Refactor incrementWithTtlScript Lua EVAL to use Redis Functions

[hongwei1]

Description

We should consider replacing the EVAL Lua script incrementWithTtlScript with Redis Functions (introduced in Redis 7.0).

Current implementation relies on EVAL/EVALSHA for rate limiting / TTL logic:

local c = redis.call('INCR', KEYS[1])
if c == 1 then
  redis.call('EXPIRE', KEYS[1], ARGV[1])
elseif redis.call('TTL', KEYS[1]) < 0 then
  redis.call('SET', KEYS[1], 1, 'EX', ARGV[1])
  c = 1
end
return {c, redis.call('TTL', KEYS[1])}

Proposed Solution

Using Redis Functions provides several benefits over EVAL:

1. Persistence & Replication: Functions are treated like data, persisting across restarts and replicating to replicas. We no longer need to handle NOSCRIPT errors on the application side.
2. Simplified Application Logic: The application simply calls FCALL increment_with_ttl 1 <key> <ttl>.

Example function definition:

#!lua name=ratelimit_lib

local function increment_with_ttl(keys, args)
  local c = redis.call('INCR', keys[1])
  if c == 1 then
    redis.call('EXPIRE', keys[1], args[1])
  elseif redis.call('TTL', keys[1]) < 0 then
    redis.call('SET', keys[1], 1, 'EX', args[1])
    c = 1
  end
  return {c, redis.call('TTL', keys[1])}
end

redis.register_function('increment_with_ttl', increment_with_ttl)

Prerequisites

• Ensure the minimum required Redis version is 7.0 in all environments.
• Ensure the Redis client library supports FCALL and FUNCTION LOAD.
• Update deployment/migration steps to load the function library into Redis upon startup.