Using Lua Scripting

1. Executing Scripts

Command	Description
`EVAL script numkeys key [key ...] arg [arg ...]`	Run inline script atomically

Command	Description
`SCRIPT LOAD script`	Returns SHA1; cache server-side
`EVALSHA sha numkeys ...`	Execute cached script

Lua Var	Purpose
`KEYS[i]`	Key names (1-indexed); enables Cluster routing
`ARGV[i]`	Arbitrary arguments

API	Behavior on Error
`redis.call(cmd, ...)`	Aborts script with error
`redis.pcall(cmd, ...)`	Returns error table; script continues

EVAL "if redis.call('GET', KEYS[1]) == ARGV[1] then \
  return redis.call('SET', KEYS[1], ARGV[2]) else return 0 end" \
  1 mykey "old" "new"

Property	Detail
Atomic	Entire script runs without interleaving
Blocking	Other commands wait — keep scripts short

Command	Returns
`SCRIPT EXISTS sha [sha ...]`	Array of 0/1 per SHA

Command	Options
`SCRIPT FLUSH [ASYNC\|SYNC]`	Clear all cached scripts

Command	Limitations
`SCRIPT KILL`	Only if script hasn't performed writes; otherwise `SHUTDOWN NOSAVE`

Function	Usage
`redis.error_reply(msg)`	Return typed error
`redis.status_reply(msg)`	Return status string

EVAL "local c = redis.call('INCR', KEYS[1]) \
  if c == 1 then redis.call('EXPIRE', KEYS[1], ARGV[1]) end \
  return c" 1 rate:user:42 60

Pattern	Use
Multi-key CAS	Read-modify-write atomically
Conditional delete	Verify owner before DEL