I'm trying to port my existing M/S app to HR because I have a gun to my head
with "Threaded Python Only for HR Apps" written on the bullets.
My system will schedule meetings automatically. Scheduling a meeting can take
some time, because a bunch of records are created, and a bunch of emails need
to go out. So the code to schedule one looked like this:
class MeetingAutoHandler(webapp.RequestHandler):
def get(self):
schedule = ScheduleModel.gql("WHERE next != :1 AND next < :2", None,
datetime.datetime.now()).get()
if schedule:
schedule.cronAuto()
taskqueue.add(url='/admin/meetingAuto', method='GET', countdown=1)
The query looks for a schedule object that needs a meeting to to be scheduled
now. There might be a few of these when the cron runs. So it does the hard
work for one of them (in cronAuto()), and schedules another call to itself to
get the next one using the task queue.
This isn't going to work in HR because that query is going to keep finding the
same meeting. I could trivially tweak this by setting the countdown=60, but
I've yet to hear any of our google overlords commit to a maximum value of when
"eventually" happens in "eventually consistent". I presume there might be
cases, like during data center transitions, when "eventually" could be a very
long time indeed. It is essentially unbounded. Right?
But I like the pattern I'm using here, and I'm trying to change as little code
as possible, so I want to put together a HR-resilient version. Here's what I
came up with:
class MeetingAutoHandler(webapp.RequestHandler):
def get(self):
now = datetime.datetime.now()
for s in db.gql("SELECT __key__ FROM ScheduleModel WHERE next != :1 AND
next < :2", None, now):
schedule = db.get(s)
if schedule.next and schedule.next < now:
schedule.cronAuto()
taskqueue.add(url='/admin/meetingAuto', method='GET', countdown=5)
return
So I'm doing a keys-only query and then doing a get() on the key. (I've never
done a keys-only GQL query before, but I think I got it right. Note to google:
There should be an option to Model.gql() to do keys-only queries!)
The way I understand HR, that get is going to get the real Model, which might
not meet the criteria in the gql, because the index might be out of date.
Right?
So I check that the model meets the criteria that I just specified. (Note to
google: It'd be cool if there was a way to test an object against a query, so I
don't have to write the same code twice!)
Finally, I pushed the next task out a bit, to make it less likely that I'll
have to look at the same objects over and over.
So what do you think? Any suggestions? (I have a couple things that work this
way, so I want to choose a good design pattern to apply to each of them.)
The complexity would be lessened if I could to this:
class MeetingAutoHandler(webapp.RequestHandler):
def get(self):
q = ScheduleModel.gql_keys_only("WHERE next != :1 AND next < :2", None,
datetime.datetime.now())
for s in q:
schedule = db.get(s)
if q.matches(schedule):
schedule.cronAuto()
taskqueue.add(url='/admin/meetingAuto', method='GET', countdown=5)
return
This would require two changes: the db.Model would need to support
gql_keys_only (that's probably trivial); GqlQuery would need a matches() method
(that's probably not trivial).
It's still a few more lines, but the complexity is about the same as the old
one.
Worth the trouble of a couple feature request issues?
-Joshua
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