Derivation Endpoint

The derivation_endpoint plugin provides a Rack app for dynamically processing uploaded files on request. This allows you to create URLs to files that might not have been generated yet, and have the endpoint process them on-the-fly.

Quick start

We first load the plugin, providing a secret key and a path prefix to where the endpoint will be mounted:

class ImageUploader < Shrine
  plugin :derivation_endpoint,
    secret_key: "<YOUR SECRET KEY>",
    prefix:     "derivations/image"
end

We can then mount the derivation endpoint for our uploader into our app's router on the path prefix we specified:

# config/routes.rb (Rails)
Rails.application.routes.draw do
  mount ImageUploader.derivation_endpoint => "derivations/image"
end

Next we can define a "derivation" block for the type of processing we want to apply to an attached file. For example, we can generate image thumbnails using the ImageProcessing gem:

gem "image_processing", "~> 1.2"

require "image_processing/mini_magick"

class ImageUploader < Shrine
  # ...
  derivation :thumbnail do |file, width, height|
    ImageProcessing::MiniMagick
      .source(file)
      .resize_to_limit!(width.to_i, height.to_i)
  end
end

Now we can generate "derivation" URLs from attached files, which on request will call the derivation block we defined.

photo.image.derivation_url(:thumbnail, "600", "400")
#=> "/derivations/image/thumbnail/600/400/eyJpZCI6ImZvbyIsInN0b3JhZ2UiOiJzdG9yZSJ9?signature=..."

In this example, photo is an instance of a Photo model which has an image attachment. The URL will render a 600x400 thumbnail of the original image.

How it works

The #derivation_url method is defined on Shrine::UploadedFile objects. It generates an URL consisting of the configured path prefix, derivation name and arguments, serialized uploaded file, and an URL signature generated using the configured secret key:

/  derivations/image  /  thumbnail  /  600/400  /  eyJmZvbyIb3JhZ2UiOiJzdG9yZSJ9  ?  signature=...
  └──── prefix ─────┘  └── name ──┘  └─ args ─┘  └─── serialized source file ───┘

When the derivation URL is requested, the derivation endpoint will first verify the signature included in query params, and proceed only if it matches the calculated signature. This ensures that only the server can generate valid derivation URLs, preventing potential DoS attacks.

The derivation endpoint then extracts the source file data, derivation name and arguments from the request URL, and calls the corresponding derivation block, passing the downloaded source file and derivation arguments.

derivation :thumbnail do |file, arg1, arg2, ...|
  file #=> #<Tempfile:...> (source file downloaded to disk)
  arg1 #=> "600" (first derivation argument in #derivation_url)
  arg2 #=> "400" (second derivation argument in #derivation_url)

  # ... do processing ...

  # return result as a File/Tempfile object or String/Pathname path
end

The derivation block is expected to return the processed file is a File/Tempfile object or a String/Pathname path. The resulting file is then rendered in the HTTP response.

Performance

By default, the processed file returned by the derivation block is not cached anywhere. This means that repeated requests to the same derivation URL will execute the derivation block each time, which can put a lot of load on your application.

For this reason it's highly recommended to put a CDN or other HTTP cache in front of your application. If you've configured a CDN, you can set the CDN host at the plugin level, and it will be used for all derivation URLs:

plugin :derivation_endpoint, host: "https://your-dist-url.cloudfront.net"

Additionally, you can have the endpoint cache derivatives to a storage. With this setup, the generated derivative will be uploaded to the storage on initial request, and then on subsequent requests the derivative will be served directly from the storage.

plugin :derivation_endpoint, upload: true

If you want to avoid having the endpoint directly serve the generated derivatives, you can have the derivation response redirect to the uploaded derivative on the storage service.

plugin :derivation_endpoint, upload: true, upload_redirect: true

For more details, see the Uploading section.

Derivation response

Mounting the derivation endpoint into the app's router is the easiest way to handle derivation requests, as routing and setting the response is done automatically.

# config/routes.rb
Rails.application.routes.draw do
  mount ImageUploader.derivation_endpoint => "derivations/image"
end

However, this approach can also be limiting if one wants to perform additional operations around derivation requests, such as authentication and authorization.

