.Net Core MemoryCache PostEvictionCallback not working properly

Question

I have set cache items with sliding expiration in a Microsoft.Extensions.Caching.Memory.MemoryCache. I want to trigger a callback everytime a cache item expires, but callback isn't triggered until I query the cache for the expired cache item.

Here is the code:

using System;
using Microsoft.Extensions.Caching.Memory;

namespace Memcache
{
    public class Program
    {
        private static MemoryCache _cache;
        private static int _cacheExpSecs;

        public static void Main(string[] args)
        {
            _cache = new MemoryCache(new MemoryCacheOptions());
            _cacheExpSecs = 2;

            var cacheEntryOptions = new MemoryCacheEntryOptions()
            .SetSlidingExpiration(TimeSpan.FromSeconds(_cacheExpSecs))
            .RegisterPostEvictionCallback(callback: EvictionCallback);

            _cache.Set(1, "One", cacheEntryOptions);
            _cache.Set(2, "Two", cacheEntryOptions);

            var autoEvent = new System.Threading.AutoResetEvent(false);

            System.Threading.Timer timer = new System.Threading.Timer(checkCache, autoEvent, 1000, 6000);

            Console.Read();
        }

        private static void checkCache(Object o)
        {
            if(_cache.Get(1)!=null)
            {
                Console.WriteLine(string.Format(@"checkCache: Cache with key {0} will be removed manually and will trigger the callback.", 1));
                _cache.Remove(1);
            }
            else
            {
                Console.WriteLine(string.Format("checkCache: Cache with key {0} is expired.", 1));
            }


            if(_cache.Get(2) != null)
            {
                Console.WriteLine(string.Format("checkCache: Cache with key {0} will expire in {1} seconds, but won't trigger the callback until we check it's value again.", 2, _cacheExpSecs));
            }
            else
            {
                Console.WriteLine(string.Format("checkCache: Cache with key {0} is expired.", 2));
            }

        }

        private static void EvictionCallback(object key, object value, EvictionReason reason, object state)
        {
            Console.WriteLine();
            Console.WriteLine("/*****************************************************/");
            Console.WriteLine(string.Format("/*  EvictionCallback: Cache with key {0} has expired.  */", key));
            Console.WriteLine("/*****************************************************/");
            Console.WriteLine();
        }
    }
}

Steve Kinyon · Accepted Answer · 2017-12-23 01:50:29Z

56

To add onto the accept answer and comments, you can force the cache to expire and evict automatically by using a expiring cancellation token.

int expirationMinutes = 60;
var expirationTime = DateTime.Now.Add(expirationMinutes);
var expirationToken = new CancellationChangeToken(
    new CancellationTokenSource(TimeSpan.FromMinutes(expirationMinutes + .01)).Token);

var cacheEntryOptions = new MemoryCacheEntryOptions()
         // Pin to cache.
         .SetPriority(CacheItemPriority.NeverRemove)
         // Set the actual expiration time
         .SetAbsoluteExpiration(expirationTime)
         // Force eviction to run
         .AddExpirationToken(expirationToken)
         // Add eviction callback
         .RegisterPostEvictionCallback(callback: CacheItemRemoved, state: this);

`

The lack of built in timer behavior, which the old one used to have, is supposed to be by design and this is what was recommended in its place. See: https://github.com/aspnet/Caching/issues/248

answered Dec 23, 2017 at 1:50

Steve Kinyon

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3 Comments

Marco Over a year ago

Thanks, this was exactly what I was looking for and can confirm that cache does not need to be hit for this to trigger.

Daniel Szabo Over a year ago

Worked great, minor fyi had to change DateTime.Now.Add(...) to DateTime.Now.AddMinutes(...) on aspnetcore 5+.

arteny Jun 15 at 17:38

But it not works for sliding expiration, it unconditional removes entry by expiration token

Scott Chamberlain · Accepted Answer · 2017-03-01 15:54:26Z

22

It is happening because the item is not evicted till you query for the item and it checks the expiration

(From the Source of MemoryCacheStore.Get(MemoryCacheKey key))

    internal MemoryCacheEntry Get(MemoryCacheKey key) {
        MemoryCacheEntry entry = _entries[key] as MemoryCacheEntry;
        // has it expired?
        if (entry != null && entry.UtcAbsExp <= DateTime.UtcNow) {
            Remove(key, entry, CacheEntryRemovedReason.Expired);
            entry = null;
        }
        // update outside of lock
        UpdateExpAndUsage(entry);
        return entry;
    }

or when Trim() is called internally due to memory pressure

(From the Source of TrimInternal(int percent))

/*SNIP*/
        trimmedOrExpired = _expires.FlushExpiredItems(true);
        if (trimmedOrExpired < toTrim) {
            trimmed = _usage.FlushUnderUsedItems(toTrim - trimmedOrExpired);
            trimmedOrExpired += trimmed;
        }
/*SNIP*/

If your system is not currently low enough on memory to trigger a trim then the only time items will be evicted is when they are attempted to be retrieved.

edited Mar 1, 2017 at 15:54

answered Mar 1, 2017 at 15:49

Scott Chamberlain

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7 Comments

mattinsalto Over a year ago

Thanks. There is ExpirationScanFrequency option in MemoryCacheOptions, but neither works.

Scott Chamberlain Over a year ago

You might want to unaccept my answer for now. I totally overlooked this was .NET Core. Checking the Core source now that my statements are still correct

Scott Chamberlain Over a year ago

Ok, the logic is still the same. ExpirationScanFrequency is the frequency it does a full scan after any kind of Get or Remove is peformed. If the time has been longer than the ExpirationScanFrequency it does a full scan of items instead of just the one it was working with, It still does not run a timer to perform the scans, they are all still done on demand when a action is perfomed.

Scott Chamberlain Over a year ago

If you wanted to you could call _cache.Compact(0) on your own timer and that would flush out the expired entries on a regular basis.

mattinsalto Over a year ago

Thank you, I'll use Compact(0). On the other hand, in .NET framework, System.Runtime.Caching.MemoryCache triggers the callback when sliding expiration cache item is expired.

arteny Jun 15 at 17:41

System.Runtime.Caching works same type for me for .NET 9 (not triggers if no any access).

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