Step 1 - Configure S3 for CORS

1. Create an Amazon S3 Bucket

2. Click on Properties -> Permissions -> Edit CORS

3. Enter a CORS configuration, such as the following. For security purposes you should restrict the origin further.

   <?xml version="1.0" encoding="UTF-8"?>
   <CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
       <CORSRule>
           <AllowedOrigin>*</AllowedOrigin>
           <AllowedMethod>PUT</AllowedMethod>
           <MaxAgeSeconds>3000</MaxAgeSeconds>
           <AllowedHeader>*</AllowedHeader>
       </CORSRule>
   </CORSConfiguration>

Step 2 - Set your AWS Keys and bucket

The application can be configured via the following environment variables.

• AWS_ACCESS_KEY_ID
• AWS_SECRET_ACCESS_KEY
• AWS_S3_BUCKET

General Flow

This project uses client-side JavaScript and server-side Ruby. In general, the completed file-upload process follows these steps:

1. A file is selected for upload by the user in their web browser;
2. JavaScript is then responsible for making a request to your web application, which produces a temporary signature with which to sign the upload request;
3. The temporary signed request is returned to the browser in JSON format;
4. JavaScript then uploads the file directly to Amazon S3 using the signed request supplied by your Rails application.

Code overview

The application flow is as follows:

1. [Client] When the user chooses a file, a callback (defined in app/assets/javascripts/products.js) contacts the server to request a signed URL that can be used for uploading to S3.
2. [Server] AWSController generates a signed URL according to http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html and returns it using a JSON hash.
3. [Client] The uploadToS3 function (defined in app/assets/javascripts/cors.js), takes the signed URL and the file, and PUTs the file onto S3.
4. [Client] Once the upload is finished, the URL of the uploaded file is appended to the form as a hidden field.
5. [Client] When the user submits the form, the file field is removed (to avoid uploading the file again). Name, price and the file URL are POSTed to the server.
6. [Server] A new product record is created.

The most important files are are:

• app/helpers/aws_helper - Builds the AWS authentication signature as described in http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html. It required the Amazon keys, as well as options about the request (request type, resource, etc).

• app/controllers/aws_controller - Given a filename and content type, returns a JSON of the form:

  {
    "put_url": "The *signed* URL that must be used to make the request from the client side]",
    "file_url": "The url the file will be uploaded to. A random number is appended to the file name to avoid name collisions."
  }

• app/assets/javascripts/cors.js - Performs the actual upload using an XMLHttpRequest. Required the signed URL, and the file to perform the request.

• app/assets/javascripts/products.js - Handles UI callbacks, such as updating the progress bar. Also handles the file-upload callback. When the user chooses a file, the server is contacted to generate a signed URL, which is then passed to the CORS script described above to complete the upload.