Design an online tic-tac-toe service over REST, with matchmaking

Two players are matched online and play tic-tac-toe from their own devices. Clients may only talk to the server through REST APIs, and the service runs in several regions. Design matchmaking, the game APIs, how each player learns about the other's move without push, game state and its storage, turn validation, disconnects, and how a game works when the two players are near different regions.