When are expired items removed from the cache and garbage collected to free up memory?

[Malgefor]

I recently switched to FusionCache from LazyCache. What I not fully understand yet about FusionCache is when expired items are actually garbage collected. The plain IMemoryCache has a method called Compact() to manually do this. How does FusionCache free up memory? And how does using FailSafe affect this?

[jodydonetti]

Hi @Malgefor

I recently switched to FusionCache from LazyCache. What I not fully understand yet about FusionCache is when expired items are actually garbage collected.

The plain IMemoryCache has a method called Compact() to manually do this. How does FusionCache free up memory?

FusionCache uses a normal IMemoryCache underneath as the L1 implementation, and if you do not pass a specific instance it will just create a new reserved MemoryCache instance.

Because of this, the actual behaviour depends on the IMemoryCache implementation which is actually used.
Now, as said, normally that would be MemoryCache: because of this, the behaviour is its standard one, meaning it's based on the ExpirationScanFrequency option, and looking at the code it seems it is done evey 1 min bydefault.
I've found more info in this old issue.

And how does using FailSafe affect this?

This is a good question: the way fail-safe works is kinda like a soft delete.
Basically, when you put something in FusionCache with a certain Duration:

• if fail-safe is disabled: an entry is saved into the underlying MemoryCache with that duration and no "special" handling. A later call to either Remove() or Expire() removes it for good, basically calling Remove() on the underlying MemoryCache, so from there you can think about a normal MemoryCache behaviour
• if fail-safe is enabled: an entry is saved into the cache with a duration equal to FailSafeMaxDuration and with an additional piece of metadata that specify when it should be considered "stale" (the specified Duration). This means that it will considered expired after Duration, but can be used as a fallback in case of problems/timeouts during factory execution. A later call to Remove() will remove it for good as above, but a call to Expire() will simply mark it as stale and keep it there

You can read more about how fail-safe work here.

Hope this helps, let me know.