direct_s3.md

doc/direct_s3.md
Last Update: 2018-10-25 17:47:40 +0200

Direct Uploads to S3

Shrine gives you the ability to upload files directly to Amazon S3 (or any other storage service that accepts direct uploads). Uploading directly to a storage service is beneficial for several reasons:

  • Accepting uploads is resource-intensive for the server, and delegating it to an external service makes scaling easier.

  • If both temporary and permanent storage are S3, promoting an S3 file to permanent storage will simply issue an S3 copy request, without any downloading and reuploading.

  • With multiple servers it's generally not possible to cache files to the disk, unless you're using a distibuted filesystem that's shared between servers.

  • On Heroku any uploaded files that aren't part of version control don't persist, they get removed each time you do a new deploy or when the dyno automatically changes the location.

  • If your request workers have a timeout configured or you're using Heroku, uploading large files to S3 or any external service inside the request-response lifecycle might not be able to finish before the request times out.

To start, let's set both temporary and permanent storage to S3, with the temporary storage uploading to the cache/ prefix:

# Gemfile
gem "shrine", "~> 2.11"
gem "aws-sdk-s3", "~> 1.2"
require "shrine/storage/s3"

s3_options = {
  bucket:            "<YOUR BUCKET>", # required
  access_key_id:     "<YOUR KEY>",
  secret_access_key: "<YOUR SECRET>",
  region:            "<REGION>",
}

Shrine.storages = {
  cache: Shrine::Storage::S3.new(prefix: "cache", **s3_options),
  store: Shrine::Storage::S3.new(**s3_options),
}

Bucket CORS configuration

In order to be able upload files directly to your S3 bucket, you'll need to update your bucket's CORS configuration, as public uploads are not allowed by default. You can do that from the AWS S3 Console by going to your bucket, clicking on the “Permissions” tab and then on “CORS Configuration”.

If you're using Uppy, this is the recommended CORS configuration for the [Aws S3 plugin] that should work for both POST and PUT uploads:

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <CORSRule>
    <AllowedOrigin>https://my-app.com</AllowedOrigin>
    <AllowedMethod>GET</AllowedMethod>
    <AllowedMethod>POST</AllowedMethod>
    <AllowedMethod>PUT</AllowedMethod>
    <MaxAgeSeconds>3000</MaxAgeSeconds>
    <AllowedHeader>Authorization</AllowedHeader>
    <AllowedHeader>x-amz-date</AllowedHeader>
    <AllowedHeader>x-amz-content-sha256</AllowedHeader>
    <AllowedHeader>content-type</AllowedHeader>
  </CORSRule>
  <CORSRule>
    <AllowedOrigin>*</AllowedOrigin>
    <AllowedMethod>GET</AllowedMethod>
    <MaxAgeSeconds>3000</MaxAgeSeconds>
  </CORSRule>
</CORSConfiguration>

Replace https://my-app.com with the URL to your app (in development you can set this to *). Once you've hit “Save”, it may take some time for the new CORS settings to be applied.

Strategy A (dynamic)

  • Best user experience

  • Single or multiple file uploads

  • Some JavaScript needed

When the user selects a file in the form, on the client side we asynchronously fetch the upload parameters from the server, and use it to upload the file to S3. It's recommended to use Uppy for client side uploads.

The presign_endpoint plugin provides a Rack application that generates these upload parameters, which we can just mount in our application. We'll make our presign endpoint also use the additional type and filename query parameters to set Content-Type and Content-Disposition for the uploaded file, as well as limit the upload size to 10 MB (see Shrine::Storage::S3#presign for the list of available options).

