Introduction

• Redis supports running Lua scripts inside Redis
  • Lua is a lightweight scripting language
• Script executes atomically (multiple commands atomic)
  • ✅ No race conditions
  • ✅ Faster (no round trips)
  • ✅ Perfect for rate limiting, locks, conditional updates
• Example use cases:
  • Rate limiting
  • API gateways
  • Distributed locks
  • Conditional updates
  • Inventory control

// Without Lua
App → Redis → App → Redis   (multiple network calls)

// With Lua:
App → Redis (script runs fully inside Redis)

Why INCR + EXPIRE is important

Use case: API rate limiting

1. You want max N requests per minute.

2. Flow:

   INCR ip:1.2.3.4
   EXPIRE ip:1.2.3.4 60
   

   • First request → INCR returns 1 → set EXPIRE to 60s
   • Subsequent requests → INCR increments → you check <= MAX
   • After 60s → key auto-deletes → counter resets

3. Atomicity concern:

   • If you do INCR → network lag → EXPIRE missed → key may never expire

4. Solution:

   • Use Lua script to combine INCR + EXPIRE atomically in one step

Example Lua script:

local current = redis.call("INCR", KEYS[1])
if current ==1 then
    redis.call("EXPIRE", KEYS[1], ARGV[1])
end
return current