How to directly upload files to Amazon S3 from your client side web app

Why you need this?

You don’t want your heavy-weight data to travel 2 legs from Client to Server to S3, incurring the cost of IO and clogging the pipe 2 times.

Instead, you want to ask your server to give your client one-time permission to upload your data directly to S3. The process is still 2 legged, but heawy-weight data travels only on 1 leg.

On Amazon S3 this is implemented with CORS (Cross Origin Resource Sharing)

We use this at Dubjoy, where customers upload their huge video files to S3 for translation and voice-over, and we don’t want our Heroku server to have anything to do with heavy-weight video files.

Steps to implement this

  1. Set up Amazon S3 bucket CORS configuration
  2. Implement client-side JavaScript (CoffeScript, JavaScript)
  3. Implement server-side upload request signing (Ruby/Sinatra, trivial to do in any other language)

1. Amazon S3 bucket CORS configuration

Set this in AWS S3 management console. Right-click on the desired bucket and select Properties. Below, on the permissions tab, click Edit CORS configuration, paste the XML below and click Save.

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="">

2. Your web app

Head to GitHub repo with CoffeScript and JavaScript Class files to include. In your app, do the following:


%input#file{ :type => 'file', :name => 'files[]'}

or HTML for the chevron-lovers

<input type='file' name='files[]' />


s3upload = s3upload ? new S3Upload
    file_dom_selector: '#files'
  s3_sign_put_url: '/signS3put'
    onProgress: (percent, message) ->
        console.log 'Upload progress: ', percent, message # Use this for live upload progress bars
    onFinishS3Put: (public_url) ->
        console.log 'Upload finished: ', public_url # Get the URL of the uploaded file
  onError: (status) ->
    console.log 'Upload error: ', status

or JavaScript for the brace-lovers

var s3upload = s3upload != null ? s3upload : new S3Upload({
  file_dom_selector: '#files',
  s3_sign_put_url: '/signS3put',
  onProgress: function(percent, message) { // Use this for live upload progress bars
    console.log('Upload progress: ', percent, message);
  onFinishS3Put: function(public_url) { // Get the URL of the uploaded file
    console.log('Upload finished: ', public_url);
  onError: function(status) {
    console.log('Upload error: ', status);

Be sure to set the right DOM selector name file_dom_selector for file input tag, #files in our case. s3_sign_put_url is an end-point on your server where you will be signing S3 PUT requests.

3. Server-side request signing

Be sure to set S3_BUCKET_NAME,S3_SECRET_KEY,S3_ACCESS_KEY. Create a bucket and get the keys under Security Credentials menu in AWS management console.


get '/signS3put' do
  objectName = params[:s3_object_name]
  mimeType = params['s3_object_type']
  expires = + 100 # PUT request to S3 must start within 100 seconds

  amzHeaders = "x-amz-acl:public-read" # set the public read permission on the uploaded file
  stringToSign = "PUT\n\n#{mimeType}\n#{expires}\n#{amzHeaders}\n/#{S3_BUCKET_NAME}/#{objectName}";
  sig = CGI::escape(Base64.strict_encode64(OpenSSL::HMAC.digest('sha1', S3_SECRET_KEY, stringToSign)))

    signed_request: CGI::escape("#{S3_URL}#{S3_BUCKET_NAME}/#{objectName}?AWSAccessKeyId=#{S3_ACCESS_KEY}&Expires=#{expires}&Signature=#{sig}"),
    url: "{S3_BUCKET_NAME}/#{objectName}"


The code is a based on these resources, but has been put in to an easy to use CoffeeScript/JavaScript Class

You can learn more about CORS here

Rok Krulec / @tantadruj