Shrine.plugin :presign_endpoint, presign_options: -> (request) {
  filename = request.params["filename"]
  type     = request.params["type"]

  {
    content_disposition:    "inline; filename=\"#{filename}\"", # set download filename
    content_type:           type,                               # set content type (required if using DigitalOcean Spaces)
    content_length_range:   0..(10*1024*1024),                  # limit upload size to 10 MB
  }
}
# config.ru (Rack)
map "/presign" do
  run Shrine.presign_endpoint(:cache)
end

# OR

# config/routes.rb (Rails)
Rails.application.routes.draw do
  mount Shrine.presign_endpoint(:cache) => "/presign"
end

The above will create a GET /presign route, which internally calls Shrine::Storage::S3#presign to return the HTTP verb (POST) and the S3 URL to which the file should be uploaded, along with the required POST parameters and request headers.

# GET /presign
{
  "method": "post",
  "url": "https://my-bucket.s3-eu-west-1.amazonaws.com",
  "fields": {
    "key": "b7d575850ba61b44c8a9ff889dfdb14d88cdc25f8dd121004c8",
    "policy": "eyJleHBpcmF0aW9uIjoiMjAxNS0QwMToxMToyOVoiLCJjb25kaXRpb25zIjpbeyJidWNrZXQiOiJ...",
    "x-amz-credential": "AKIAIJF55TMZYT6Q/20151024/eu-west-1/s3/aws4_request",
    "x-amz-algorithm": "AWS4-HMAC-SHA256",
    "x-amz-date": "20151024T001129Z",
    "x-amz-signature": "c1eb634f83f96b69bd675f535b3ff15ae184b102fcba51e4db5f4959b4ae26f4"
  },
  "headers": {}
}

Uppy's AWS S3 plugin would then make a request to this endpoint and use these parameters to upload the file directly to S3. Once the file has been uploaded, you can generate a JSON representation of the uploaded file on the client side, and write it to the hidden attachment field (or send it directly in an AJAX request).

{
  "id": "302858ldg9agjad7f3ls.jpg",
  "storage": "cache",
  "metadata": {
    "size": 943483,
    "filename": "nature.jpg",
    "mime_type": "image/jpeg",
  }
}
  • id – location of the file on S3 (minus the :prefix)

  • storage – direct uploads typically use the :cache storage

  • metadata – hash of metadata extracted from the file

Once the form is submitted, this JSON data will then be assigned to the attachment attribute instead of the raw file. See this walkthrough for adding dynamic direct S3 uploads from scratch, as well as the Roda / Rails demo app for a complete example of multiple direct S3 uploads.

Strategy B (static)

  • Basic user experience

  • Only for single uploads

  • No JavaScript needed

An alternative to the previous strategy is to generate an S3 upload form on page render. The user can then select a file and submit it directly to S3. For generating the form can use Shrine::Storage::S3#presign, which returns URL and form fields that should be used for the upload.

presign_data = Shrine.storages[:cache].presign(
  SecureRandom.hex,
  success_action_redirect: new_album_url
)

form action: presign_data[:url], method: "post", enctype: "multipart/form-data" do |f|
  presign_data[:fields].each do |name, value|
    f.input :hidden, name: name, value: value
  end
  f.input :file, name: "file"
  f.button "Submit"
end

Note the additional :success_action_redirect option which tells S3 where to redirect to after the file has been uploaded. If you're using the Rails form builder to generate this form, you might need to also tell S3 to ignore the additional utf8 and authenticity_token fields that Rails generates:

presign_data = Shrine.storages[:cache].presign(
  SecureRandom.hex,
  allow_any: ["utf8", "authenticity_token"],
  success_action_redirect: new_album_url
)

# ...

Let's assume we specified the redirect URL to be a page which renders the form for a new record. S3 will include some information about the upload in form of GET parameters in the URL, out of which we only need the key parameter:

cached_file = {
  storage: "cache",
  id: params["key"][/^cache\/(.+)/, 1], # we subtract the storage prefix
  metadata: {},
}

form @album, action: "/albums" do |f|
  f.input :image, type: :hidden, value: cached_file.to_json
  f.button "Save"
end

Shrine metadata

When attaching a file that was uploaded directly to S3, by default Shrine will not extract metadata from the file, instead it will simply copy over any metadata assigned on the client side. This is the default behaviour because extracting metadata requires retrieving file content, which in this case means additional HTTP requests.

