App Engine tasks which rewrite data for a single entity type

RewriteDataTask.java

package blah;

import com.google.appengine.api.taskqueue.DeferredTask;
import com.google.common.base.Function;
import com.google.common.collect.Iterables;
import com.googlecode.objectify.Key;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import st.voo.tick.Queues;

import static st.voo.tick.OfyService.ofy;

/**
 * Task which kicks off rewriting a large chunk of data
 */
@Slf4j
@RequiredArgsConstructor
public class RewriteDataTask<T> extends DeferredTask {
	private static final long serialVersionUID = 1L;

	final Class<T> type;

	/** */
	@Override
	public void run() {
		log.warn("Starting rewrite data");

		log.info("############# SPAWNING rewrite for " + type);
		Iterable<Key<T>> keys = ofy().load().type(type).keys().iterable();
		Queues.deflt().add(Iterables.transform(keys, new Function<Key<T>, RewriteSingleTask<T>>() {
			@Override
			public RewriteSingleTask<T> apply(Key<T> key) {
				return makeTask(key);
			}
		}));

		log.warn("Finished spawning rewrite data");
	}

	private RewriteSingleTask<T> makeTask(Key<T> key) {
		return new RewriteSingleTask<T>(key) {
			private static final long serialVersionUID = 1L;

			@Override
			protected void process(T thing) {
				RewriteDataTask.this.process(thing);
			}
		};
	}

	protected void process(T thing) {
		// Do nothing by default
	}
}

RewriteSingleTask.java

package blah;

import com.google.appengine.api.taskqueue.DeferredTask;
import com.googlecode.objectify.Key;
import com.googlecode.objectify.VoidWork;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;

import static st.voo.tick.OfyService.ofy;

/**
 * Task which rewrites one single piece of data
 */
@Slf4j
@RequiredArgsConstructor
public class RewriteSingleTask<T> extends DeferredTask {
	private static final long serialVersionUID = 1L;

	final Key<T> key;

	/** */
	@Override
	public void run() {
		ofy().transact(new VoidWork() {
			@Override
			public void vrun() {
				T thing = ofy().load().key(key).safe();
				log.info("Processing: " + thing);
				process(thing);
				ofy().save().entity(thing);
			}
		});
	}

	protected void process(T thing) {
		// Do nothing by default
	}
}

The transaction is necessary, otherwise you risk losing changes when a resave conflicts with normal business activity.