I'm curious about the impact of using a Scheduler when creating a Caffeine cache.

[jojaeng2]

Hi, I am currently considering using Caffeine as follows:

centerAuthCaches = Caffeine.newBuilder()
            .expireAfterWrite(Duration.ofSeconds(10L))
            .refreshAfterWrite(Duration.ofSeconds(5L))
            .scheduler(Scheduler.systemScheduler())
            .buildAsync((key, executor) -> mappingFunction(key));

I have reviewed the expireAfterWrite and refreshAfterWrite options, but I have some questions about the Scheduler.
I understand that the Scheduler roughly performs the role of an executor for tasks like cache eviction, using a thread pool. Is that correct?
If so, what differences would occur if I declare a Scheduler versus not declaring one? It seems that the code would function similarly even without declaring a Scheduler.

[ben-manes]

It doesn’t look like you need the scheduler. By default the various activity on the cache will cause it to periodically evict expired entries. That could mean an idle cache won’t evict for a while. When a listener is attached, sometimes there is a desire to promptly notify upon expiration instead of waiting. The scheduler has the next expiration event for its delay so that it can trigger the cleanup. An example might be to disconnect idle users after a session timeout (expireAfterAccess). If you don’t use a listener or need percise eviction timings, then you can skip it and piggyback on regular activity.

[jojaeng2]

Thank you for your answer. There is no difference in performance, but can I think that there may be some time difference in the scheduler processing the activities (ex:evit) that occur in caffeine?

[ben-manes]

Not really. It just triggers cleanUp() based on the next expiration time. The scheduler doesn’t actually run it either, it just submits it to the executor. We maintain expiration order so that finding expired entries is O(1), so scheduling is just a peeking ahead. There is not much about it all.