Lua script syntax in Redis

Introduction

Lua scripts in Redis let you run multiple commands together safely and quickly without interruptions.

You want to update several keys in Redis at once without other commands interfering.

You need to perform a calculation or check before changing data in Redis.

You want to reduce the number of round trips between your app and Redis server.

You want to ensure a group of commands run as a single atomic operation.

You want to reuse a small program inside Redis for common tasks.

Syntax

Redis

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

script is the Lua code you want to run, given as a string.

numkeys tells Redis how many keys you pass next, so it knows which arguments are keys.

Examples

This script gets the value of 'mykey' from Redis.

Redis

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

This script sets 'mykey' to 'newvalue' and then returns the new value.

Redis

EVAL "redis.call('SET', KEYS[1], ARGV[1]); return redis.call('GET', KEYS[1])" 1 mykey newvalue

This script increments the 'counter' key by 1 and returns the new count.

Redis

EVAL "local val = redis.call('INCR', KEYS[1]); return val" 1 counter

Sample Program

This script checks if the key 'greeting' exists. If not, it sets it to 'Hello' and returns 'Hello'. If it exists, it returns the current value.

Redis

EVAL "local current = redis.call('GET', KEYS[1]); if not current then redis.call('SET', KEYS[1], ARGV[1]); return ARGV[1] else return current end" 1 greeting Hello

Important Notes

KEYS array holds the keys passed after numkeys; ARGV array holds the other arguments.

Lua scripts run atomically in Redis, so no other commands run during the script.

Keep scripts short and simple to avoid blocking Redis for too long.

Summary

Lua scripts let you run multiple Redis commands together safely and quickly.

Use EVAL with your Lua code, number of keys, keys, and arguments.

Scripts can read and write keys, and return results.