Real-world Google App Engine
Aral Balkan (@aral, http://aralbalkan.com)

This is not an introduction to
Google App Engine!

Google App Engine is really
good at scaling...
... a request/response model within various
limitations and subject to certain quotas.

Request/response
No long running processes or CRON jobs

Fake it!

return HttpResponseRedirect
(“/recurse/lastKey/”)
No long-running processes
Workaround #1: Server-side redirect

<meta http-equiv=\"refresh\"
content=\"0;url=/recurse/lastKey/\" />
No long-running processes
Workaround #2: Meta refresh

$.post(
url,
{lastKey: lastKey},
callback,
\" json \");
No long-running processes
Workaround #3: JavaScript

Sortable keys
A welcome addition (1.1.6, Nov 21, 2008)

if 'last_key' in request.REQUEST:
last_key = request.REQUEST['last_key']
else:
# Handle error and return ...
Get the last key value passed by the client.

if last_key == 'none':
query = Model.gql('ORDER BY __key__')
else:
key_ref = db.Key(last_key)
query = Model.gql('WHERE __key__ > :1
ORDER BY __key__', key_ref)
rows = query.fetch(ROWS_PER_GO+1)
Sort by key, using last key if provided and fetch
the number of rows you want to affect.

if len(rows) == ROWS_PER_GO+1:
# There's more, we need to recurse again
new_last_key =
str(rows[ROWS_PER_GO].key())
...
else:
# This is the last iteration.
Check if you need to recurse.
If so, set the new last key value.

No CRON jobs
Cannot run periodical tasks

WebBasedCron.com
Workaround: use an external CRON service.

Size matters
1MB limit on any object

aws.amazon.com/s3/
Workaround: Use Amazon S3 & CloudFront

infinitebits.org
FTP to S3 service by Benjamin Allfree

pyamf.appspot.com/shell/
Ajax shell: http://shell.appspot.com/

Growing pains
1,000 file limit per application

Growing pains
Workaround: zipimport

Zipping up Django
http://aralbalkan.com/1757

Quotas

Quotas
CPU time: 46.30 hours
Bandwidth: 10GB (out/in, 20GB total)
Secure bandwith: 2GB (out/in, 4GB total)
2000 emails
0.5GB stored data

Evil Quotas

Evil quota #1
Short-term high CPU usage quota

Evil quota #2
Short-term email limit (officially 2, empirically 30)

Sailthru.com triggermail
$1.50 cpm mass mail, $6.00 transactional

Web Hooks (Jeff Lindsay)
http://webhooks.pbwiki.com/
smtp2web
http://smtp2web.com/
astrotrain
http://github.com/entp/astrotrain/tree/master

Secret evil quota solution
(Ssshh, don’t tell anyone)

Please sir, may I have some more?

Django or webapp?
Which framework should you use?

webapp Django
“Official” framework Lots more
functionality for free
Not very active
Very active
Guido doesn’t use it
Guido uses it
Allows subclassing
No subclassing of
You’ll likely use bits views
of it anyway
Great community
Plain Plain rocks!

Django
Which helper should you use?
(Depends on your needs)

Google App Engine Helper
The more “official” helper

app-engine-patch
The community “helper” led by Waldemar
Kornewal (@wkornewald)

Helper Patch
More official Community-run
Slow updates Frequent updates
Django models Webapp models
Admin panel
Self-contained,
reusable apps.
Context-issues
solved in GAE 1.1.8

GAE! What is it good for?
Use Cases and Don’t Use Cases

UCs DUCs
APIs Ecommerce
Facebook/Open (no SSL)
Social apps Sites that require
Static sites admin features (no
long running
processes)
CDN (file size limit of
1MB)

Why Google App Engine is
broken and what Google
must do to fix it.
http://aralbalkan.com/1504

App Engine’s Achilles' heel
A missing modus operandi

Request/response Admin
Needs to scale Does not