[ovirt-devel] Caching of data from the database done properly

Hi Everyone, I wanted to discuss a practice which seems to be pretty common in the engine which I find very limiting, dangerous and for some things it can even be a blocker. There are several places in the engine where we are using maps as cache in singletons to avoid reloading data from the database. Two prominent ones are the QuotaManager[1] and the MacPoolPerCluster[2]. While it looks tempting to just use a map as cache, add some locks around it and create an injectable singleton, this has some drawbacks: 1) We have an autoritative source for our data and it offers transactions to take care of inconsistencies or parallel updates. Doing all that in a service again duplicates this. 2) Caching on the service layer is definitely not a good idea. It can introduce unwanted side effects when someone invokes the DAOs directly. 3) The point is more about the question if a cache is really needed: Do I just want that cache because I find it convenient to do a #getMacPoolForCluster(Guid clusterId) in a loop instead of just loading it once before the loop, or do my usage requirements really force me to use a cache? If you really need a cache, consider the following: 1) Do the caching on the DAO layer. This guarantees the best consistency across the data. 2) Yes this means either locking in the DAOs or a transactional cache. But before you complain, think about what in [1] and [2] is done. We do exactly that there, so the complexity is already introduced anyway. 3) Since we are working with transactions, a custom cache should NEVER cache writes (really just talking about our use case here). This makes checks for existing IDs before adding an entity or similar checks unnecessary, don't duplicate constraint checks like in [2]. 4) There should always be a way to disable the cache (even if it is just for testing). 5) If I can't convince you to move the cache to the DAO layer, still add a way to disable the cache. For as long as there is no general caching solution with something like ehcache or infinispan, in my eyes such small things matter a lot to keep a project maintainable. That are some of the best practices I have seen around caching database data. It would be great if we could agree on something like that. Maybe there is already an agreement on something and I am just not aware of it. Looking forward to hear your feedback. Roman [1] https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules... [2] https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules...