Concurrency issue between get(K key, Callable<? extends V> valueLoader) and invalidate(Object key)

Hi,

I encountered a concurrency issue between the “get(K key, Callable<? extends V> valueLoader)” and “invalidate(Object key)” methods with a basic (and i suppose a common) usage of the cache which let a stale value in the cache.

Here is the use case:

• The cache is initially empty
• 1 thread is getting a value in the callable with a given key while an other thread is invalidating the same given key

Thread 1

Bean myBean = cache.get(ID, new Callable<Bean>() {
    @Override
    public Bean call() throws Exception {
        Bean bean = loadFromDB(ID); // (1)
        return bean; // (4)
    }
});

Thread 2

// Update just one property of the bean in DB
updatePartialDataInDB(ID, "newValue1"); // (2)
// Then, we need to invalidate the cache
cache.invalidate(ID); // (3)

The execution sequence order is marked with // (number) After the point // 4, I have the old object in myBean variable which is fine. However, If i try to get again the bean with the identifier ID, I expect to have no value in the cache but a fresh bean reloaded from the DB. Unfortunately, that is not the case and a stale value is kept in the cache.

Is that a wanted behavior?

In LocalCache$Segment#remove(Object key, int hash) line 3081 to 3087 (guava 17.0):

RemovalCause cause;
if (entryValue != null) {
    cause = RemovalCause.EXPLICIT;
} else if (valueReference.isActive()) {
    cause = RemovalCause.COLLECTED;
} else {
    // currently loading
    return null;
}

Shouldn’t a particular process be done before return null ?

Thanks

[txshtkckr]

This is causing some very serious issues for us. I would argue that anyone using a lazy-loading cache would expect a happens-before relationship between invalidate and a following call to get, but since it can piggy-back on an existing load operation that fetched data before invalidate was called, that relationship does not exist.

I started poking around in here to try to fix it, but as @lowasser hinted, it isn’t trivial. The main problems are

1. Whether to remove the LoadingValueReference, block on it, or somehow mark it as stale; and
2. How to rework the removal notifications

This is sufficiently painful that at this point I’m inclined to switch to another library, instead.

[findepi]

@ben-manes thanks for the update about Caffeine.

I am still looking for the Guava team’s perspective on the problem. In particular, would a patch be accepted? what solution categories have been discarded already? What are acceptable drawbacks, if any, for a potential fix?

@lowasser 's comment from 2014 (https://github.com/google/guava/issues/1881#issuecomment-67080198) suggests a patch would be welcome, but there was also a concern about value-in-flight invalidation (https://github.com/google/guava/issues/1881#issuecomment-67076595). Of course, both are couple years old, so they not necessarily describe current state.