SET NX & XX — Conditional Writes

Redis SET supports flags that make writes conditional.

NX — Only If Not Exists

SET key value NX sets the key only if it doesn't already exist.

• Key doesn't exist → sets it, returns +OK\r\n
• Key exists → does nothing, returns $-1\r\n (null)

This is how distributed locks work — SET lock_key owner NX EX 30 atomically acquires a lock.

XX — Only If Exists

SET key value XX sets the key only if it already exists.

• Key exists → updates it, returns +OK\r\n
• Key doesn't exist → does nothing, returns $-1\r\n

Parsing Flags

Flags appear after the value: SET key value [NX|XX]. Parse the remaining arguments after key and value to detect flags. Flags are case-insensitive.