Instead of mounting the endpoint into the router, you can also call the derivation endpoint from a controller. In this case the endpoint needs to receive the Rack env hash, so that it can infer derivation parameters from the request URL. The return value is a 3-element array, containing the status, headers, and body that should be returned in the HTTP response:

# config/routes.rb
Rails.application.routes.draw do
  get "/derivations/image/*rest" => "derivations#image"
end

# app/controllers/derivations_controller.rb
class DerivationsController < ApplicationController
  def image
    # we can perform authentication here
    set_rack_response ImageUploader.derivation_response(request.env)
  end

  private

  def set_rack_response((status, headers, body))
    self.status = status
    self.headers.merge!(headers)
    self.response_body = body
  end
end

For even more control, you can generate derivation responses in custom routes. Once you retrieve the Shrine::UploadedFile object, you can call #derivation_response directly on it, passing the derivation name and arguments, as well as the Rack env hash.

# config/routes.rb
Rails.application.routes.draw do
  resources :photos do
    member do
      get "thumbnail" # for example
    end
  end
end

# app/controllers/photos_controller.rb
class PhotosController < ApplicationController
  def thumbnail
    # we can perform authorization here
    photo = Photo.find(params[:id])
    image = photo.image

    set_rack_response image.derivation_response(:thumbnail, 300, 300, env: request.env)
  end

  private

  def set_rack_response((status, headers, body))
    self.status = status
    self.headers.merge!(headers)
    self.response_body = body
  end
end

Shrine.derivation_endpoint, Shrine.derivation_response, and UploadedFile#derivation_response methods all accept additional options, which will override options set on the plugin level.

ImageUploader.derivation_endpoint(disposition: "attachment")
# or
ImageUploader.derivation_response(env, disposition: "attachment")
# or
uploaded_file.derivation_response(:thumbnail, env: env, disposition: "attachment")

Dynamic settings

For most options passed to plugin :derivation_endpoint, Shrine.derivation_endpoint, Shrine.derivation_response, or Shrine::UploadedFile#derivation_response, the value can also be a block that returns a dynamic result. The block will be evaluated within the context of a Shrine::Derivation instance, allowing you to access information about the current derivation:

plugin :derivation_endpoint, disposition: -> {
  self   #=> #<Shrine::Derivation>

  name   #=> :thumbnail
  args   #=> ["500", "400"]
  source #=> #<Shrine::UploadedFile>

  # ...
}

For example, we can use it to specify that thumbnails should be rendered inline in the browser, while other derivatives will be force downloaded.

plugin :derivation_endpoint, disposition: -> {
  name == :thumbnail ? "inline" : "attachment"
}

Host

Derivation URLs are relative by default. To generate absolute URLs, you can pass the :host option:

plugin :derivation_endpoint, host: "https://example.com"

Now the generated URLs will include the specified URL host:

uploaded_file.derivation_url(:thumbnail)
#=> "https://example.com/.../thumbnail/eyJpZCI6ImZvbyIsInN?signature=..."

You can also pass :host per URL:

uploaded_file.derivation_url(:thumbnail, host: "https://example.com")
#=> "https://example.com/.../thumbnail/eyJpZCI6ImZvbyIsInN?signature=..."

Prefix

If you're mounting the derivation endpoint under a path prefix, the derivation URLs will need to include that path prefix. This can be configured with the :prefix option:

plugin :derivation_endpoint, prefix: "transformations/image"

Now generated URLs will include the specified path prefix:

uploaded_file.derivation_url(:thumbnail)
#=> ".../transformations/image/thumbnail/eyJpZCI6ImZvbyIsInN?signature=..."

You can also pass :prefix per URL:

uploaded_file.derivation_url(:thumbnail, prefix: "transformations/image")
#=> ".../transformations/image/thumbnail/eyJpZCI6ImZvbyIsInN?signature=..."

Expiration

By default derivation URLs are valid indefinitely. If you want URLs to expire after a certain amount of time, you can set the :expires_in option:

plugin :derivation_endpoint, expires_in: 90

Now any URL will stop being valid 90 seconds after it was generated:

uploaded_file.derivation_url(:thumbnail)
#=> ".../thumbnail/eyJpZCI6ImZvbyIsInN?expires_at=1547843568&signature=..."

You can also pass :expires_in per URL:

uploaded_file.derivation_url(:thumbnail, expires_in: 90)
#=> ".../thumbnail/eyJpZCI6ImZvbyIsInN?expires_at=1547843568&signature=..."

Response headers

Content Type

The derivation response includes the Content-Type header. By default default its value will be inferred from the file extension of the generated derivative (using Rack::Mime). This can be overriden with the :type option:

plugin :derivation_endpoint, type: -> { "image/webp" if name == :webp }

The above will set Content-Type response header value to image/webp for :webp derivatives, while for others it will be inferred from the file extension if possible.

