C#•13mo ago

Cache not clearing, what am I missing?

Does anyone know why this cache is not clearing? I am trying to implement simple rate limiting an and old MVC application. If a POST endpoint is hit too many times, return a 500/429. My approach:

private bool IsAllowed(string controllerName, string actionName, string ipAddress)
{
    var isAllowed = true;
    var maxRequests = 3;
    var timeframeMins = 3;
    var cacheKey = $"{controllerName}-{actionName}-{ipAddress}";  
                
    MemoryCache cache = MemoryCache.Default;
    CacheItemPolicy policy = new CacheItemPolicy
    {
        AbsoluteExpiration = DateTimeOffset.UtcNow.AddMinutes(timeframeMins),
        Priority = System.Runtime.Caching.CacheItemPriority.Default,
    };

    if (cache.Get(cacheKey) == null)
    {
        cache.Add(cacheKey, 1, policy);
    }
    else
    {
        var hitCount = (int)cache.Get(cacheKey);
        hitCount++;
        cache[cacheKey] = hitCount;

        if (hitCount > maxRequests)
        {
            isAllowed = false;
        }
    }

    return isAllowed;
}

private bool IsAllowed(string controllerName, string actionName, string ipAddress)
{
    var isAllowed = true;
    var maxRequests = 3;
    var timeframeMins = 3;
    var cacheKey = $"{controllerName}-{actionName}-{ipAddress}";  
                
    MemoryCache cache = MemoryCache.Default;
    CacheItemPolicy policy = new CacheItemPolicy
    {
        AbsoluteExpiration = DateTimeOffset.UtcNow.AddMinutes(timeframeMins),
        Priority = System.Runtime.Caching.CacheItemPriority.Default,
    };

    if (cache.Get(cacheKey) == null)
    {
        cache.Add(cacheKey, 1, policy);
    }
    else
    {
        var hitCount = (int)cache.Get(cacheKey);
        hitCount++;
        cache[cacheKey] = hitCount;

        if (hitCount > maxRequests)
        {
            isAllowed = false;
        }
    }

    return isAllowed;
}

This cache item never expires. The same thing happens if I am using the HttpContext cache approach: ...

HttpContext.Current.Cache.Insert(cacheKey,
                   1,
                   null,
                   DateTimeOffset.UtcNow.AddMinutes(timeframeMins),
                   Cache.NoSlidingExpiration,
                   CacheItemPriority.Low)

...

HttpContext.Current.Cache[cacheKey] = hitCount;

HttpContext.Current.Cache.Insert(cacheKey,
                   1,
                   null,
                   DateTimeOffset.UtcNow.AddMinutes(timeframeMins),
                   Cache.NoSlidingExpiration,
                   CacheItemPriority.Low)

...

HttpContext.Current.Cache[cacheKey] = hitCount;

etc. Any idea? I am testing this by running an external script that hits the end point multiple times. I can trigger the rejection, but the cache never clears, so the limiting/blocking always remains in place.

1 Reply

ZimmertyZimOP•13mo ago

Still not sure what's going on here. I0 am guessing the cache resets or adds time somehow every time I update the hitCount, it is my understanding that shouldn't be the case. I've just created a second cache item and that tracks the same expiry time. This one clears as expected, so if it's empty I can reset the original cache item (doesn't matter if has become null at that point as a new one is just created):

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Cache not clearing, what am I missing?