What is Lua scripting in Redis and when should you use it?

Redis supports executing Lua scripts atomically on the server via EVAL script numkeys key [key ...] arg [arg ...]. The entire script runs as a single atomic unit — no other command can execute between Lua operations. This is stronger than MULTI/EXEC because Lua allows conditional logic: you can read a value inside the script and branch based on it before writing, which is impossible in a MULTI block (where all commands are queued before any result is known). Common uses: complex atomic operations like "check inventory and decrement if available," rate limiting with fine-grained logic, and distributed lock implementation. Use EVALSHA to execute a pre-loaded script by its SHA1 hash, avoiding repeated network transfer of the script body.