ANTOOZA-BACKEND/Infrastructure/RateLimit/RedisSlidingWindowLimiter.cs

using Application.Abstractions.RateLimit;
using StackExchange.Redis;

namespace Infrastructure.RateLimit;

/// <summary>
/// Redis sorted-set 기반 sliding window rate limiter.
/// Lua script 로 원자성 보장 (read-modify-write 한 번에).
/// </summary>
public sealed class RedisSlidingWindowLimiter(IConnectionMultiplexer redis) : IRateLimiter
{
    private readonly IDatabase _db = redis.GetDatabase();

    private const string LuaScript = @"
local key = KEYS[1]
local now = tonumber(ARGV[1])
local minScore = tonumber(ARGV[2])
local windowSeconds = tonumber(ARGV[3])
local limit = tonumber(ARGV[4])
local member = ARGV[5]

redis.call('ZREMRANGEBYSCORE', key, '-inf', minScore)
local count = redis.call('ZCARD', key)

if count < limit then
    redis.call('ZADD', key, now, member)
    redis.call('EXPIRE', key, windowSeconds)
    return { 1, limit - count - 1 }
else
    return { 0, 0 }
end
";

    public async Task<RateLimitResult> CheckAsync(string key, int limit, TimeSpan window, CancellationToken ct = default)
    {
        var now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
        var windowMs = (long)window.TotalMilliseconds;
        var minScore = now - windowMs;
        var fullKey = $"ratelimit:{key}";
        var uniqueMember = $"{now}:{Guid.NewGuid():N}";

        var raw = await _db.ScriptEvaluateAsync(
            LuaScript,
            keys: [fullKey],
            values: [now, minScore, (long)window.TotalSeconds, limit, uniqueMember]
        );

        var array = (RedisResult[])raw!;
        var allowed = (long)array[0] == 1;
        var remaining = (int)(long)array[1];
        var resetAt = DateTimeOffset.FromUnixTimeMilliseconds(now + windowMs).UtcDateTime;

        return new RateLimitResult(allowed, limit, remaining, resetAt);
    }
}