cache2k, read through and blocking

Question

I have used cache2k with read through in a web application to load blog posts on demand. However, I am concerned about blocking for the read through feature. For example, if multiple threads (requests) ask the cache for the same key, is it possible for the read through method to be called multiple times to load the same key/value into the cache?

I get the impression from the documentation that the read through feature does block concurrent requests for the same key until the load has completed, but may I have mis-read the documentation. I just want to check that this is the behaviour.

The method which initializes the cache looks like this:

private void initializeURItoPostCache()
{
    final CacheLoader<String, PostImpl> postFileLoader = new CacheLoader<String, PostImpl>(){
        @Override public PostImpl load(String uri)
        {
            // Fetch the data and create the post object
            final PostImpl post = new PostImpl();
            //.. code omitted
            return post;
        }
    };

    // Initialize the cache with a read-through loader
    this.cacheUriToPost = new Cache2kBuilder<String, PostImpl>(){}
    .name("cacheBlogPosts")
    .eternal(true)
    .loader(postFileLoader)
    .build();
}

The following method is used to request a post from the cache:

public Post getPostByURI(final String uri)
{
    // Check with the index service to ensure the URI is known (valid to the application)
    if(this.indexService.isValidPostURI(uri))
    {
        // We have a post associated with the given URI, so
        // request it from the cache
        return this.cacheUriToPost.get(uri);
    }
    return EMPTY_POST;
}

Many thanks in advance, and a happy and prosperous New Year to all.

Answer 1

When multiple requests to the same key will provoke a cache loader call, cache2k will only invoke the loader once. Other threads wait until the load is finished. This behavior is called blocking read through. To cite from the Java Doc:

Blocking: If the loader is invoked by Cache.get(K) or other methods that allow transparent access concurrent requests on the same key will block until the loading is completed. For expired values blocking can be avoided by enabling Cache2kBuilder.refreshAhead(boolean). There is no guarantee that the loader is invoked only for one key at a time. For example, after Cache.clear() is called load operations for one key may overlap.

This behavior is very important for caches, since it protects against the Cache stampede. An example: A high traffic website receives 1000 requests per second. One resource takes quite long to generate, about 100 milliseconds. When the cache is not blocking out the multiple requests when there is a cache miss, there would be at least 100 requests hitting the loader for the same key. "at least" is an understatement, since your machine will probably not handle 100 requests at the same speed then one.

Keep in mind that there is no hard guarantee by the cache. The loader must still be able to perform correctly when called for the same key at the same time. For example blocking read through and Cache.clear() lead to competing requirements. The Cache.clear() should be fast, which means we don't want to wait for ongoing load operations to finish.