App Engine Dev Server datastore not updating fast enough?

Question

Problem: Data Store Update is too slow -- after submitting a form that creates an entity, need to hit reload on the resulting page that shows the entity in the datastore.

Expected Behavior: Entity should appear in query for it, as I'm using NDB for my datastore which caches things automatically.

Problem Repro steps

Create a default project in GoogleAppEngineLauncher 1.8.3 for Mac (my OS version is 10.8.4) and paste the code below into "main.py"
Run the project and visit the root URL.
Enter a number in the form and click submit.
You'll see the text "Here's a list of entities:...end list."
Hit Reload on your browser.
Now you'll see the text "Here's a list of entities: [Number You Entered] ...end list."

Explanation of Expected Behavior

Since NDB automatically uses memcache, Steps 4 and 5 should not happen. After you click Submit on the form, the number you entered should show up. I observed this behavior with the regular appengine DB as well, and I know I can get around it with memcache.

Here is some code that you can drop in to the default main.py created by the AppEngineLauncher that replicates this issue:

import webapp2
from google.appengine.ext import ndb

class SmallModel(ndb.Model):
    n = ndb.IntegerProperty(required=True)
    stamp = ndb.DateTimeProperty(auto_now_add=True)

class MainHandler(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hello world. Simple form. <form method="post"><input name="n" type="number"><input type="submit"></form>')
    def post(self):
        entity = SmallModel(n=int(self.request.get('n')))
        entity.put()
        self.redirect('/list')

class List(webapp2.RequestHandler):
    def get(self):
        self.response.out.write("here's a list of entities:")
        entities = SmallModel.query()
        for entity in entities.iter():
            self.response.out.write(" %s " % entity.n)
        self.response.out.write("...end list.")

app = webapp2.WSGIApplication([
    ('/', MainHandler),
    ('/list',List)
], debug=True)

Any help / advice? Thank you in advance! I've been able to repro this issue in the two browsers I tested -- Chrome and Safari.

Aerodyno Aerodyno · Accepted Answer · 2013-09-02T21:46:03

I managed to get my expected behavior, and my updated code is below. I took a hint from the "guestbook" example code which has the behavior I expect, and set a parent key. I also delved a bit deeper into the NDB docs. Setting a parent key enables the consistency I expect, but limits writes to once per second (I'm assuming that means for all children of a given parent key, NOT all entities for a given model).

Here's what I changed to eliminate steps 4 and 5. I merely set a parent key on the new entity, and do an ancestor query using this parent key. (And of course, this is super minimal code used to illustrate a point, I have no real interest in a model that creates a list of numbers. ;))

import webapp2
from google.appengine.ext import ndb

def parent_key():
    return ndb.Key('My','Entities')

class SmallModel(ndb.Model):
    n = ndb.IntegerProperty(required=True)
    stamp = ndb.DateTimeProperty(auto_now_add=True)

class MainHandler(webapp2.RequestHandler):
    def get(self):
        self.response.write('Hello world. Simple form. <form method="post"><input name="n" type="number"><input type="submit"></form>')
    def post(self):
        entity = SmallModel(parent=parent_key(),n=int(self.request.get('n')))
        entity.put()
        self.redirect('/list')

class List(webapp2.RequestHandler):
    def get(self):
        self.response.out.write("here's a list:")
        entities = SmallModel.query(ancestor=parent_key())
        for entity in entities.iter():
            self.response.out.write(" %s " % entity.n)
        self.response.out.write("...end list.")

app = webapp2.WSGIApplication([
    ('/', MainHandler),
    ('/list',List)
], debug=True)

Thanks for reading and participating.

App Engine Dev Server datastore not updating fast enough?

4 Answers