can this use case be ever worked out in ehcache?

Question

i want to use ehcache not only as a cache but as a container of dirty objects that i want to flush to my database when the objects get evicted/expired. during normal processing, i use a key to lookup in ehcache. if key is not present, i read data from database and put it in ehcache. the value is actually a complex object that i modify. when the ttl/idle time/overflow condition happens, i see that CacheEventListener callbacks get invoked. but there is a big problem. the notifyElementExpired is called after the key-value are removed from cache. so there is a race condition. if i do the task of flushing the dirty value to cache in notifyElementExpired and at the same time, in another thread, read for the same key happens, then there is a synchronization issue. the 2nd thread will not find the object in ehcache, and hence will go to database while the other thread is still getting ready to flush.

i tried experimenting with write-through ehcache and i dont think thats also working.

is there a solution here??

i would really appreciate good solutions to this problem even if it involves some other caching mechanism other than ehcache.

thanks

Answer 1

If you're ok with a purely in-memory cache, I'd suggest extending the Google Guava library's CacheLoader, ex:

public class DBLoader extends CacheLoader<String, String> {

    @Override
    public String load(String key) throws Exception {
        // load a value from the database
        return value;
    }
}

Then in usage, something like:

private LoadingCache<String, String> dbCache = CacheBuilder.newBuilder()
.expireAfterWrite(CACHE_EXPIRE_IN_SECONDS, TimeUnit.SECONDS)
.build(new DBLoader());

String value = dbCache.get(someKey);

You'll need to tidy that up with proper exception handling of course.

I find guava to be much more straightforward than getting ehcache configured properly.