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title
Moving static assets to S3

At Famigo, we serve lots of static assets. Every application we review has an icon, some screenshots (with thumbnails), and some associated content like logos and background images. An app like iBlast Moki weighs in with more than 200 KB of images.

Up until last week, we weren't serving up those images in the best way. Our web server runs Django with MongoDB as the database. We also use lighttpd to serve some static assets, like our logo. Any time you wanted to see an app's icon, it went through Django and MongoDB before giving it to you.

Not any more! We realized our page would load much faster if more things went through lighttpd. Then, we realized app icons don't change all that often; we could probably let Amazon S3 handle them. Finally, we got smart and put everything on Amazon CloudFront. The end result? Most of our requests got ten times faster! (From about 200 ms to 20 ms on average.) Here's how we did it.

Since we're using Django, we'll use Amazon's S3 library for Python to communicate with S3. Add that module and a few constants to your project for you access key, secret key, and bucket. Amazon lets you use vanity URLs (like "bucket.s3.amazonaws.com"), provided that your bucket is unique. We set our desired bucket name and concatenated it with our access key to make it unique.

AWS_ACCESS_KEY = '...'
AWS_SECRET_KEY = '...'
AWS_BUCKET = 'famigo-static'
AWS_BUCKET = '{0}-{1}'.format(AWS_ACCESS_KEY, AWS_BUCKET).lower()

On to the hard part: moving everything from the database to S3. Create a Django admin command to do this. The first thing we need to do is connect to S3 and make sure our bucket exists.

import httplib, S3
connection = S3.AWSAuthConnect(AWS_ACCESS_KEY, AWS_SECRET_KEY)
if connection.check_bucket_exists(AWS_BUCKET) != httplib.OK:
    connection.create_bucket(AWS_BUCKET)

For every app in our database, we want to get its icon and send it over to S3. We also want to make it publicly readable so people can access it without a token. And, just like our bucket, our object keys need to be unique. We'll be using the app's package name as its key.

for application in Application.objects:
    key = '{0}-icon'.format(application.package_name)
    content = application.icon.read()
    connection.put(AWS_BUCKET, key, content, {'x-amz-acl': 'public-read'})

Now the icon is stored on Amazon's server. We'll need a way to get it back, though. Amazon's S3 library has a URL generator that does exactly that.

generator = S3.QueryStringAuthGenerator(AWS_ACCESS_KEY, AWS_SECRET_KEY)
for application in Application.objects:
    url = generator.make_bare_url(AWS_BUCKET, key) # with key as before
    application.icon_url = url
    application.save()

The final step is using the new URL in templates. Assuming you were using Django's URL tag already, this is a piece of cake. Replace all instances of {% raw %}{% url application_icon application.id %}{% endraw %} with {% raw %}{{ application.icon_url }}{% endraw %}. If you're not using the URL tag, you'll have to jump through a few more hoops, but the end result should be the same.

That's it! You are now serving static assets through Amazon S3. Getting CloudFront set up to serve them through a CDN is an optional step, and one I won't cover. It involves lots of administration and very little code.

Update (Oct 31): As noted in a Reddit comment, it's a good idea to set Expires (or Cache-Control) headers on your static assets. When using S3, you want to add another entry to the headers dictionary. It should end up looking like this:

{
    'Cache-Control': 'max-age=604800, public',
    'x-amz-acl': 'public-read',
}

For a more detailed look at caching, I suggest Mark Nottingham's caching tutorial.