I found this: https://developers.google.com/appengine/docs/python/datastore/overview#Cross_Group_Transactions "For example, to increment a counter field in an object, you need to read the value of the counter, calculate the new value, and then store it back. Without a transaction, it is possible for another process to increment the counter between the time you read the value and the time you update it, causing your application to overwrite the updated value. Doing the read, calculation, and write in a single transaction ensures that no other process can interfere with the increment."
On Friday, November 30, 2012 12:19:29 PM UTC-6, Joe Barnhart wrote: > > Think hard before locking a table while a user is dawdling over his > reservation. > > The feature you are dealing with is database "isolation > level<http://en.wikipedia.org/wiki/Isolation_%28database_systems%29>" > and is the subject that fills volumes of database theory. From what you > describe I think you want the "repeatable read" level. I do not know > enough about GAE to tell you which levels it supports. But here is an > article on Google BigTable > Isolation<https://developers.google.com/appengine/articles/transaction_isolation>which > may help. > > -- Joe B. > > On Thursday, November 29, 2012 2:18:47 AM UTC-8, José Manuel López Muñoz > wrote: >> >> I'm making a room reservation system in Web2Py over Google App Engine. >> When a user is booking a Room the system must be sure that this room is >> really available and no one else have reserved it just a moment before. To >> be sure I make a query to see if the room is available, then I make the >> reservation. The problem is how can I do this transaction in a kind of >> "Mutual exclusion" to be sure that this room is really for this user? >> >> Thank you!! :) >> > --
