.Net Core MemoryCache PostEvictionCallback无法正常工作 [英] .Net Core MemoryCache PostEvictionCallback not working properly

问题描述

我已经在Microsoft.Extensions.Caching.Memory.MemoryCache中设置了具有过期时间的缓存项。
我想在每次缓存项过期时触发回调，但是直到我查询缓存中已过期的缓存项时才触发回调。

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.

这里是代码：

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();
        }
    }
}

推荐答案

之所以发生这种情况，是因为直到您查询该项目并检查到期之后，该项目才被逐出

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

（来自 MemoryCacheStore.Get（MemoryCacheKey键） ）

    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;
    }

或 Trim（） 由于内存压力在内部被调用

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

（来自 TrimInternal（int percent） ）

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

如果系统当前不低足够的内存来触发修整，则只有在尝试检索项目时才将其逐出。

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.

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