-- KEYS[1]: rate limit key
-- ARGV[1]: capacity (max tokens)
-- ARGV[2]: rate (tokens per second)
-- ARGV[3]: current timestamp
 
local key = KEYS[1]
local capacity = tonumber(ARGV[1])
local rate = tonumber(ARGV[2])
local now = tonumber(ARGV[3])
 
local info = redis.call("HMGET", key, "tokens", "last_update")
local tokens = tonumber(info[1])
local last_update = tonumber(info[2])
 
if tokens == nil then
    tokens = capacity
    last_update = now
else
    local delta = math.max(0, now - last_update)
    tokens = math.min(capacity, tokens + delta * rate)
end
 
if tokens >= 1 then
    tokens = tokens - 1
    redis.call("HMSET", key, "tokens", tokens, "last_update", now)
    redis.call("EXPIRE", key, math.ceil(capacity / rate))
    return 1 -- Allowed
else
    return 0 -- Rate Limited
end

%%{init: {"theme": "default", "flowchart": {"useMaxWidth": false}}}%%
flowchart TD
    A[Client Request] --> B{Rate Limiter}
    B -- Allowed --> C[API Gateway]
    C --> D[Backend Service]
    B -- Denied --> E[429 Too Many Requests]