Hi David,
Even if a put request to the datastore is not run in a transaction,
the operation is automatically retried. Contention is not unique to
transactions. The benefit of using transactions, is that if one write
in the transaction times out (due to too much contention or some other
issue) the other parts of the transaction will not be applied. For
more details see:
http://code.google.com/appengine/docs/datastore/transactions.html#Using_Transactions
Happy coding,
Jeff
On Sep 18, 6:25 pm, DXD <[EMAIL PROTECTED]> wrote:
> I appreciate any clarifications on my situation as follows. I have an
> entity group whose the root entity is called "root". When a particular
> URL is requested, a new entity is added to this group as a direct
> child of root. The code looks similar to this:
>
> def insert():
> root = Root.get_by_key_name('key_name')
> child = Child(parent=root)
> child.put()
>
> Note that the insert() function is not run in a transaction (not
> called by db.run_in_transaction()).
>
> I spawned many concurrent requests to this URL. The log shows that
> there are many failed requests with either "TransactionFailedError:
> too much contention on these datastore entities. please try again" or
> "DeadlineExceededError". Since I'm still a bit unclear about the
> internal working of the datastore, these are my explanations for what
> happened. Pls correct me where I'm wrong:
>
> 1. when one child entity is being inserted, it locks the entire group.
> All other concurrent requests are blocked, and their child.put()
> statement exclusively is retried a number of times. Say the limit
> number of retry is r.
>
> 2. If child.put() is retried r times but still doesn't go through, it
> gives up and yields the "too much contention" error.
>
> 3. If child.put() does not yet reach r times of retry, but its session
> already reaches the time limit t, then it fails yielding the
> "DeadlineExceededError".
>
> If my explanations are correct, isn't it true that the insert()
> function is exactly equivalent to this version?:
>
> def insert():
> root = Root.get_by_key_name('key_name')
> child = Child(parent=root)
> def txn()
> child.put()
> db.run_in_transaction(txn)
>
> Or more generally, is it true that all API operations that write to
> the datastore have exactly the same effect with transaction
> (automatically retried if failed, and so on)?
>
> Thanks for clarifications,
> David.
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