Transactions cross entity groups

[Cornel]

(I've posted this message one day ago but it did not appear on the
group discussion list, so i will post it again. Hope i am not spamming
or anything)


Hello. I'm using app engine to write a business application. I've read
that during a transaction one can modify only entities within the same
entity group. How would one approach the following scenario? :

Consider the "Account object" with the "Owner" and "Credit" fields.

If i want to make a credit transfer between accounts A and B
(A.credit--; B.credit++), it must be done in a single transaction.
That can only happen if A and B are in the same entity group (from
what i understand)

Since a credit transfer can be done between any two random accounts, i
must put them all in the same entity group; but this way, two
unrelated transfers (A to B and C to D let's say) cannot be done at
the same time anymore. Which again is not desired (i understand that
having a single big entity group is bad practice)

I think this is a pretty general problem (not related only to this
scenario), so how is it solved?

[Jeff S (Google)]

Hi Cornel,

I recall a session at Google I/O from Daniel Wilkerson which describes this scenario and an approach for solving it:

http://code.google.com/events/io/sessions/DesignDistributedTransactionLayerAppEngine.html

The above describes an implementation which does not require A and B to be in the same entity group or have changes execute withing a single transaction.

Thank you,

Jeff

[Craig]

For the benefit of anyone coming to read this thread years later, GAE 1.5.5 will be supporting cross group transactions:

https://groups.google.com/d/msg/google-appengine/f9YVplAlRs4/xFnah8fGTNMJ