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File Processing

Shrine allows you to process attached files eagerly or on-the-fly. For example, if your app is accepting image uploads, you can generate a predefined set of of thumbnails when the image is attached to a record, or you can have thumbnails generated dynamically as they're needed.

How you're going to implement processing is entirely up to you. For images it's recommended to use the ImageProcessing gem, which provides wrappers for processing with MiniMagick and libvips. Here is an example of generating a thumbnail with ImageProcessing:

$ brew install imagemagick
# Gemfile
gem "image_processing", "~> 1.8"
require "image_processing/mini_magick"

thumbnail = ImageProcessing::MiniMagick
  .source(image)              # input file
  .resize_to_limit(600, 400)  # resize macro
  .colorspace("grayscale")    # custom operation
  .convert("jpeg")            # output type
  .saver(quality: 90)         # output options
  .call                       # run the pipeline

thumbnail #=> #<Tempfile:...> (a 600x400 thumbnail of the source image)

Eager processing

Let's say we're handling images, and want to generate a predefined set of thumbnails with various dimensions. We can use the derivatives plugin to upload and save the processed files:

Shrine.plugin :derivatives
require "image_processing/mini_magick"

class ImageUploader < Shrine
  Attacher.derivatives do |original|
    magick = ImageProcessing::MiniMagick.source(original)

    {
      large:  magick.resize_to_limit!(800, 800),
      medium: magick.resize_to_limit!(500, 500),
      small:  magick.resize_to_limit!(300, 300),
    }
  end
end
photo = Photo.new(image: file)
photo.image_derivatives! # calls derivatives processor
photo.save

After the processed files are uploaded, their data is saved into the <attachment>_data column. You can then retrieve the derivatives as Shrine::UploadedFile objects:

photo.image(:large)            #=> #<Shrine::UploadedFile ...>
photo.image(:large).url        #=> "/uploads/store/lg043.jpg"
photo.image(:large).size       #=> 5825949
photo.image(:large).mime_type  #=> "image/jpeg"

Conditional derivatives

The Attacher.derivatives block is evaluated in context of a Shrine::Attacher instance:

Attacher.derivatives do |original|
  self    #=> #<Shrine::Attacher>

  file    #=> #<Shrine::UploadedFile>
  record  #=> #<Photo>
  name    #=> :image
  context #=> { ... }

  # ...
end

This gives you the ability to branch the processing logic based on the attachment information:

Attacher.derivatives do |original|
  magick = ImageProcessing::MiniMagick.source(original)
  result = {}

  if record.is_a?(Photo)
    result[:jpg]  = magick.convert!("jpeg")
    result[:gray] = magick.colorspace!("grayscale")
  end

  if file.mime_type == "image/svg+xml"
    result[:png] = magick.loader(transparent: "white").convert!("png")
  end

  result
end

The type_predicates plugin provides convenient predicate methods for branching based on the file type.

Backgrounding

Since file processing can be time consuming, it's recommended to move it into a background job.

A) Creating derivatives with promotion

The simplest way is to use the backgrounding plugin to move promotion into a background job, and then create derivatives as part of promotion:

Shrine.plugin :backgrounding
Shrine::Attacher.promote_block do
  PromoteJob.perform_async(self.class.name, record.class.name, record.id, name, file_data)
end
class PromoteJob
  include Sidekiq::Worker

  def perform(attacher_class, record_class, record_id, name, file_data)
    attacher_class = Object.const_get(attacher_class)
    record         = Object.const_get(record_class).find(record_id) # if using Active Record

    attacher = attacher_class.retrieve(model: record, name: name, file: file_data)
    attacher.create_derivatives # calls derivatives processor
    attacher.atomic_promote
  rescue Shrine::AttachmentChanged, ActiveRecord::RecordNotFound
    # attachment has changed or the record has been deleted, nothing to do
  end
end

B) Creating derivatives separately from promotion

Derivatives don't need to be created as part of the attachment flow, you can create them at any point after promotion:

