MemoryCache: "Cannot update the item that is not yet expired."

[mholzmann-medi]

I use the MemoryCache implementation and have problems in a multi-threaded environment.

When I call a Query() method with the same cacheKey from different threads, I get a "Cannot update the item that is not yet expired." exception. Why can't I update a cached item when it is not expired yet?

Does this mean that I can't use the caching within a multi-threaded environment?

Thanks.

[mikependon]

To ensure that caching mechanism is intact and will not be polluted, we did restrict adding an item to the MemoryCache object with the same key (that has not yet expired), unless of course you explicitly expire the existing cache item and add a new one manually.

But, your problem is not related to the persistency of the cache item. Upon the initial investigation, it sounds to us that the Add method has been invoked twice at the same time and that hits the validation code during caching, in which technically should not happen.

ConcurrentDictionary worked as expected (thread-safe) in your scenario, but our validation has not.

We will look into this and revert back you ASAP. We will also include the fix to this (if there is) on the next major release.

[mikependon]

Temporary solution, can you identify the 2nd thread and delay it for just 1 sec?

[mholzmann-medi]

Thanks for your quick reply.

I quickly implemented a MemoryCache class clone and modified the Add<T>(CacheItem<T> item, bool throwException = true) method to re-add new CacheItem:

if (!item.IsExpired() && throwException == true)
  throw new InvalidOperationException("Cannot update the item that is not yet expired.");

this.cache.AddOrUpdate(item.Key, key => new CacheItem<T>(item.Key, item.Value, item.CacheItemExpiration ?? Constant.DefaultCacheItemExpirationInMinutes), (key, val) => new CacheItem<T>(item.Key, item.Value, item.CacheItemExpiration ?? Constant.DefaultCacheItemExpirationInMinutes));

The disadvantage here is currently that I can't set the Value and CacheItemExpiration properties, so a new instance must be created. But now it works as expected.

[mikependon]

You should be able to set the expiration of the cache item by accessing actual CacheItem itself.

RepoDB/RepoDb.Core/RepoDb/CacheItem.cs

Line 97 in 964f455

public DateTime Expiration { get; set; }

FYI, we do understand your scenario about multi-threaded calls on the same item from the database. This problem needs to be fixed fundamentally.

We will consider your fix on our fix to this issue.

[mikependon]

FYI: The issue can be replicated by this project solution (Issue1017.zip).

[mikependon]

The fix has been added on the main branch. It will be a part of the next release (beta or actual).

Note: There is a behavioral change on this. When calling the MemoryCache.Add method, the existing cached item (if present) information will be updated by the latest and updated one.

[mholzmann-medi]

@mikependon Can you tell me when the next release will be? Thanks a lot.