Systems – Its lightweight design makes it suitable for IoT and embedded devices.
5. Configuration & Automation – Applications use Lua for customization through config files and automation.

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🔹 Example of a Lua Script

-- Hello World in Lua
print("Hello, World!")

-- Simple loop
for i = 1, 5 do
   print("Number: " .. i)
end

-- Function
function add(a, b)
   return a + b
end

print("Sum: " .. add(10, 20))

Output:

Hello, World!
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5
Sum: 30

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👉 In short, Lua scripts are small programs that automate tasks, extend applications, or add logic inside larger systems.

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Redis supports Lua because sometimes you need to run multiple Redis commands atomically (all-or-nothing) or perform logic close to the database to reduce network calls.

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🔹 Why Lua in Redis?

• Atomic execution → A Lua script in Redis runs as a single atomic operation (no other command runs in between).
• Reduce round-trips → Instead of sending multiple commands one by one, you put the logic in Lua and send once.
• More powerful logic → You can add loops, conditions, and calculations directly in Redis.

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🔹 Running a Lua Script in Redis

You use the EVAL command.

Example 1 – Simple script

EVAL "return 'Hello from Lua!'" 0

📌 Output:

"Hello from Lua!"

Here:

• "return 'Hello from Lua!'" is the Lua script.
• 0 means no Redis keys are passed in.

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Example 2 – Working with keys and values

Let’s say you want to set a key only if it doesn’t exist.

EVAL "if redis.call('EXISTS', KEYS[1]) == 0 then
         return redis.call('SET', KEYS[1], ARGV[1])
      else
         return 'Key already exists'
      end" 1 mykey myvalue

Explanation:

• 1 → number of keys (here mykey is the key).
• mykey → the key passed to the script (KEYS[1]).
• myvalue → argument passed (ARGV[1]).

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Example 3 – Increment a counter safely

EVAL "return redis.call('INCRBY', KEYS[1], ARGV[1])" 1 user:123:score 10

📌 This increments user:123:score by 10. If you run again, it adds another 10.

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🔹 Benefits in Redis

• You can do transactions with logic.
• Avoid writing multiple round-trips in your app code.
• Very fast since script runs inside Redis itself.

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👉 So, Lua scripting in Redis is like writing “mini-programs” inside Redis for atomic, efficient operations.