This document discusses using Python and the Google App Engine (GAE) to build mobile backends. It summarizes several key frameworks and techniques for building RESTful APIs and caching data on GAE. These include Flask for routing and views, JSON responses, caching models with Memcache, handling references and aggregations, image handling with Blobstore, and geospatial querying with GeoPt. Maintaining aggregated data and cached references may require periodic cleanup scripts.

1. Python alapú mobil
backend GAE felett
by Gyuri Horak

2. GAE Python
● Python 2.7
● WSGI, webapp2
● db, memcache, cloud-sql, url fetch,
blobstore, users, unit testing ...
● django (django-nonrel, django+cloud-sql)

3. Példaprojekt
● backend iOS, Android és weboldal számára
● REST/JSON

4. Flask
Flask microframework, @mitsuhiko
● url routing
● class based views
● debugger, gae-profiler
● ...

5. JSONView
● Flask.views.MethodView
● success és failure metódusok
● X-Location header
● néhány Exception kezelése

6. ExceptionHandlerViewMeta
class ReferenceErrorMeta(MethodViewType):
def __new__(mcs, name, bases, odict):
if 'get' in odict:
odict['get'] = mcs.wrapmethod(odict['get'])
return MethodViewType.__new__(mcs, name, bases, odict)
@staticmethod
def wrapmethod(method):
@wraps(method)
def wrapper(self, *args, **kwargs):
try:
return method(self, *args, **kwargs)
except db.ReferencePropertyResolveError:
return self.failure(10, 'Reference object seems to be deleted.')
return wrapper

7. DB: a lekérdezés pénz
● NE előre optimalizálj!
● cache: memcache, request scope cache
● aggregálni, okosan
● Mérj! (GAE profiler, datastore stats)

8. CachedModel
class CachedModel(db.Model):
@classmethod
def get_from_cache(id):
...
@classmethod
def get(cls, key):
obj = cls.get_from_cache(key.id())
if not obj:
obj = super(CachedModel, cls).get(key)
return obj
def put(self):
super(CachedModel, self).put()
self.update_in_cache()
def delete(self):
super(CachedModel, self).delete()
self.remove_in_cache()
def __eq__(self, other):
return self.key() == other.key()

9. class CachedReferenceProperty(db.ReferenceProperty):
def __get__(self, model_instance, model_class):
...
# instance = get(reference_id) # WTF?
instance = self.reference_class.get(reference_id)
if instance is None:
# Feature check vs. isinstance
if hasattr(model_instance, 'update_references'):
model_instance.update_references()
raise db.ReferencePropertyResolveError(...)
CachedReferenceProperty

10. class CachedReferenceProperty(db.ReferenceProperty):
def __get__(self, model_instance, model_class):
...
# instance = get(reference_id) # WTF?
instance = self.reference_class.get(reference_id)
if instance is None:
# Feature check vs. isinstance
if hasattr(model_instance, 'update_references'):
model_instance.update_references()
raise db.ReferencePropertyResolveError(...)
Aggregáció

11. Aggregáció
Sérülhet, akár rajtunk kívül álló okok miatt
is.
● cron jobok takarítani / update-elni
● javítás, amikor hibát találunk

12. Képkezelés
● blobstore
○ create_upload_url()
○ files API (experimental)
● images ~ PIL
● get_serving_url()
○ lehet, hogy nincs kész

13. Geomodel
● db.GeoPt(lat, lon)
● geocells
● bounding box és proximity lekérdezés
localmind is ezt használja clustering
megoldásként