Great question! Let's unpack this by first recalling what a pure function is, then applying that definition to your rate-limiting script.

What Makes a Function "Pure"? #

A pure function has two core traits:

1. Deterministic: Given the exact same input parameters, it will always return the exact same output—no matter when or how many times you run it. It doesn't depend on external, mutable state.
2. No Side Effects: It doesn't modify any external state (like database records, cache keys, or global variables) beyond returning a value.

Analyzing Your Rate-Limiting Script #

Your script is designed to handle race conditions from key expiration by:

• Checking if the rate-limit key exists (i.e., hasn't expired)
• If the key doesn't exist, initializing it with the max request count minus one, plus setting its TTL
• If the key does exist, decrementing the remaining request count
• Returning the updated remaining count

Let's map this to pure function rules:

1. It's not deterministic:
   Suppose your input parameters are ("api:checkout", 10, 60) (key name, max requests, TTL in seconds).

   • First run: Key is expired (doesn't exist), so you initialize it to 9 and return 9.
   • 59 seconds later, same input: Key still exists, so you decrement to 8 and return 8.
   • 2 seconds after that, key expires again: Same input returns 9 once more.
     Same inputs → different outputs, which breaks determinism.

2. It has side effects:
   Your script modifies Redis state directly: it either creates a new key with a TTL or updates an existing key's value. Pure functions don't alter external state—they only compute and return values.

A Quick Note on Redis Lua Atomicity #

It's important to distinguish between atomicity and purity here. Your script solves the WATCH/MULTI race condition because Redis executes Lua scripts atomically, but atomicity doesn't make it a pure function. Atomicity ensures no other operations interfere during execution; purity is about input/output consistency and lack of side effects.

Final Verdict #

Your Lua script is not a pure function. Its behavior depends on the dynamic state of your Redis keys (whether they've expired or how many requests are left), and it modifies Redis state as part of its execution—both of which violate the core rules of pure functions.

内容的提问来源于stack exchange，提问作者usbpc102