Any time you write/update an entity in a group (share a common ancestor) you lock the entire entity group all the way to the top most parent. All branches under that parent... the entire tree.
Multi eg transactions will allow you to write to multiple groups but it will not reduce contention in this scenario at all. Mike On Sep 13, 2:23 pm, Brian Olson <[email protected]> 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. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
