Java – Google Guava’s CacheLoader loadAll() vs reload() semantics

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Two things I really like about Guava 11's CacheLoader (thanks, Google!) are loadAll(), which allows me to load multiple keys at once, and reload(), which allows me to reload a key asynchronously when it's "stale" but an old value exists. I'm curious as to how they play together, since reload() operates on but a single key.

Concretely, extending the example from CachesExplained:

LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
   .maximumSize(1000)
   .refreshAfterWrite(1, TimeUnit.MINUTES)
   .build(
       new CacheLoader<Key, Graph>() {
         public Graph load(Key key) { // no checked exception
           return getGraphFromDatabase(key);
         }

         public Map<Key, Graph> loadAll(Iterable<? extends K> keys) {
           return getAllGraphsFromDatabase(keys);
         }

         public ListenableFuture<Graph> reload(final Key key, Graph prevGraph) {
           if (neverNeedsRefresh(key)) {
             return Futures.immediateFuture(prevGraph);
           } else {
             // asynchronous!
             return ListenableFutureTask.create(new Callable<Graph>() {
               public Graph call() {
                 return getGraphFromDatabase(key);
               }
             });
           }
         }
       });

…where "getAllGraphsFromDatabase()" does an aggregate database query rather than length(keys) individual queries.

How do these two components of a LoadingCache play together? If some keys in my request to getAll() aren't present in the cache, they are loaded as a group with loadAll(), but if some need refreshing, do they get reloaded individually with load()? If so, are there plans to support a reloadAll()?

Best Answer

Here's how refreshing works.

Refreshing on a cache entry can be triggered in two ways:

Explicitly, with cache.refresh(key).
Implicitly, if the cache is configured with refreshAfterWrite and the entry is queried after the specified amount of time after it was written.

If an entry that is eligible for reload is queried, then the old value is returned, and a (possibly asynchronous) refresh is triggered. The cache will continue to return the old value for the key while the refresh is in progress. (So if some keys in a getAll request are eligible for refresh, their old values will be returned, but the values for those keys will be (possibly asynchronously) reloaded.)

The default implementation of CacheLoader.reload(key, oldValue) just returns Futures.immediateFuture(load(key)), which (synchronously) recomputes the value. More sophisticated, asynchronous implementations are recommended if you expect to be doing cache refreshes.

I don't think we're inclined to provide reloadAll at the moment. I suspect it's possible, but things are complicated enough as it is, and I think we're inclined to wait until we see specific demand for such a thing.

Related Solutions

Javascript – How to force the browser to reload cached CSS and JavaScript files

This solution is written in PHP, but it should be easily adapted to other languages.

The original .htaccess regex can cause problems with files like json-1.3.js. The solution is to only rewrite if there are exactly 10 digits at the end. (Because 10 digits covers all timestamps from 9/9/2001 to 11/20/2286.)

First, we use the following rewrite rule in .htaccess:

RewriteEngine on
RewriteRule ^(.*)\.[\d]{10}\.(css|js)$ $1.$2 [L]

Now, we write the following PHP function:

/**
 *  Given a file, i.e. /css/base.css, replaces it with a string containing the
 *  file's mtime, i.e. /css/base.1221534296.css.
 *
 *  @param $file  The file to be loaded.  Must be an absolute path (i.e.
 *                starting with slash).
 */
function auto_version($file)
{
  if(strpos($file, '/') !== 0 || !file_exists($_SERVER['DOCUMENT_ROOT'] . $file))
    return $file;

  $mtime = filemtime($_SERVER['DOCUMENT_ROOT'] . $file);
  return preg_replace('{\\.([^./]+)$}', ".$mtime.\$1", $file);
}

Now, wherever you include your CSS, change it from this:

<link rel="stylesheet" href="/css/base.css" type="text/css" />

To this:

<link rel="stylesheet" href="<?php echo auto_version('/css/base.css'); ?>" type="text/css" />

This way, you never have to modify the link tag again, and the user will always see the latest CSS. The browser will be able to cache the CSS file, but when you make any changes to your CSS the browser will see this as a new URL, so it won't use the cached copy.

This can also work with images, favicons, and JavaScript. Basically anything that is not dynamically generated.

Can someone help me understand Guava CacheLoader

Guava's Cache type is generally intended to be used as a computing cache. You don't usually add values to it manually. Rather, you tell it how to load the expensive to calculate value for a key by giving it a CacheLoader that contains the necessary code.

A typical example is loading a value from a database or doing an expensive calculation.

private final FooDatabase fooDatabase = ...;

private final LoadingCache<Long, Foo> cache = CacheBuilder.newBuilder()
    .maximumSize(10)
    .build(new CacheLoader<Long, Foo>() {
      public Foo load(Long id) {
        return fooDatabase.getFoo(id);
      }
    });

public Foo getFoo(long id) {
  // never need to manually put a Foo in... will be loaded from DB if needed
  return cache.getUnchecked(id);
}

Also, I tried the example you gave and mycache.get("key123") returned "value123" as expected.

Best Answer

Related Solutions

Javascript – How to force the browser to reload cached CSS and JavaScript files

Can someone help me understand Guava CacheLoader

Related Topic