See this section or the rationale and instructions on how to opt in.

Object data

When the cached S3 object is copied to permanent storage, the destination S3 object will by default inherit any object data that was assigned to the cached object via presign parameters. However, S3 will by default also ignore any new object parameters that are given to the copy request.

Whether object data will be copied or replaced depends on the value of the :metadata_directive parameter:

  • "COPY" - destination object will inherit source object data and any new data will be ignored (default)

  • "REPLACE" - destination object will not inherit any of the source object data and will accept new data

You can use the upload_options plugin to change the :metadata_directive option when S3 objects are copied:

plugin :upload_options, store: -> (io, context) do
  { metadata_directive: "REPLACE" } if io.is_a?(Shrine::UploadedFile)
end

Clearing cache

Directly uploaded files won't automatically be deleted from your temporary storage, so you'll want to periodically clear them. One way to do that is by setting up recurring script which calls Shrine::Storage::S3#clear!:

s3 = Shrine.storages[:cache]
s3.clear! { |object| object.last_modified < Time.now - 7*24*60*60 } # delete files older than 1 week

Alternatively you can add a bucket lifeycle rule to do this for you. This can be done either from the AWS Console or via an API call:

require "aws-sdk-s3"

client = Aws::S3::Client.new(
  access_key_id:     "<YOUR KEY>",
  secret_access_key: "<YOUR SECRET>",
  region:            "<REGION>",
)

client.put_bucket_lifecycle_configuration(
  bucket: "<YOUR BUCKET>",
  lifecycle_configuration: {
    rules: [{
      expiration: { days: 7 },
      filter: { prefix: "cache/" },
      id: "cache-clear",
      status: "Enabled"
    }]
  }
)

Eventual consistency

When uploading objects to Amazon S3, sometimes they may not be available immediately. This can be a problem when using direct S3 uploads, because usually in this case you're using S3 for both cache and store, so the S3 object is moved to store soon after caching.

Amazon S3 provides eventual consistency for some operations, so it is possible that new data will not be available immediately after the upload, which could result in an incomplete data load or loading stale data. COPY operations where the cluster and the bucket are in different regions are eventually consistent. All regions provide read-after-write consistency for uploads of new objects with unique object keys. For more information about data consistency, see Amazon S3 Data Consistency Model in the Amazon Simple Storage Service Developer Guide.

This means that in certain cases copying from cache to store can fail if it happens immediately after uploading to cache. If you start noticing these errors, and you're using backgrounding plugin, you can tell your backgrounding library to perform the job with a delay:

Shrine.plugin :backgrounding

Shrine::Attacher.promote do |data|
  PromoteJob.perform_in(3, data) # tells a Sidekiq worker to perform in 3 seconds
end

Checksums

You can have AWS S3 verify the integrity of the uploaded data by including a checksum generated on the client side in the upload request. For that we'll need to include the checksum in the presign request, which we can pass in via the checksum query parameter. The :content_md5 parameter is not supported in POST presigns, so for this we'll need to switch to PUT.

Shrine.plugin :presign_endpoint, presign_options: -> (request) do
  {
    method: :put,
    content_md5: request.params["checksum"],
  }
end

See this walkthrough for a complete JavaScript implementation of checksums.

Note that PUT presigns don't support the :content_length_range option, but they support :content_length instead. So, if you want to limit the upload size during direct uploads, you can pass an additional size query parameter to the presign request on the client side, and require it when generating presign options:

Shrine.plugin :presign_endpoint, presign_options: -> (request) do
  {
    method: :put,
    content_length: request.params.fetch("size"),
    content_md5: request.params["checksum"],
  }
end

Testing

To avoid network requests in your test and development environment, you can use Minio. Minio is an open source object storage server with AWS S3 compatible API which you can run locally. See how to set it up in the Testing guide.