Testing is happening presently for multi-entity-group transactions I
believe. I saw a thread on it about a week ago.
Steve
On 11-09-13 03:23 PM, Brian Olson wrote:
Re-reading the documentation, this kinda makes sense, but it bit me
recently so I want to tell the story and see what others think.
I make an entity Parent(). Some time later I make an entity
Child(parent=some_parent) and I do this in a transaction. I do this a
bunch, concurrently from task-queue entries.
I was surprised to learn that simply creating a Child in a
transaction, without otherwise doing anything to the parent, neither
.get() nor .put(), locks the parent and all its children.
def txn_make_child(some_parent):
foo = Child(parent=some_parent)
foo.put()
# also transactionally enqueue a task to operate on the Child
instance foo
Code very much like that was failing out due to too many transaction
retries. I didn't expect /any/ transaction contention, because I
thought I was just creating an object and enqueueing a task, and those
were the only two things in the transaction in my head. But it turns
out the above code locks some_parent and all its children. Boo.
I think I was expecting things like this to lock parent and all its
children:
def txn_p_c_example(parent_key, child_key):
parent = db.get(parent_key)
child = db.get(child_key)
# now they're clearly both involved, and involving the parent winds
up locking all the children. I can accept that.
parent.put()
child.put()
I was able to re-code it to make Child have no ancestor, but there are
still times when I would much rather still commit parent and child at
exactly the same time.
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