It turns out the technique I described in my previous post has two issues.
1. Because only one transaction can be running at a time within a process,
and the entity used as the mutex really cannot be in the same entity group
as the primary entity, you can get into a situation where the mutex ent
OK, here is something that looks promising;
http://squeeville.com/2009/01/30/add-a-unique-constraint-to-google-app-engine/
This shows a solution in python, but the concept is applicable to Java.
Basically, for the unique field, you create another database entity and
derive a primary Key based
I'm struggling with a bug that is closely related to this thread. I think
transactions all by themselves will not work here. This is because the
transactions are optimistic. So, if the transaction looks like this (pseudo
code)
pm.currentTransaction.begin();
Object preexistingEntity = pm.quer
You could use transactions if all Servers are in the same entity
group. In a transaction you can query for existing A specifying the
ancestor in the query. When you find no other Server with the key you
can store it - also with the same ancestor - then commit the
transaction. If another
hello everyone
I have several fields which must be unique because of business
logic.how can app engine make the unique key for the several fields?
For example, in a Server "A" ,I query a value and find there is no
existing value , so I insert value into Server "A" successfully.
But almost the sa
