In-memory cache timespan not working

[eaidland]

Hi!
First of all thanks for a great package 🥇

Version: DoubleCache 2.0.0-beta8 (Nuget).

When calling our API we have a middelware that uses DoubleCache. The idea is to get key from in-memory, then redis and if non then fetch from keyvault. We have set the absolute expiration to 1 minute.

1. First run get the value from KeyVault, since the key isn't in in-mem nor redis.
2. Saves value (encryptet) to in-mem and redis.
3. Second run fetches from in-mem.
4. Server restart
5. Third run fetches from redis, then save in mem.
6. Change secret in KeyVault
7. Wait for the absolute expiration (over 1 min)
8. Fourth run the in-mem still remembers the old value

Our conclution is that the local memory doesn't respect the expiration value.

string value = await _cache.GetAsync("mykey", async () => await _myService.GetKeyValue(myValue), TimeSpan.FromMinutes(1));

To get the solution to work we had to override the MemoryCache.Default

System.Runtime.Caching.MemoryCache.Default.Set("mykey", value, DateTimeOffset.UtcNow.AddMinutes(1));

So is this a bug or are we using the package wrong?

Thanks!

[cbigsby]

Hi eaidland,

I took a look at the code and it does look like a bug.

When we have an in-memory cache-miss we fetch the item from redis and store it in the in-memory cache with the TTL provided (ie: 1 minute in your example) instead of the time remaining on the cached item.

If you are only changing the underlying data through the same service that is using DoubleCache you can either remove the old cached item or update the cached item with the new one as a workaround.

I'm not exactly sure how to fix this bug ATM because the ICacheAside doesn't expose the remaining TTL for the cached items, so there's no way for the in-memory cache to know how much longer the data will be in the Redis cache before it should expire. Perhaps @Ulriksen has an idea on that.

Could you clarify if the cached item stays in-memory if you wait 3+ minutes after step 5? Also, do you know if this behavior also happens on DoubleCache 1.x? 2.0 is still in development so I'm not sure if it should be used for production apps yet.

[Ulriksen]

I believe the cache items needs a wrapper object, we already use one in v2-beta for local cache to support null values.