An Entity is an individual record that gets stored and retrieved from the datastore. The Kind is the unique string identifier of the type of entity. For example, "Joe" is an Entity with age=42, dob=10-12-2000, and Kind "Person".

There's a 30 second deadline for web requests, and a 10 second deadline for URLFetch requests. You'll want to make sure you aren't exceeding either of these.

Cold-start latency is determined by the time it takes to load all the classes needed to handle the request. If you upload a JAR file, but nothing references it, it won't be loaded, and thus won't affect your cold-start latency.

Only those jars under WEB-INF/lib will be uploaded to GAE. You can prevent GWT jars from being uploaded by not placing them under WEB-INF/lib, rather by externally linking to them in your project build path.

You must have two things in consideration: the appid and version must match, and you should enable datastore writes.

Well java.lang.System is whitelisted, so won't java.lang.System.out.println(...) work?

Google App Engine applications written in Java can write information to the log files using java.util.logging.Logger. Log data for an application can be viewed and analyzed using the Administration Console, or downloaded using appcfg.sh request_logs. More info in the Logging documentation.

I propose two possiblities: Duplicate Line_Item. Line_Item appointment_key name price finished ... A Line_Item should have a finished state, when the item was finished or not by the employee. If an employee hadn't finished all line items, mark them as unfinished, create a new appointment and copy all items that were unfinished. You can index on the appointment_key field on all Line_Items, which is a Good Thing. However, the duplicated data may be a problem. Dynamic fields for Line_Item: Line_Item duplicate_key appointment_key name price finished ... Create a new field, duplicate_key, for Line_Item which points to another Line_Item or to null (reserve this key!). Null means that the Line_Item is original, any other value means that this Line_Item is a duplicate of the Line_Item the field points to. All fields of Line_Item marked as a duplicate inherit the fields of the original Line_Item, except the appointment_key: so it will take less storage. Also this solution should have appointment_key indexed, to speed up lookup times. This requires one additional query per duplicated Line_Item, which may be a problem. Now, it's a clear choice: either better speed or better storage. I would go for the first, as it reduces complexity of your model, and storage is never a problem with modern systems. Less complexity generally means less bugs and less development/testing costs, which justifies the cost of the storage requirement.

That's a restriction of App Engine's mail API: The sender address can be either the email address of a registered administrator for the application, or the email address of the current signed-in user (the user making the request that is sending the message). If you've got Google Apps running on that domain, you should have (or be able to create) an @thatdomain.com email addresses that you can register as an administrator of the App Engine app in question, which will then let you send email "from" that address.

I have an Entity with an id type of Long, and the id gets filled in after I call makePersistent(). Here is what the code looks like:     GameEntity game = new GameEntity();    log.warning("before makePersistent id is " + game.getId());    pm.makePersistent(game);    log.warning("after makePersistent id is " + game.getId()); Here is a snippet of the GameEntity class: @PersistenceCapable(identityType = IdentityType.APPLICATION)public class GameEntity {    @PrimaryKey    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)    private Long id; And the output shows what you'd expect: WARNING 6428 - before makePersistent id is nullWARNING 6444 - after makePersistent id is 6 UPDATE: It occurred to me belatedly that you might want an actual Key object. You can create that yourself if you have the id: public Key getKey() {    return KeyFactory.createKey(GameEntity.class.getSimpleName(), id);}