GoogleCloudPlatform/appengine-java-vm-guestbook-extras — pull request #3 by toldervoll (2014-11-16 09:17:56+00:00)
Title: Add datastore-indexes.xml

Without this file the app will not run on appspot.com and the final
step in stage 1 of the tutorial will fail. SignGuestbookServlet
refers to the data store kind "Greeting", and this needs to be
declared as explained in
https://cloud.google.com/appengine/docs/java/config/indexconfig.