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 * Implements background tasks for * <a href="http://code.google.com/appengine/docs/java/overview.html">Google App * Engine for Java</a>, based on the * <a href="http://code.google.com/appengine/articles/deferred.html">Python 'deferred' * library</a>; simplifies use of the <a href="http://code.google.com/appengine/docs/java/taskqueue/overview.html"> * Task Queue Java API</a> by automatically handling the serialization and * deserializtion of complex task arguments.

You create a java project only with the shared files (NOT a GAE project). You DON'T copy any libraries into the lib folder. You should add the required third-party components into your main project. Then you rightclick on the shared project's *source* folder, go to "Build Path -> Remove from build path" item in the context menu. You may, by the way, rename the source folder, so that it has a name different from the default source folder name. Next you rightclick on the main project's folder, go to "Build Path -> Link source".

 If you're following the how-to article on unit testing ( http://code.google.com/appengine/docs/java/howto/unittesting.html) you'll need to update your TestEnvironment class to look like this one: http://code.google.com/p/datanucleus-appengine/source/browse/branches... (lines 34 - 66) We're working on getting the docs updated right now.

One thing you get used to on appengine is that any bulk data work requires the task queue.  You can use a little bit of framework and make all of these transforms (including deleting data) a question of just writing a simple task class and firing it off.  You'll want a copy of the Deferred servlet: http://code.google.com/p/gaevfs/source/browse/trunk/src/com/newatlant... Fair warning:  I found that I needed to change the code to make it perform base64 encoding all the time, not just on the dev instance.

One thing you get used to on appengine is that any bulk data work requires the task queue.  You can use a little bit of framework and make all of these transforms (including deleting data) a question of just writing a simple task class and firing it off.  You'll want a copy of the Deferred servlet: http://code.google.com/p/gaevfs/source/browse/trunk/src/com/newatlant... Fair warning:  I found that I needed to change the code to make it perform base64 encoding all the time, not just on the dev instance.

Cron jobs / task queues Instead of using a framework like Quartz to schedule jobs, Google App Engine takes care of executing jobs for you. You simply enter a cron-like expression and a URL to call and your job is configured. You also have a task queue at your disposal. Your application code can add tasks to a task queue which will be executed later in the future, asynchronously. An example use case is that you don’t want clients to wait for an email to be sent before he sees the next page. Instead you can put the email task on the task queue and the email will be sent asynchronously.

When an email is received Google App Engine does a post on an URL in your application you configured. The HTTP body of the POST request contains the exact mime message as it was received by Google. To parse this mime message you can use the MimeMessage class provided by the JDK.

 Twig is an alternative to the standard   persistence interfaces JDO and JPA, specifically designed to make the   most of the underlying datastore's unique features.

Twig is the only interface to support direct unowned relationships so   your code is not dependent on low-level datastore classes.  It is the   only interface to support OR queries by merging together multiple   queries, sorting them and filtering out duplicates at the lowest level   for performance.

Async datastore calls are not yet part of the low-level API.  Twig   uses the underlying Protocol Buffer layer classes to call   ApiProxy.makeAsyncCall() instead of makeSyncCall.  All the code is   open source so you can check out how its done.

we can build the start of an open source App Engine server clone that's suitable for many purposes:  initially just for regression testing & local development (like the "dev_appserver" that comes with the App Engine Python SDK), but perhaps in the future (once Hypertable/Hbase/etc are ready) a full stack to give to ISPs to let them run App Engine apps on their own.

The Blobstore API provides two types of functions:   An ability to upload and save the blob automaticallyThe BlobstoreService which is provided by the com.google.appengine.api.blobstore.BlobstoreService allows us to specify a URL where users can upload their large files. You can think of this url as the action element in the HTML form. The implementation at this URL is internal to the BlobstoreService. But what it does is significant. It will extract out the file contents that you uploaded and store it as a Blob in the database. Each blob that is stored in the database is associated with the a Blob Key. This Blob key is then provided to your url and you can then use the Blob Key to do anything within your application. In our case, we form a url that is tweeted to the users who can then view the picture that we uploaded. An ability to serve or retrieve the blob.The BlobstoreService also provides an ability to serve or retrieve the blob that was saved successfully. It will provide the blob as a response that could then use as a source in an <img> element for example. All you need to do is provide it the Blob Key and the response stream and in return, it will provide the content properly encoded as per its type that you could then use.

The mapper API isn't just limited to mapping over datastore entities, either. You can map over lines in a text file in the blobstore, or over the contents of a zip file in the blobstore. It's even possible to write your own data sources