Joshua, have you considered memcache as a write-through cache? Memcache should be strongly consistent with the caveat that when the data center changes or any irregularity is detected, we flush it the cache.
-- Ikai Lan Developer Programs Engineer, Google App Engine plus.ikailan.com | twitter.com/ikai On Thu, Sep 8, 2011 at 4:36 AM, Joshua Smith <[email protected]>wrote: > In the dev appserver with --high_replication, if I do this: > > 1. Create a blank entity > 2. Edit that entity > 3. put that entity > 4. Generate a list of entities > > That list, more often than not, shows me the blank entity. The entity is > in the index, but it does not reflect the put in step #3. This is > consistent with the "eventual consistency" model I've read about. > > If, in step 4, I generate a list of all entity KEYS, and then db.get() > those entities, I never see the blank entity. This is also consistent with > the documentation. > > So, I believe that while my pattern requires an extra db roundtrip, it is > much less like to show information that will lead to a support call. > > -Joshua > > On Sep 8, 2011, at 12:47 AM, Robert Kluin wrote: > > > This is done on the backend, if I remember correctly. It doesn't gain > > you anything. > > > > > > > > On Wed, Sep 7, 2011 at 19:28, Joshua Smith <[email protected]> > wrote: > >> Continuing the dialog with myself :) > >> > >> I've added this method to one of my classes that extends db.Model() and > it is working well with the dev appserver in --high_replication mode: > >> > >> @classmethod > >> def gql_with_get(cls, query_string, *args, **kwds): > >> return db.get(db.GqlQuery('SELECT __key__ FROM %s %s' % (cls.kind(), > query_string), *args, **kwds)) > >> > >> You use it just like gql().fetch(). For example: > >> > >> boards = BoardModel.gql_with_get("WHERE towns = :1 ORDER BY name", > tid) > >> > >> It doesn't fix the index (things might be out of order, for instance), > but otherwise, it cures the problem of seeing stale data in HR. > >> > >> On Sep 7, 2011, at 12:22 PM, Joshua Smith wrote: > >> > >>> Another thought: The reason I was doing only one meeting per request > was because of the old 30 second limit on crons. But cron handlers can be > 10 minutes now, which is plenty of time to schedule all the meetings. > Therefore, I suppose I could do this, right? > >>> > >>> now = datetime.datetime.now() > >>> for schedule in db.get(db.gql("SELECT __key__ FROM ScheduleModel > WHERE next != :1 AND next < :2", None, now)): > >>> if schedule.next and schedule.next < now: > >>> schedule.cronAuto() > >>> > >>> Is wrapping a GET around a KEYS-ONLY query guaranteed to get me the > real-deal results (except, of course, for the fact that the index might be > out-of-date, so I might miss recent changes to who is in/out of the query > parameters)? Is this an efficient way to express this, or should I be doing > a fetch() on the gql first? > >>> > >>> It seems like it's possible to use a technique like this to get a > more-consistent result in cases where that's desirable. It at least would > get you a consistent data for a subset of things matching your query. In > principle, you could even re-sort the results if there is an ORDER clause. > Seems like this would be something useful in the db API... > >>> > >>> -Joshua > >>> > >>> On Sep 7, 2011, at 11:18 AM, Joshua Smith wrote: > >>> > >>>> > >>>> 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 > >>>> > >>>> -- > >>>> You received this message because you are subscribed to the Google > Groups "Google App Engine" group. > >>>> To post to this group, send email to > [email protected]. > >>>> To unsubscribe from this group, send email to > [email protected]. > >>>> For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > >>>> > >>> > >>> -- > >>> You received this message because you are subscribed to the Google > Groups "Google App Engine" group. > >>> To post to this group, send email to [email protected] > . > >>> To unsubscribe from this group, send email to > [email protected]. > >>> For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > >>> > >> > >> -- > >> You received this message because you are subscribed to the Google > Groups "Google App Engine" group. > >> To post to this group, send email to [email protected]. > >> To unsubscribe from this group, send email to > [email protected]. > >> For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > >> > >> > > > > -- > > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > > To post to this group, send email to [email protected]. > > To unsubscribe from this group, send email to > [email protected]. > > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > > > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > > -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. 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