EVAL, Lua & Function Stats (Server-side Scripting)

Code

# EVAL: run Lua inline. Keys start at KEYS[1], args at ARGV[1]
127.0.0.1:6379> EVAL "return redis.call('SET', KEYS[1], ARGV[1])" 1 mykey hello
OK
127.0.0.1:6379> EVAL "return redis.call('GET', KEYS[1])" 1 mykey
"hello"

# Atomic conditional set (only if not exists)
127.0.0.1:6379> EVAL \
  "if redis.call('EXISTS', KEYS[1]) == 0 then \
     return redis.call('SET', KEYS[1], ARGV[1]) \
   else return 0 end" 1 lock:resource token-abc

# Cache the script — call by SHA1 (avoids resending the body)
127.0.0.1:6379> SCRIPT LOAD "return redis.call('GET', KEYS[1])"
"e0e1f9fabfc9d4800c877a703b823ac0578ff833"
127.0.0.1:6379> EVALSHA e0e1f9fabfc9d4800c877a703b823ac0578ff833 1 mykey
"hello"
127.0.0.1:6379> SCRIPT EXISTS e0e1f9fabfc9d4800c877a703b823ac0578ff833

# Functions (Redis 7+) — named, library-style, replaces EVAL for new apps
127.0.0.1:6379> FUNCTION LOAD '#!lua name=mylib \
  redis.register_function("myget", function(keys, args) \
    return redis.call("GET", keys[1]) end)'
127.0.0.1:6379> FCALL myget 1 mykey

# List / dump functions
127.0.0.1:6379> FUNCTION LIST
127.0.0.1:6379> FUNCTION DUMP

Explanation

EVAL runs Lua server-side: scripts execute atomically (no other command interleaves), making them ideal for atomic compare-and-set, rate limiting, or multi-key logic. KEYS[] and ARGV[] separate key names (so cluster can route) from arguments. SCRIPT LOAD caches the body and returns its SHA1; EVALSHA runs by hash — clients use this to avoid resending the script each call. Redis 7+ introduces FUNCTION LOAD with named, versionable libraries (FCALL to invoke) — preferred for new code over anonymous EVAL. Scripts must be fast: a long script blocks the whole server. Don't read non-key state (system time, random) — results must be deterministic for replication.

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