LazyCache：定期刷新缓存项

Question

我正在使用LazyCache 并希望刷新缓存，例如每小时一次，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

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;
}

不过，RegisterPostEvictionCallback 不会在缓存项过期时调用，而只会在对该项的下一个请求发生时调用（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。

很遗憾，LazyCache.GetOrAddAsync 没有 CancellationToken 作为参数。 在预定时间触发重新加载缓存的最佳方式是什么？第一个用户的等待时间最短？

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，但没有成功。但是，相同的答案通过使用计时器具有替代（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;
        }

 }

}

Answer 2

现在可以使用 LazyCache 2.1 实现自动刷新，使用 LazyCacheEntryOptions 和 ExpirationMode.ImmediateExpiration 这实际上只是时间延迟取消令牌的包装器。您可以在 LazyCache 测试套件的以下测试中看到这一点：

        [Test]
        public async Task AutoRefresh()
        {
            var key = "someKey";
            var refreshInterval = TimeSpan.FromSeconds(1);
            var timesGenerated = 0;

            // this is the Func what we are caching 
            ComplexTestObject GetStuff()
            {
                timesGenerated++;
                return new ComplexTestObject();
            }

            // this sets up options that will recreate the entry on eviction
            MemoryCacheEntryOptions GetOptions()
            {
                var options = new LazyCacheEntryOptions()
                    .SetAbsoluteExpiration(refreshInterval, ExpirationMode.ImmediateExpiration);
                options.RegisterPostEvictionCallback((keyEvicted, value, reason, state) =>
                {
                    if (reason == EvictionReason.Expired  || reason == EvictionReason.TokenExpired)
                        sut.GetOrAdd(key, _ => GetStuff(), GetOptions());
                });
                return options;
            }

            // get from the cache every 2s
            for (var i = 0; i < 3; i++)
            {
                var thing = sut.GetOrAdd(key, () => GetStuff(), GetOptions());
                Assert.That(thing, Is.Not.Null);
                await Task.Delay(2 * refreshInterval);
            }

            // we refreshed every second in 6 seconds so generated 6 times
            // even though we only fetched it every other second which would be 3 times
            Assert.That(timesGenerated, Is.EqualTo(6));
        }