You can also set :type per URL:

uploaded_file.derivation_url(:webp, type: "image/webp")
#=> ".../webp/eyJpZCI6ImZvbyIsInN?type=image%2Fwebp&signature=..."

Content Disposition

The derivation response includes the Content-Disposition header. By default the disposition is set to inline, with download filename generated from derivation name, arguments and source file id. These values can be changed with the :disposition and :filename options:

plugin :derivation_endpoint,
  disposition: -> { name == :thumbnail ? "inline" : "attachment" },
  filename:    -> { [name, *args].join("-") }

With the above settings, visiting a thumbnail URL will render the image in the browser, while other derivatives will be treated as an attachment and be downloaded.

The :filename and :disposition options can also be set per URL:

uploaded_file.derivation_url(:pdf, disposition: "attachment", filename: "custom-filename")
#=> ".../thumbnail/eyJpZCI6ImZvbyIsInN?disposition=attachment&filename=custom-filename&signature=..."

Cache Control

The endpoint uses the Cache-Control response header to tell clients (browsers, CDNs, HTTP caches) how long they can cache derivation responses. The default cache duration is 1 year from the initial request, or if :expires_in is used it's the time until the URL expires. The header value can be changed with the :cache_control option:

plugin :derivation_endpoint, cache_control: "public, max-age=#{7*24*60*60}" # 7 weeks

Note that Cache-Control is added to response headers only when using Shrine.derivation_endpoint or Shrine.derivation_response, it's not added when using Shrine::UploadedFile#derivation_response.

Uploading

By default the generated derivatives aren't saved anywhere, which means that repeated requests to the same derivation URL will call the derivation block each time. If you don't want to rely on solely on your HTTP cache, you can enable the :upload option, which will make derivatives automatically cached on the Shrine storage:

plugin :derivation_endpoint, upload: true

Now whenever a derivation is requested, the endpoint will first check whether the derivative already exists on the storage. If it doesn't exist, it will fetch the original uploaded file, call the derivation block, upload the derivative to the storage, and serve the derivative. If the derivative does exist on checking, the endpoint will download the derivative and serve it.

The default upload location for derivatives is <source id>/<name>-<args>. This can be changed with the :upload_location option:

plugin :derivation_endpoint, upload: true, upload_location: -> {
  # e.g. "derivatives/9a7d1bfdad24a76f9cfaff137fe1b5c7/thumbnail-1000-800"
  ["derivatives", File.basename(source.id, ".*"), [name, *args].join("-")].join("/")
}

Since the default upload location won't have any file extension, the derivation response won't know the appropriate Content-Type header value to set, and the generic application/octet-stream will be used. It's recommended to use the :type option to set the appropriate Content-Type value.

The target storage used is the same as for the source uploaded file. The :upload_storage option can be used to specify a different Shrine storage:

plugin :derivation_endpoint, upload: true,
                             upload_storage: :thumbnail_storage

Additional storage-specific upload options can be passed via :upload_options:

plugin :derivation_endpoint, upload: true,
                             upload_options: { acl: "public-read" }

Additional storage-specific download options for the uploaded derivation result can be passed via :upload_open_options:

plugin :derivation_endpoint, upload: true,
                             upload_open_options: { response_content_encoding: "gzip" }

Redirecting

You can configure the endpoint to redirect to the uploaded derivative on the storage instead of serving it through the endpoint (which is the default behaviour) by setting both :upload and :upload_redirect to true:

plugin :derivation_endpoint, upload: true,
                             upload_redirect: true

In that case additional storage-specific URL options can be passed in for the redirect URL:

plugin :derivation_endpoint, upload: true,
                             upload_redirect: true,
                             upload_redirect_url_options: { public: true }

Cache busting

The derivation endpoint response instructs browsers, CDNs and other clients to cache the response for a long time. This saves server resources and improves response times. However, if the derivation block is modified, the derivation URLs will remain unchanged, which means that old cached derivatives might still be served.

If you want to ensure derivation URLs don't point to old cached derivatives, you can add a "version" query parameter to the URL, which will make HTTP caches treat it as a new URL. You can do this via the :version option:

plugin :derivation_endpoint, version: -> { 1 if name == :thumbnail }

With the above settings, all :thumbnail derivation URLs will include version in the query string:

uploaded_file.derivation_url(:thumbnail)
#=> ".../thumbnail/eyJpZCI6ImZvbyIsInN?version=1&signature=..."

