Atomic Operations with Lua Scripts

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Execute complex multi-step operations atomically in Redis using server-side Lua scripting to eliminate race conditions.

Prerequisites

-Redis installed and running
-Basic understanding of Lua syntax

Steps

Run an inline Lua script

Execute a simple Lua script that sets and returns a value atomically.

$ redis-cli EVAL "redis.call('SET', KEYS[1], ARGV[1]) return redis.call('GET', KEYS[1])" 1 mykey hello

KEYS[1] is the first key argument, ARGV[1] is the first non-key argument. Always pass keys via KEYS for cluster compatibility.

Atomic compare-and-swap

Implement a compare-and-swap operation that only updates a key if it matches an expected value.

$ redis-cli EVAL "if redis.call('GET', KEYS[1]) == ARGV[1] then redis.call('SET', KEYS[1], ARGV[2]) return 1 else return 0 end" 1 mykey hello world

Load a script for reuse

Load a Lua script into Redis and get its SHA hash for repeated execution without resending the script body.

$ redis-cli SCRIPT LOAD "redis.call('SET', KEYS[1], ARGV[1]) redis.call('EXPIRE', KEYS[1], ARGV[2]) return 1"

Save the returned SHA hash and use EVALSHA to call the script by hash, reducing bandwidth.

Execute a loaded script by SHA

Run a previously loaded script using its SHA hash.

$ redis-cli EVALSHA <sha-hash> 1 cached:item '{"data":"value"}' 3600

Atomic rate limiter in Lua

Implement a precise rate limiter as a single atomic Lua script.

$ redis-cli EVAL "local count = redis.call('INCR', KEYS[1]) if count == 1 then redis.call('EXPIRE', KEYS[1], ARGV[2]) end if count > tonumber(ARGV[1]) then return 0 else return 1 end" 1 ratelimit:user1001 10 60

Full Script

full-script.sh

#!/bin/bash
# Redis Lua Scripting Examples
set -euo pipefail

echo "=== Inline script: SET + GET ==="
redis-cli EVAL "redis.call('SET', KEYS[1], ARGV[1]) return redis.call('GET', KEYS[1])" 1 lua:test "hello-lua"

echo ""
echo "=== Compare-and-swap ==="
redis-cli SET cas:key "old-value"
RESULT=$(redis-cli EVAL "if redis.call('GET', KEYS[1]) == ARGV[1] then redis.call('SET', KEYS[1], ARGV[2]) return 1 else return 0 end" 1 cas:key "old-value" "new-value")
echo "CAS result (1=swapped): $RESULT"
echo "New value: $(redis-cli GET cas:key)"

echo ""
echo "=== Load script and use EVALSHA ==="
SHA=$(redis-cli SCRIPT LOAD "redis.call('SET', KEYS[1], ARGV[1]) redis.call('EXPIRE', KEYS[1], tonumber(ARGV[2])) return 1")
echo "Script SHA: $SHA"
redis-cli EVALSHA "$SHA" 1 lua:cached "cached-data" 300
echo "Cached value: $(redis-cli GET lua:cached)"
echo "TTL: $(redis-cli TTL lua:cached)"

echo ""
echo "=== Atomic rate limiter ==="
RATE_SCRIPT="local count = redis.call('INCR', KEYS[1]) if count == 1 then redis.call('EXPIRE', KEYS[1], tonumber(ARGV[2])) end if count > tonumber(ARGV[1]) then return 0 else return 1 end"
for i in {1..12}; do
  ALLOWED=$(redis-cli EVAL "$RATE_SCRIPT" 1 lua:ratelimit 10 60)
  echo "Request $i: $([ "$ALLOWED" = "1" ] && echo "ALLOWED" || echo "DENIED")"
done

echo ""
echo "=== Cleanup ==="
redis-cli DEL lua:test cas:key lua:cached lua:ratelimit

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#!/bin/bash # Redis Lua Scripting Examples set -euo pipefail echo "=== Inline script: SET + GET ===" redis-cli EVAL "redis.call('SET', KEYS[1], ARGV[1]) return redis.call('GET', KEYS[1])" 1 lua:test "hello-lua" echo "" echo "=== Compare-and-swap ===" redis-cli SET cas:key "old-value" RESULT=$(redis-cli EVAL "if redis.call('GET', KEYS[1]) == ARGV[1] then redis.call('SET', KEYS[1], ARGV[2]) return 1 else return 0 end" 1 cas:key "old-value" "new-value") echo "CAS result (1=swapped): $RESULT" echo "New value: $(redis-cli GET cas:key)" echo "" echo "=== Load script and use EVALSHA ===" SHA=$(redis-cli SCRIPT LOAD "redis.call('SET', KEYS[1], ARGV[1]) redis.call('EXPIRE', KEYS[1], tonumber(ARGV[2])) return 1") echo "Script SHA: $SHA" redis-cli EVALSHA "$SHA" 1 lua:cached "cached-data" 300 echo "Cached value: $(redis-cli GET lua:cached)" echo "TTL: $(redis-cli TTL lua:cached)" echo "" echo "=== Atomic rate limiter ===" RATE_SCRIPT="local count = redis.call('INCR', KEYS[1]) if count == 1 then redis.call('EXPIRE', KEYS[1], tonumber(ARGV[2])) end if count > tonumber(ARGV[1]) then return 0 else return 1 end" for i in {1..12}; do ALLOWED=$(redis-cli EVAL "$RATE_SCRIPT" 1 lua:ratelimit 10 60) echo "Request $i: $([ "$ALLOWED" = "1" ] && echo "ALLOWED" || echo "DENIED")" done echo "" echo "=== Cleanup ===" redis-cli DEL lua:test cas:key lua:cached lua:ratelimit