3

Take the following code for example (ignore the lack of usefulness in its functionality, as it's just a simple example to include the things I need):

@transaction.commit_on_success
def test_for_success(username)
    person = Person.objects.select_for_update().get(username=username)
    response = urllib2.urlopen(URL_TO_SOME_SLOW_API, some_data)
    if '<YES>' in response.read():
        person.successes += 1
        person.save()

My question pertaining the example has to do with when the queries hit the database. Clearly the first query will lock the Person row, and then I'm calling a slow API, which could take 3 seconds to respond, causing the row to be locked for 3 seconds. Am I understanding this correctly, and in the case of slow API hits happening in my transaction, if I move the location of my queries so that a SELECT FOR UPDATE doesn't happen until after all the slow API requests, will this have the seemingly obvious effect of not locking my rows for seconds at a time (the case for select_for_update in my application is unavoidable)? Or, am I misunderstanding, and somehow none of the SQL actually hits the database until the end of the transaction?

orokusaki
  • 55,146
  • 59
  • 179
  • 257

1 Answers1

1

Your assumptions about your code are correct. If you look at the select_for_update() docs, this action does lock those rows in the database until they are unlocked. This would in effect lock out for the duration of your urllib request.

If you were to move the database call into the conditional after the request, you are correct again that the database would be locked for a much shorter amount of time (though if that is called alot will still have some clients who block on the call due to contention).

g19fanatic
  • 10,567
  • 6
  • 33
  • 63
  • Ah, the worst part is that I cannot lock the row, update it to reflect the post-API state (inventory-related, so I have to block concurrent capturing of the same quantities), then call the API, then rollback, if the API doesn't tell me what I want to know. Well, I can, just that it will still block until the function returns. Ugh, databases :( – orokusaki Aug 27 '12 at 16:31