You can also bump the :version per URL:

uploaded_file.derivation_url(:thumbnail, version: 1)
#=> ".../thumbnail/eyJpZCI6ImZvbyIsInN?version=1&signature=..."

Accessing source file

If you want to access the source UploadedFile object when deriving, you can set :include_uploaded_file to true.

plugin :derivation_endpoint, include_uploaded_file: true

Now the source UploadedFile will be passed as the second argument of the derivation block:

derivation :thumbnail do |file, uploaded_file, width, height|
  uploaded_file             #=> #<Shrine::UploadedFile>
  uploaded_file.id          #=> "9a7d1bfdad24a76f9cfaff137fe1b5c7.jpg"
  uploaded_file.storage_key #=> "store"
  uploaded_file.metadata    #=> {}

  # ...
end

By default original metadata that were extracted on attachment won't be available in the derivation block. This is because metadata we want to have available would need to be serialized into the derivation URL, which would make it longer. However, you can opt in for the metadata you need with the :metadata option:

plugin :derivation_endpoint, metadata: ["filename", "mime_type"]

Now filename and mime_type metadata values will be available in the derivation block:

derivation :thumbnail do |file, uploaded_file, width, height|
  uploaded_file.metadata #=>
  # {
  #  "filename" => "nature.jpg",
  #  "mime_type" => "image/jpeg"
  # }

  uploaded_file.original_filename #=> "nature.jpg"
  uploaded_file.mime_type         #=> "image/jpeg"

  # ...
end

Downloading

When a derivation is requested, the original uploaded file will be downloaded to disk before the derivation block is called. If you want to pass in additional storage-specific download options, you can do so via :download_options:

plugin :derivation_endpoint, download_options: {
  sse_customer_algorithm: "AES256",
  sse_customer_key:       "secret_key",
  sse_customer_key_md5:   "secret_key_md5",
}

If the source file has been deleted, the error the storage raises when attempting to download it will be propagated by default. For Shrine.derivation_endpoint and Shrine.derivation_response you can have these errors converted to 404 responses by adding them to :download_errors:

plugin :derivation_endpoint, download_errors: [
  Errno::ENOENT,              # raised by Shrine::Storage::FileSystem
  Aws::S3::Errors::NoSuchKey, # raised by Shrine::Storage::S3
]

Skipping download

If for whatever reason you don't want the uploaded file to be downloaded to disk for you, you can set :download to false.

plugin :derivation_endpoint, download: false

In this case the UploadedFile object is yielded to the derivation block instead of the raw file:

derivation :thumbnail do |uploaded_file, width, height|
  uploaded_file #=> #<Shrine::UploadedFile>

  # ...
end

One use case for this is delegating processing to a 3rd-party service:

require "down/http"

derivation :thumbnail do |uploaded_file, width, height|
  # generate the thumbnail using ImageOptim.com
  down = Down::Http.new(method: :post)
  down.download("https://im2.io/<USERNAME>/#{width}x#{height}/#{uploaded_file.url}")
end

Derivation API

In addition to generating derivation responses, it's also possible to operate with derivations on a lower level. You can access that API by calling UploadedFile#derivation, which returns a Derivation object.

derivation = uploaded_file.derivation(:thumbnail, 500, 500)
derivation #=> #<Shrine::Derivation: @name=:thumbnail, @args=[500, 500] ...>
derivation.name   #=> :thumbnail
derivation.args   #=> [500, 500]
derivation.source #=> #<Shrine::UploadedFile>

When initializing the Derivation object you can override any plugin options:

uploaded_file.derivation(:grayscale, upload_storage: :other_storage)

`#url`

Derivation#url method (called by UploadedFile#derivation_url) generates the URL to the derivation.

derivation.url #=> "/thumbnail/500/400/eyJpZCI6ImZvbyIsInN0b3JhZ2UiOiJzdG9yZSJ9?signature=..."

`#response`

Derivation#response method (called by UploadedFile#derivation_response) generates appropriate status, headers, and body for the derivative to be returned as an HTTP response.

status, headers, body = derivation.response
status  #=> 200
headers #=>
# {
#   "Content-Type" => "image/jpeg",
#   "Content-Length" => "12424",
#   "Content-Disposition" => "inline; filename=\"thumbnail-500-500-k9f8sdksdfk2414\"",
#   "Accept_Ranges" => "bytes"
# }
body    #=> #each object that yields derivative content

`#processed`

Derivation#processed method returns the processed derivative. If :upload is enabled, it returns a Shrine::UploadedFile object pointing to the derivative, processing and uploading the derivative if it hasn't been already.

