django 1.6.1中交易的奇怪行为

I am using transaction.atomic as a context manager for transactions in django 1.6. There is a block of code which I want to be in a transaction which has a couple of network calls and some database writes. I am seeing very weird behaviour. Every once in while (maybe 1 in 20 times) I have noticed a partial rollback happening without any exception having been raised and the view executing without any errors. My application is hosted on heroku and we use heroku postgres v9.2.8. Pseudo code:I am using transaction.atomic as a context mana