Problem: some users can create a document, and choose date and time in the future, for the execution of some code on that document. The users can also cancel that scheduled event. I'm looking for the best implementation in GAE. I'm thinking to two possible implementations:

-A push Task for any document, setting the eta parameter, I can save the result of Taskqueue.add(..) in the document, in case the user want to delete the task before its execution. I don't know if there is a max number of tasks that I can launch, and the max number of days for the eta(30) can be a problem.

-I can save in the DataStore datetime and key of the document and use cron to check every 10 mins if there's a any entry to process. I fear that this approach may be expensive. Am i right?

Because your users can schedule an event as well as cancel - I would recommend saving the scheduled request to the datastore and have a cron job check if there is anything scheduled at that time.

To reduce the number of times the job runs, you can change the precision of the date/time a user can select for the job (such as 15 minute increments, or hourly). As long as you construct the scheduled request entity properly - it shouldn't be an expensive hit to the datastore. To reduce datastore hits, you can fetch a range of upcoming scheduled events in one query and put them in memcache - as long as you manage that cache when a user modifies one.