# Infrastructure

Django 1.3 running on Google App Engine via Django-nonrel

http://code.google.com/appengine/articles/django-nonrel.html

## Motivation

* Cut infrastructure time & costs while avoiding vendor lock-in
* Easy scaling until its clear what the major choke points will be

## Benefits

* Perks of appengine without the lockin
* No database migrations
* Simpler deployment
* Linear scaling instead of 1-server to 3-server to X-server jumps

## Drawbacks

* Scales differently than django on a relational DB
** Exploding index problem needs to be understood by everyone writing models and queries
* Missing many common and convenient django queries and features
** No filtering on null foreign key: model.objects.filter(foreign_key_field__isnull=True)
** No __iexact, __beginswith, etc
** No ManyToManyField


# Setup

Follow guide at http://www.allbuttonspressed.com/projects/djangoappengine


# Development

Run development server using django manage.py commands, *not* appengine's dev_appserver.py commands or GUI

* python manage.py syncdb
* python manage.py createsuperuser
* python manage.py runserver
* python manage.py shell

http://localhost:8000
http://localhost:8000/admin/


# Deployment

Deploy using modified commands in django's manage.py, *not* appengine's command line or GUI. 

#. Commit, pull, and push git version
#. Deploy
** python manage.py deploy
#. Load fixtures & update index.yaml
** python manage.py remote syncdb
#. Tag stable release in git for future rollback purposes


# Rollback & disaster recovery

For major recoveries, roll back to most recent git version tagged as a stable release and then follow deploy steps

For minor fixes, e.g. setting a field's default after changing the models, *carefully* use shell

* python manage.py remote shell
* *Never* delete objects or erase fields through shell, use /admin/ for that for built-in undo


# Common errors (for those used to django)

index.yaml needs an entry - syncdb (local or remote)

index.yaml changes are in progress - just wait, can take a few minutes. You can monitor the progress in the appengine dashboard

Doing a query, models claims not to have a foreign key field which it really does have - nonrel throws an does not exist error when you do a query on or try to access a null foreign key field. Current solution is to add a default model to the initial_data fixture and set the model's foreign key to default to the fixture's pk, treating that default value as you would normally treat isnull.

Exploding index problem - no clear heuristics about when it's going to happen. Google it and stay aware. Sanity check index.yaml every so often to see if it has exploded.