Google AppEngine

Just my 2 cents: I was "forced" to use an "unowned relationship" for my application because I had very large objects: My parent object "Journey" contained thousands of "JourneyPoints" in a List element. A call to "makePersistent" constantly hit the 30s deadline. By modeling it as an unowned relationship and distributing the parent key by hand to the JourneyPoints everything was fine again and the performance was very good.

class DeleteFull():     def execute(self):         deleting = model_class_name.all().order('__key__').fetch(100)         while deleting:             a = []             key = deleting[-1].key()             for item in deleting:                 a.append(item)             db.delete(a)             deleting = model_class_name.all().filter('__key__ >', key).order('__key__').fetch(100) This purged everything, but it took a hell of a long time.

GaeVFS is an Apache Commons VFS plug-in that implements a distributed, writeable virtual file system for Google App Engine (GAE) for Java. GaeVFS is implemented using the GAE datastore and memcache APIs. The primary goal of GaeVFS is to provide a portability layer that allows you to write application code to access the file system--both reads and writes--that runs unmodified in either GAE or non-GAE servlet environments.

GaeVFS ships with a servlet that handles file uploads and abstracts how it writes them to its virtual file system. 

GaeVFS is built on top of Apache Commons VFS, and you use their FileObject instead of the standard File class.  There are some examples on the GaeVFS wiki, but I had to play around for a bit before I figured out some of the differences I needed to know. 

So the summary is: do not store nulls into Lists, otherwise you will get into a problem in future.

A join model is a data model that models the relationship between two other models. For example, you might have Person entities and Group entities, and you want Persons to be in Groups and Groups to "have" Persons. The relationship between Persons and Groups is one of Memberships. A Person is "in" a Group "through" a Membership. They may belong to multiple Groups, i.e. have multiple Memberships.

class Membership(db.Model): person = db.ReferenceProperty(Person) group = db.ReferenceProperty(Group)

If certain properties of your joined entities (Person and Group in this example) don't change much, but get queried often via the join model, you may find it worth caching those properties on the join model itself.

You can study the JavaMail API in more detail to understand how to do things like sending attachments in your email too. You can do that using the Google App Engine Email Service.

Google app engine uses the concept of static files. This is a performance optimization. Using the file appengine-web.xml you can configure the way google handles static files. You can include and exclude certain files using their extension or name. More information can be found here at google. This all works nice in the online version, however there seems to be a problem with the development server. Some solutions try to configure the local version as well, still that did not work for me. I decided to look for a servlet that serves static files.

That is it, now you can test your stuff locally and all your scripts, images, styles are loaded by your application. Of course you have to remove this servlet before uploading your application. Hope it helps people with their local debugging of jquery scripts or other javascript things.