Using String or Long for Google App Engine Datastore Key

Question

What is the difference of GAE datastore Long id and String key in Key.

Using the Key

KeyFactory.createKey(kind, key);

KeyFactory.createKey(kind, id);

So in this case either can be used as Identifier. If I create a Key from KeyFactory.createKey(kind, key) where key is a String I can see the id field of the key is 0. In this case getId() will be 0? And in contrast, if what is set is KeyFactory.createKey(kind, someLongValue) then getName() will be a empty String?

Answer 1

An app engine Key can have EITHER a unique long ID, or a unique String name - they are mutually exclusive. If you set a long ID, the name will be null. Which you decide to use for any Entity kind really depends on your use case.

A benefit of using long IDs is that you can have app engine auto-generate them for you. That is, you can create your Entity without any value for its ID, and then when you save it it will be given a valid unique long ID by the datastore.

If you use a String name, you must create the unique name before the Entity is saved. This is useful in some cases if you have some property on the Entity that makes for a natural fit for the unique name (for example, and SKU for a part). Also, some people generate a random UUID for the String name - this ensures that all names are globally unique, not just unique among a particular Entity kind and ancestor path.