Question

In development I have an empty datastore.
Online I have a datastore with millions of entities.

In the development datastore (locally) I put one new entity (generating a new key).
Then I export the entity and put it in the online datastore (with the key generated locally). What is the risk that the key generated locally to have already been assigned to an entity in the online datastore?

Or would it be simpler to avoid collision by creating the keys locally like this:

for (int i = 0; i < data.size(); i++) {
    Key k = KeyFactory.createKey(kind, new Date() + i);
    // continue to creating and inserting entities...
}

Thanks.

Answer 1

From https://developers.google.com/appengine/docs/java/datastore/entities:

Instead of using key name strings or generating numeric IDs
automatically, advanced applications may sometimes wish to assign
their own numeric IDs manually to the entities they create. Be aware,
however, that there is nothing to prevent the Datastore from assigning
one of your manual numeric IDs to another entity. The only way to
avoid such conflicts is to have your application obtain a block of IDs
with the methods DatastoreService.allocateIds() or
AsyncDatastoreService.allocateIds(). The Datastore’s automatic ID
generator will keep track of IDs that have been allocated with these
methods and will avoid reusing them for another entity, so you can
safely use such IDs without conflict.

You must either generate all of the numeric keys manually (and in such a way that they will never collide) or use allocateIds(). There is specifically no guarantee that anything you generate manually will not collide with an existing key, unless you use that function. The generated key ids are not like an auto increment field in a relational db which increment by one each time.