[google-appengine] Unexpected Entity Group transaction contention

[Brian Olson]

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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Re: [google-appengine] Unexpected Entity Group transaction contention

[Steve Sherrie]

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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