uploaded_file = derivation.processed
uploaded_file    #=> #<Shrine::UploadedFile>
uploaded_file.id #=> "bcfd0d67e4a8ec2dc9a6d7ddcf3825a1/thumbnail-500-500"

`#generate`

Derivation#generate method calls the derivation block and returns the result.

result = derivation.generate
result #=> #<Tempfile:...>

Internally it will download the source uploaded file to disk and pass it to the derivation block (unless :download was disabled). You can also pass in an already downloaded source file:

derivation.generate(source_file)

`#upload`

Derivation#upload method uploads the given file to the configured derivation location.

uploaded_file = derivation.upload(file)
uploaded_file    #=> #<Shrine::UploadedFile>
uploaded_file.id #=> "bcfd0d67e4a8ec2dc9a6d7ddcf3825a1/thumbnail-500-500"

If not given any arguments, it generates the derivative before uploading it.

`#retrieve`

Derivation#retrieve method returns the uploaded derivative file. If the file exists on the storage, it returns an UploadedFile object, otherwise nil is returned.

uploaded_file = derivation.retrieve
uploaded_file    #=> #<Shrine::UploadedFile>
uploaded_file.id #=> "bcfd0d67e4a8ec2dc9a6d7ddcf3825a1/thumbnail-500-500"

`#delete`

Derivation#delete method deletes the uploaded derivative file from the storage.

derivation.delete

`#option`

Derivation#option returns the value of the specified plugin option.

derivation.option(:upload_location)
#=> "bcfd0d67e4a8ec2dc9a6d7ddcf3825a1/thumbnail-500-500"

Plugin Options

Name	Description	Default
`:cache_control`	Hash of directives for the `Cache-Control` response header	`{ public: true, max_age: 3652460*60 }`
`:disposition`	Whether the browser should attempt to render the derivative (`inline`) or prompt the user to download the file to disk (`attachment`)	`inline`
`:download`	Whether the source uploaded file should be downloaded to disk when the derivation block is called	`true`
`:download_errors`	List of error classes that will be converted to a `404 Not Found` response by the derivation endpoint	`[]`
`:download_options`	Additional options to pass when downloading the source uploaded file	`{}`
`:expires_in`	Number of seconds after which the URL will not be available anymore	`nil`
`:filename`	Filename the browser will assume when the derivative is downloaded to disk	`<name>-<args>-<source id basename>`
`:host`	URL host to use when generated URLs	`nil`
`:include_uploaded_file`	Whether to include the source uploaded file in the derivation block arguments	`false`
`:metadata`	List of metadata keys the source uploaded file should include in the derivation block	`[]`
`:prefix`	Path prefix added to the URLs	`nil`
`:secret_key`	Key used to sign derivation URLs in order to prevent tampering	required
`:type`	Media type returned in the `Content-Type` response header in the derivation response	determined from derivative's extension when possible
`:upload`	Whether the generated derivatives will be cached on the storage	`false`
`:upload_location`	Location to which the derivatives will be uploaded on the storage	`<source id>/<name>-<args>`
`:upload_options`	Additional options to be passed when uploading derivatives	`{}`
`:upload_open_options`	Additional options to be passed when downloading the uploaded derivative	`{}`
`:upload_redirect`	Whether the derivation response should redirect to the uploaded derivative	`false`
`:upload_redirect_url_options`	Additional options to be passed when generating the URL for the uploaded derivative	`{}`
`:upload_storage`	Storage to which the derivations will be uploaded	same storage as the source file
`:version`	Version number to append to the URL for cache busting	`nil`

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Derivation Endpoint

Quick start

How it works

Performance

Derivation response

Dynamic settings

Host

Prefix

Expiration

Response headers

Content Type

Content Disposition

Cache Control

Uploading

Redirecting

Cache busting

Accessing source file

Downloading

Skipping download

Derivation API

`#url`

`#response`

`#processed`

`#generate`

`#upload`

`#retrieve`

`#delete`

`#option`

Plugin Options

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Derivation Endpoint

Quick start

How it works

Performance

Derivation response

Dynamic settings

Host

Prefix

Expiration

Response headers

Content Type

Content Disposition

Cache Control

Uploading

Redirecting

Cache busting

Accessing source file

Downloading

Skipping download

Derivation API

#url

#response

#processed

#generate

#upload

#retrieve

#delete

#option

Plugin Options

`#url`

`#response`

`#processed`

`#generate`

`#upload`

`#retrieve`

`#delete`

`#option`