Programming Google App Engine with Python

Chapter 8. Datastore Transactions

With web applications, many users access and update data concurrently. Often, multiple users need to read or write to the same unit of data at the same time. This requires a data system that can give some assurances that simultaneous operations will not corrupt any user’s view of the data. Most data systems guarantee that a single operation on a single unit of data maintains the integrity of that unit, typically by scheduling operations that act on the unit to be performed in a sequence, one at a time.

Many applications need similar data integrity guarantees when performing a set of multiple operations, possibly over multiple units of data. Such a set of operations is called a transaction. A data system that supports transactions guarantees that if a transaction succeeds, all the operations in the transaction are executed completely. If any step of the transaction fails, then none of its effects are applied to the data. The data remains in a consistent and predictable state before and after the transaction, even if other processes are attempting to modify the data concurrently.

For example, say you want to post a message to the bulletin board in the town square inviting other players to join your guild. The bulletin board maintains a count of how many messages have been posted to the board, so readers can see how many messages there are without reading every message object in the system. Posting a message requires three datastore operations: