Tuesday, September 14, 2010

Tapestry5: Caching Method Results

Assume you have methods that (almost) always return the same result for the same input arguments. If preparing method result is a heavy operation and/or it consumes time, it is reasonable to cache these results.

One way of building method cache in Tapestry5 is by implementing MethodAdvice interface like this:

public class CacheMethodResultAdvice implements MethodAdvice {

private static final Logger logger = LoggerFactory.getLogger(CacheMethodResultAdvice.class);

private final Cache cache;
private final Class<?> advisedClass;
private final Object nullObject = new Object();

public CacheMethodResultAdvice(Class<?> advisedClass, Cache cache) {
this.advisedClass = advisedClass;
this.cache = cache;
}

@Override
public void advise(Invocation invocation) {
String invocationSignature = getInvocationSignature(invocation);

String entityCacheKey = String.valueOf(invocationSignature.hashCode());

Object result;

if (cache.containsKey(entityCacheKey))
{
result = cache.get(entityCacheKey);

logger.debug("Using invocation result ({}) from cache '{}'", invocationSignature, result);

invocation.overrideResult(result);
}
else
{
invocation.proceed();

if (!invocation.isFail())
{
result = invocation.getResult();

cache.put(entityCacheKey, result);
}
}
}

private String getInvocationSignature(Invocation invocation) {
StringBuilder builder = new StringBuilder(150);
builder.append(advisedClass.getName());
builder.append('.');
builder.append(invocation.getMethodName());
builder.append('(');
for (int i = 0; i < invocation.getParameterCount(); i++) {
if (i > 0) {
builder.append(',');
}
Class<?> type = invocation.getParameterType(i);
builder.append(type.getName());
builder.append(' ');

Object param = invocation.getParameter(i);
builder.append(param != null ? param : nullObject);
}
builder.append(')');

return builder.toString();
}

}


Implementation of getInvocationSignature(...) is not ideal, but you may improve it to match your requirements. One issue I see here is building invocation signature for null-value parameters in a clustered environment (which is GAE). In this implementation method nullObject.toString() will return something like java.lang.Object@33aa9b. And this value will vary in different instances of your application. You may replace nullObject with just "null" string. Just keep in mind that "null" != null.

To make this advice working you should declare it in your AppModule.java:

    @SuppressWarnings("unchecked")
@Match("IPResolver")
public static void adviseCacheIPResolverMethods(final MethodAdviceReceiver receiver, Logger logger, PerthreadManager perthreadManager) {
try {
Map props = new HashMap();

// IP address of URL may change, keep it in cache for one day
props.put(GCacheFactory.EXPIRATION_DELTA, 60 * 60 * 24);

CacheFactory cacheFactory = CacheManager.getInstance().getCacheFactory();
Cache cache = cacheFactory.createCache(props);

LocalMemorySoftCache cache2 = new LocalMemorySoftCache(cache);

// We don't want local memory cache live longer than memcache
// Since we don't have any mechanism to set local cache expiration
// we will just reset this cache after each request
perthreadManager.addThreadCleanupListener(cache2);

receiver.adviseAllMethods(new CacheMethodResultAdvice(IPResolver.class, cache2));
} catch (CacheException e) {
logger.error("Error instantiating cache", e);
}
}

@Match("LocationResolver")
public static void adviseCacheLocationResolverMethods(final MethodAdviceReceiver receiver, Cache cache) {
// Assume that location of IP address will never change,
// so we don't have to set any custom cache expiration parameters
receiver.adviseAllMethods(new CacheMethodResultAdvice(LocationResolver.class, cache));
}


These declarations tell Tapestry5 to add our advice to all methods of services that implement IPResolver and LocationResolver interfaces.

Note that we able to use caches with different settings for different methods/services like in example above (see comments in code).

See also: