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javacachingguava
Richard
Richard

Reputation: 10432

Pre-load values for a Guava Cache

I have a requirement where we are loading static data from a database for use in a Java application. Any caching mechanism should have the following functionality:

Lazy loading of all the data isn't an option as the application will be deployed to multiple geographical locations and will have to communicate with a single database. Lazy loading the data will make the first request for a specific element too slow where the application is in a different region to the database.

I have been using the MapMaker API in Guava with success but we are now upgrading to the latest release and I can't seem to find the same functionality in the CacheBuilder API; I can't seem to find a clean way of loading all data at start-up.

One way would be to load all keys from the database and load those through the Cache individually. This would work but would result in N+1 calls to the database, which isn't quite the efficient solution I'm looking for.

public void loadData(){
    List<String> keys = getAllKeys();
    for(String s : keys)
        cache.get(s);
}

Or the other solution is to use a ConcurrentHashMap implementation and handle all of the threads and missing entries myself? I'm not keen on doing this as the MapMaker and CacheBuilder APIs provide the key-based thread locking for free without having to provide extra testing. I'm also pretty sure the MapMaker/CacheBuilder implementations will have some efficiencies that I don't know about/haven't got time to investigate.

public Element get(String key){
    Lock lock = getObjectLock(key);
    lock.lock();
    try{
        Element ret = map.get(key)
        if(ret == null){
            ret = getElement(key); // database call
            map.put(key, e);
        }
        return ret;
    }finally {
        lock.unlock();
    } 
}

Can anyone think of a better solution to my two requirements?

Feature Request

I don't think pre-loading a cache is an uncommon requirement, so it would be nice if the CacheBuilder provided a configuration option to pre-load the cache. I think providing an Interface (much like CacheLoader) which will populate the cache at start-up would be an ideal solution, such as:

CacheBuilder.newBuilder().populate(new CachePopulator<String, Element>(){

    @Override
    public Map<String, Element> populate() throws Exception {
        return getAllElements();
    }

}).build(new CacheLoader<String, Element>(){

    @Override
    public Element load(String key) throws Exception {       
        return getElement(key);
    }

});

This implementation would allow the Cache to be pre-populated with all relevant Element objects, whilst keeping the underlying CustomConcurrentHashMap non-visible to the outside world.

Upvotes: 24

Views: 17217

Answers (2)

Etienne Neveu
Etienne Neveu

Reputation: 12692

I'd load all static data from the DB, and store it in the Cache using cache.asMap().put(key, value) ([Guava 10.0.1 allows write operations on the Cache.asMap() view][1]).

Of course, this static data might get evicted, if your cache is configured to evict entries...

The CachePopulator idea is interesting.

Upvotes: 5

fry
fry

Reputation: 591

In the short-term I would just use Cache.asMap().putAll(Map<K, V>).

Once Guava 11.0 is released you can use Cache.getAll(Iterable<K>), which will issue a single bulk request for all absent elements.

Upvotes: 8

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