问题
Assuming the IDs have not been used by calling put() for an Entity. How long would the allocated IDs stick around for? Are they ever put back into use by datastore? Or are they allocated forever?
回答1:
The documentation says,
These keys are guaranteed not to have been returned previously by the data store's internal ID generator, nor will they be returned by future calls to the internal ID generator.
I'll go out on a limb and say that the use of 'guarantee' and 'future' above means forever.
来源：https://stackoverflow.com/questions/40768129/when-using-allocateids-how-long-do-unused-ids-remain-allocated