Question

I am currently implementing cache. I have completed basic implementation, like below. What I want to do is to run a thread that will remove entry that satisfy certain conditions.

class Cache {
    int timeLimit = 10; //how long each entry needs to be kept after accessed(marked)
    int maxEntries = 10; //maximum number of Entries
    HashSet<String> set = new HashSet<String>();   
    public void add(Entry t){
        ....
    }

    public Entry access(String key){
        //mark Entry that it has been used
        //Since it has been marked, background thread should remove this entry after timeLimit seconds.
        return set.get(key);
    }
    ....
}

My question is, how should I implement background thread so that the thread will go around the entries in set and remove the ones that has been marked && (last access time - now)>timeLimit ?

edit

Above is just simplified version of codes, that I did not write synchronized statements.

Answer 1

Why are you reinventing the wheel? EhCache (and any decent cache implementation) will do this for you. Also much more lightweight MapMaker Cache from Guava can automatically remove old entries.

If you really want to implement this yourself, it is not really that simple.

1. Remember about synchronization. You should use ConcurrentHashMap or synchronized keyword to store entries. This might be really tricky.

2. You must store last access time somehow of each entry somehow. Every time you access an entry, you must update that timestamp.

3. Think about eviction policy. If there are more than maxEntries in your cache, which ones to remove first?

4. Do you really need a background thread?

   This is surprising, but EhCache (enterprise ready and proven) does not use background thread to invalidate old entries). Instead it waits until the map is full and removes entries lazily. This looks like a good trade-off as threads are expensive.

5. If you have a background thread, should there be one per cache or one global? Do you start a new thread while creating a new cache or have a global list of all caches? This is harder than you think…

Once you answer all these questions, the implementation is fairly simple: go through all the entries every second or so and if the condition you’ve already written is met, remove the entry.