Question

I’m looking to use Google’s App Engine (Java) to provide the backend to an Android messaging app I’m currently writing, I’m just starting out with GAP but have a little experience with Java (through Android).

The first time someone uses the app it will send some sign-up data to the server, this will stored in the GAE datastore, and a unique id returned to the phone (or an error message if something broken).

As I can’t see something that looks like key = datastore.giveMeAUniqueKey or datastore.hasThisBeenUsedBefore(key) I guess I’m going to have to generate a random key and see if it’s been taken (I’m not that sure how to do that to be honest).

Any ideas (either answers to the specific question, or pointer to useful “getting started” resources)?

Thanks.

Answer 1

If this value is not security sensitive (ie, it’s just a user ID and you have some other method to authenticate the phone), just do an insert and take the key of the newly inserted entity. The datastore will assign a guarenteed-unique key automatically if you insert a new entity without providing one. Alternately, you can explicitly request an ID with the allocate_ids call.

If the value is security sensitive (it’s a session nonce or something used for authentication), use the SecureRandom class to generate a sequence of random bytes. Do not use this as a key for an entity such as a user object; this would preclude changing the session ID if the user’s session is compromised. Have a separate user ID used for that purpose, and use this secure nonce only for the authentication step.

Note that simply looping creating IDs, testing for conflicts, and inserting is not safe without using a transaction; it’s easier (and faster, and cheaper…) just to use app engine’s built in ID assignment system.