LazyCache hitting Db frequently

[doaji]

Hello Alastair,

I am currently using the 0.7.1.44 version in my .net application. i added to some logging inside my Db call method and i noticed that Db calls are being made rather frequently. I currently have a sliding expiration policy for 7 days, but the method is called multiple times same day.

here is a portion of today's logs

2020-01-16 00:07:41.7333 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS
2020-01-16 00:22:51.7508 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS
2020-01-16 00:34:34.8084 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS
2020-01-16 01:30:59.2897 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS
2020-01-16 03:31:42.4026 INFO 1/16/2020 3:31:42 AM Removing Cache: CachedActivityFeed:5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6-POSTS CacheSpecificEviction
2020-01-16 11:27:02.1429 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS
2020-01-16 14:37:11.0202 INFO Calling ActivityFeedDB: 5a10c9e6-eb2c-41f0-9fc8-20c0fe6643f6:POSTS

these are snippets of my code:

    private static readonly CacheItemPolicy policy = new CacheItemPolicy()
    {
        Priority = CacheItemPriority.Default,
        RemovedCallback = new CacheEntryRemovedCallback(Factory.OnCacheExpired),
        SlidingExpiration = new TimeSpan(7, 0, 0, 0, 0)
    };

    public static CachedActivityFeed GetItem(Guid? activityid, int? id, Site_Activities.Activities type)
    {
        try
        {
            return Factory.GetOrAdd(GetCacheID(activityid, id, type), () => Construct(activityid, id, type), policy);
        }
        catch (Exception)
        {

            return null;
        }

    }

    public static T GetOrAdd<T>(string key, Func<T> func, CacheItemPolicy cacheItemPolicy)
    {
        var result = TempCache.cache.GetOrAdd(key, func, cacheItemPolicy);
        return result;
    }

public static IAppCache cache = new CachingService();

Am i doing something wrong implementation wise? or is this a know bug thats fixed with an upgrade?

Great Lib!

[alastairtree]

Some code seems to be missing - for example I cant see any code that generates the logs you included. Perhaps a couple of suggestions:

• Check that your key GetCacheID(activityid, id, type) is always returning the same key for each item.
• Try creating a new CacheItemPolicy on each call into GetOrAdd - it might be that by reusing the same policy instance MemoryCache will use the sliding expiry based on the first request and not at the scond time the item is cached.

Failing that create and share a minimal reproducable console app that recreates your issue to help others solve your problem.

[doaji]

thanks for the response Alastair,

the GetCacheID is a pretty standard method that concats the values passes to make sure the key is consistent
` private static string GetCacheID(Guid? gid, int? id, Site_Activities.Activities type)
{
if (gid.HasValue)
{
return $"CachedActivityFeed:{gid}-{type}";
}
else
{
return $"CachedActivityFeed:{id}-{type}";
}

    }`

this is also the code for logging removals of the cache
` public static void Delete(string key)
{
LogHelper.Logger.Info($"{DateTime.Now} Removing Cache (Code): {key}");
TempCache.cache.Remove(key);
}

    public static void OnCacheExpired(CacheEntryRemovedArguments arguments)
    {
        LogHelper.Logger.Info($"{DateTime.Now} Removing Cache: {arguments.CacheItem.Key} {arguments.RemovedReason}");
    }`

I have updated the code to create a new policy every time GetOrAdd is called
`return Factory.GetOrAdd(GetCacheID(activityid, id, type), () => Construct(activityid, id, type), Factory.GetCacheItemPolicy(TimeSpan.FromDays(7),Factory.OnCacheExpired));

    public static CacheItemPolicy GetCacheItemPolicy(TimeSpan slidingTime, CacheEntryRemovedCallback removeCallback)
    {
        return new CacheItemPolicy()
        {
            Priority = CacheItemPriority.Default,
            SlidingExpiration = slidingTime, RemovedCallback=new CacheEntryRemovedCallback(removeCallback)
        };
    }

`
. Will test it out

[doaji]

so update on my digging so far, i downloaded the sc for 0.7.1.44 and when i removed the call to EnsureRemovedCallbackDoesNotReturnTheLazy(policy); in the GetOrAdd method it seems to work fine in production. looks like the policy modification is causing the issue. i also noticed you are doing lazy execution in this version, but in the recent versions you are using SemaphoreSlim, is there any advantage of using that over the lazy load?

[alastairtree]

Because of the changes to underlying APIs in MemoryCache when I moved LazyCache to version 2, some additional locking was required and because it is async in places SemaphoreSlim is required. It might not be optimal but is reliable and 'fast enough' for my use cases.
As I am not using the 0.7 version any longer I have minimal time to support it, but if you or anyone would like to submit a PR for the bug may be able to release a patch for it. Best course of action may just be to upgrade to the newer version 2 of LazyCache though following the upgrade guide.

[alastairtree]

Did you manage to update to LazyCache 2? Are you still having issues? Or can we close this?