SetFailSafe(true) and exception handling

[tjmoore]

Hi, I'm trying to understand the behaviour when fail safe is enabled.

Am I right in thinking any exception at all from the factory method will be treated as downtime/failure, consumed and return from cache, not exposing the exception to the caller?

What if the factory method may throw a genuine exception, such as a "not found" exception in a database query? This will effectively be ignored?

e.g.

• First call not cached yet - item doesn't exist - throws "not found" which should be thrown to the caller okay ?
• First call not cached yet - item exists - caches fine and returns item
• Subsequent calls return from cache
• Cache expires, data changed in database so item no longer exists, so it should throw "not found", but this will be caught in fail safe mode and return stale cached data ?

I want to use the factory timeout feature to allow re-caching to occur in the background where that operation is slow, but it looks like that only works with fail safe enabled and it looks like that will hide exceptions we want. Out database is file based (LiteBD) and less likely to ever be "down" so not that bothered about fail safe, but lags in caching have serious impacts on performance with the frequency of requests we have to handle, which is why I'm interested in FusionCache.

Ideally not found would return null not throw an exception, but have complex calls with multiple items in params and one or more may not exist so an exception is thrown detailing which doesn't exist, for example.

Also, kind of related? #203

Though don't want to trigger fail safe without throwing an exception, but bypass fail safe in some situations.

[jodydonetti]

Hi @tjmoore and thanks for using FusionCache!

Am I right in thinking any exception at all from the factory method will be treated as downtime/failure, consumed and return from cache, not exposing the exception to the caller?

Yes and no: IF you enable fail-safe AND there's a stale (expired) value to use as a fallback (AND/OR you haven't specified a fail-safe default value in the call) then yes, no exception will be exposed to the caller since that's the whole point of fail-safe.

BUT if there's no stale value (like, the first time a factory is called and you haven't specified a fail-safe default value) then the factory exception will be exposed to the caller.

What if the factory method may throw a genuine exception, such as a "not found" exception in a database query? This will effectively be ignored?

Yes, since throwing an exception when something is not found is typically considered not a good practice.

e.g.

• First call not cached yet - item doesn't exist - throws "not found" which should be thrown to the caller okay ?
• First call not cached yet - item exists - caches fine and returns item
• Subsequent calls return from cache
• Cache expires, data changed in database so item no longer exists, so it should throw "not found", but this will be caught in fail safe mode and return stale cached data ?

The behaviour you described is in fact how it works, but again I would suggest not to throw exceptions when "something is not in the database" or similar. The reason is that even "xyz doesn't exist" should be cached, maybe for less time if you want, but still cached.

Otherwise think about your service and what would happen if I start repeatedly asking for "/product/99999999" (just an example): you just opened a venue for a good old DoS attack, and your database will be bombarded constantly by external calls.

I want to use the factory timeout feature to allow re-caching to occur in the background where that operation is slow, but it looks like that only works with fail safe enabled and it looks like that will hide exceptions we want. Out database is file based (LiteBD) and less likely to ever be "down" so not that bothered about fail safe, but lags in caching have serious impacts on performance with the frequency of requests we have to handle, which is why I'm interested in FusionCache.

If you really want your data layer to throw when not found, I would suggest to try/catch that exception inside the factory and return null and/or cache that for less time, for example by using Adaptive Caching.

Does this make sense?

Also, kind of related? #203

Not really, I think: that issues is for activating fail-safe even without throwing, whereas you here are asking for kinda the opposite: being able to throw without activating fail-safe.

Though don't want to trigger fail safe without throwing an exception, but bypass fail safe in some situations.
Adaptive caching can do that, I would suggest try playing with it and see if it works.

Hope this helps, let me know.

[tjmoore]

Thanks. That helps a lot. I think we've only got a few cases of throwing exceptions such as not found. In most cases we return null for example. I think we can adjust it to only throw exceptions where the database is in a bad state/unavailable.

My main challenge now is handling a few slow queries on the database.

I'm actually needing conditional refresh when we know the database has been updated and background caching when database queries are slow.

[jodydonetti]

Awesome, glad it helped!

Let me know if you need something more, hopefully I'll answer faster next time 😅