CODEWITHSUNDEEP
google-app-enginegoogle-cloud-endpointsgoogle-cloud-datastoreobjectify
Micro
Micro

Reputation: 10891

Allocate a random Id to use in Entity constructor Google App Engine

When you insert an Entity into datastore with a @id Long id; property, the datastore automatically creates a random (or what seems like a random) Long value as the id that looks like: 5490350115034675.

I would like to set the Long id myself but have it be randomly generated from datastore.

I found this piece of code that seems to do just that:

 Key<MyEntity> entityKey = factory().allocateId(MyEntity.class);
 Long commentId = entityKey.getId();

Then I can pass in the commentId into the constructor of MyEntity and subsequently save it to the datastore.

When I do that however, I do not seem to get a randomly generated id, it seems to follow some weird pattern where the first allocated id is 1 and the next one is 10002, then 20001 and so on.

Not sure what all that means and if it is safe to continue using... Is this the only way to do this?

Upvotes: 0

Views: 554

Answers (2)

stickfigure
stickfigure

Reputation: 13556

When you use the autogenerated ids (ie Long), GAE uses the 'scattered' id generator which gives you ids from a broad range of the keyspace. This is because high volume writing (thousands per second) of more-or-less contiguous values in an index results in a lot of table splitting, hurting performance.

When you use allocateId(), you get an id from the older allocator that was used before scattered ids. They aren't necessarily contiguous or monotonic but they tend to start small and grow.

You can mix and match; allocations will never conflict.

I presume, however, that you want random-looking ids because you want them to be hard to guess. Despite their appearance at first glance, the scattered id allocator does not produce unguessable ids. If you want sparse ids that will prevent someone from scanning your keyspace, you need to explicitly add a random element. Or just use UUID.randomUUID() in the first place.

Upvotes: 2

Andrei Volgin
Andrei Volgin

Reputation: 41089

App Engine allocates IDs using its own internal algorithm designed to improve datastore performance. I would trust App Engine team to do their magic.

Introducing your own scheme for allocating IDs is not as simple - you have to account for eventual consistency, etc. And it's unlikely that you will gain anything, performance-wise, from all this effort.

Upvotes: 1

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