Turn based server for iOS/Android App using Google App Engine

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http://stackoverflow.com – I am working on a multiplayer turn based word game for iOS/Android and need to make a server. I have researched and found google app engine. I believe I will use GAE with python but I am open to other suggestions or possible problems. My plan is to send HTTP requests to the server from the client and have the server respond with JSON. However, I am not sure how to have the server contact a client when the other player had finished their move. Do I check the server every x seconds or is there a better way to do this? Is there any other information I should know about GAE or turn based servers (HowTos)