caching - LazyCache:定期刷新缓存的项目

Question

我正在使用 LazyCache并希望刷新缓存，例如每小时，但理想情况下，我希望缓存项过期后的第一个调用者不要等待缓存重新加载。我写了以下内容

public async Task<List<KeyValuePair<string, string>>> GetCarriersAsync()
{

    var options = new MemoryCacheEntryOptions
    {
        AbsoluteExpirationRelativeToNow = new TimeSpan(1,0,0),// consider to config
    }.RegisterPostEvictionCallback(
         async  (key, value, reason, state) =>
        {
            await GetCarriersAsync();//will save to cache
            _logger.LogInformation("Carriers are reloaded: " );
        });
    Func<Task<List<KeyValuePair<string, string>>>> cacheableAsyncFunc = () => GetCarriersFromApi();
    var cachedCarriers = await _cache.GetOrAddAsync($"Carriers", cacheableAsyncFunc, options);

    return cachedCarriers;
}

然而 RegisterPostEvictionCallback 在缓存项过期时不会被调用，而只会在对该项的下一个请求发生时调用(并且调用者需要等待一个冗长的操作)。

线程 Expiration almost never happens on it's own in the background #248解释说
这是设计使然，并建议解决方法来指定 CancellationTokenSource.CancelAfter(TimeSpan.FromHours(1)) 而不是 SetAbsoluteExpiration。

不幸的是 LazyCache.GetOrAddAsync 没有 CancellationToken 作为参数。
在预定时间触发缓存重新加载且第一个用户等待时间最短的最佳方法是什么？

Answer 1

我发现了类似的问题 In-Memory Caching with auto-regeneration on ASP.Net Core建议调用AddExpirationToken(new CancellationChangeToken(new CancellationTokenSource(_options.ReferenceDataRefreshTimeSpan).Token) .
我试过了，但没有让它工作。然而，同样的答案通过使用计时器有替代(和推荐)选项。我创建了一个 RefreshebleCache 类，用于不同的可缓存选项，如下所示:

   var refreshebleCache = new RefreshebleCache<MyCashableObjectType>(_cache, _logger);
   Task<MyCashableObjectType> CacheableAsyncFunc() => GetMyCashableObjectTypeFromApiAsync();
   var cachedResponse = await refreshebleCache.GetOrAddAsync("MyCashableObject", CacheableAsyncFunc,
                        _options.RefreshTimeSpan);

RefreshebleCache 实现:

/// <summary>
    /// Based on https://stackoverflow.com/questions/44723017/in-memory-caching-with-auto-regeneration-on-asp-net-core
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class RefreshebleCache<T>
    {

        protected readonly IAppCache _cache;
        private readonly ILogger _logger;
        public bool LoadingBusy = false;
        private string _cacheKey;
        private TimeSpan _refreshTimeSpan;
        private Func<Task<T>> _functionToLoad;
        private Timer _timer;

        public RefreshebleCache(IAppCache cache, ILogger logger)
        {

            _cache = cache;
            _logger = logger;
        }

        public async Task<T>  GetOrAddAsync (string cacheKey , Func<Task<T>> functionToLoad, TimeSpan refreshTimeSpan)
        {
            _refreshTimeSpan= refreshTimeSpan;
            _functionToLoad = functionToLoad;
            _cacheKey = cacheKey;
            var timerCachedKey = "Timer_for_"+cacheKey;
            //if removed from cache, _timer could continue to work, creating redundant calls
            _timer =  _appCache.GetOrAdd(timerCachedKey, () => 
             CreateTimer(refreshTimeSpan), 
  SetMemoryCacheEntryOptions(CacheItemPriority.NeverRemove));
            var cachedValue = await LoadCacheEntryAsync();
            return  cachedValue;
        }
        private Timer CreateTimer(TimeSpan refreshTimeSpan)
        {
            Debug.WriteLine($"calling CreateTimer for {_cacheKey} refreshTimeSpan {refreshTimeSpan}"); //start first time in refreshTimeSpan
            return new Timer(TimerTickAsync, null, refreshTimeSpan, refreshTimeSpan);
        }

    
        private async void TimerTickAsync(object state)
        {
            if (LoadingBusy) return;
            try
            {
                LoadingBusy = true;
                Debug.WriteLine($"calling LoadCacheEntryAsync from TimerTickAsync for {_cacheKey}");
                var loadingTask = LoadCacheEntryAsync(true);
                await loadingTask;
            }
            catch(Exception e)
            {
                _logger.LogWarning($" {nameof(T)} for {_cacheKey} was not reloaded.    {e} ");
            }
            finally
            {
                LoadingBusy = false;
            }
        }
        private async Task<T> LoadCacheEntryAsync(bool update=false)
        {
            var cacheEntryOptions = SetMemoryCacheEntryOptions();

            Func<Task<T>> cacheableAsyncFunc = () => _functionToLoad();
            Debug.WriteLine($"called LoadCacheEntryAsync for {_cacheKey} update:{update}");
            T cachedValues = default(T);
            if (update)
            {
                cachedValues =await cacheableAsyncFunc();
                if (cachedValues != null)
                {
                    _cache.Add(_cacheKey, cachedValues, cacheEntryOptions);
                }

                //    _cache.Add(_cacheKey, cacheableAsyncFunc, cacheEntryOptions);
            }
            else
            {
                 cachedValues = await _cache.GetOrAddAsync(_cacheKey, cacheableAsyncFunc, cacheEntryOptions);
            }
            return cachedValues;
        }
        private MemoryCacheEntryOptions SetMemoryCacheEntryOptions(CacheItemPriority priority= CacheItemPriority.Normal)
       {
          var cacheEntryOptions = new MemoryCacheEntryOptions
          {
            Priority = priority 
          };
          return cacheEntryOptions;
        }

 }

}