DerivativesJob.perform_async(
  attacher.class.name,
  attacher.record.class.name,
  attacher.record.id,
  attacher.name,
  attacher.file_data,
)
class DerivativesJob
  include Sidekiq::Worker

  def perform(attacher_class, record_class, record_id, name, file_data)
    attacher_class = Object.const_get(attacher_class)
    record         = Object.const_get(record_class).find(record_id) # if using Active Record

    attacher = attacher_class.retrieve(model: record, name: name, file: file_data)
    attacher.create_derivatives # calls derivatives processor
    attacher.atomic_persist
  rescue Shrine::AttachmentChanged, ActiveRecord::RecordNotFound
    attacher&.destroy_attached # delete now orphaned derivatives
  end
end

C) Creating derivatives concurrently

You can also generate derivatives concurrently:

class ImageUploader < Shrine
  THUMBNAILS = {
    large:  [800, 800],
    medium: [500, 500],
    small:  [300, 300],
  }

  Attacher.derivatives do |original, name:|
    thumbnail = ImageProcessing::MiniMagick
      .source(original)
      .resize_to_limit!(*THUMBNAILS.fetch(name))

    { name => thumbnail }
  end
end
ImageUploader::THUMBNAILS.each_key do |derivative_name|
  DerivativeJob.perform_async(
    attacher.class.name,
    attacher.record.class.name,
    attacher.record.id,
    attacher.name,
    attacher.file_data,
    derivative_name,
  )
end
class DerivativeJob
  include Sidekiq::Worker

  def perform(attacher_class, record_class, record_id, name, file_data, derivative_name)
    attacher_class = Object.const_get(attacher_class)
    record         = Object.const_get(record_class).find(record_id) # if using Active Record

    attacher = attacher_class.retrieve(model: record, name: name, file: file_data)
    attacher.create_derivatives(name: derivative_name)
    attacher.atomic_persist do |reloaded_attacher|
      # make sure we don't override derivatives created in other jobs
      attacher.merge_derivatives(reloaded_attacher.derivatives)
    end
  rescue Shrine::AttachmentChanged, ActiveRecord::RecordNotFound
    attacher.derivatives[derivative_name].delete # delete now orphaned derivative
  end
end

URL fallbacks

If you're creating derivatives in a background job, you'll likely want to use some fallbacks for derivative URLs while the background job is still processing. You can do that with the default_url plugin.

Shrine.plugin :default_url

A) Fallback to original

You can fall back to the original file URL when the derivative is missing:

Attacher.default_url do |derivative: nil, **|
  file&.url if derivative
end
photo.image_url(:large) #=> "https://example.com/path/to/original.jpg"
# ... background job finishes ...
photo.image_url(:large) #=> "https://example.com/path/to/large.jpg"

B) Fallback to derivative

You can fall back to another derivative URL when the derivative is missing:

Attacher.default_url do |derivative: nil, **|
  derivatives[:optimized]&.url if derivative
end
photo.image_url(:large) #=> "https://example.com/path/to/optimized.jpg"
# ... background job finishes ...
photo.image_url(:large) #=> "https://example.com/path/to/large.jpg"

C) Fallback to on-the-fly

You can also fall back to on-the-fly processing, which should generally provide the best user experience.

THUMBNAILS = {
  small:  [300, 300],
  medium: [500, 500],
  large:  [800, 800],
}

Attacher.default_url do |derivative: nil, **|
  file&.derivation_url(:thumbnail, *THUMBNAILS.fetch(derivative)) if derivative
end
photo.image_url(:large) #=> "../derivations/thumbnail/800/800/..."
# ... background job finishes ...
photo.image_url(:large) #=> "https://example.com/path/to/large.jpg"

On-the-fly processing

Having eagerly created image thumbnails can be a pain to maintain, because whenever you need to add a new version or change an existing one, you need to retroactively apply it to all existing attachments (see the Managing Derivatives guide for more details).

Sometimes it makes more sense to generate thumbnails dynamically as they're requested, and then cache them for future requests. This strategy is known as processing "on-the-fly" or "on-demand", and it's suitable for short-running processing such as creating image thumbnails or document previews.

Shrine provides on-the-fly processing functionality via the derivation_endpoint plugin. You set it up by loading the plugin with a secret key and a path prefix, mount its Rack app in your routes on the configured path prefix, and define processing you want to perform:

# config/initializers/shrine.rb (Rails)
# ...
Shrine.plugin :derivation_endpoints, secret_key: "<YOUR_SECRET_KEY>"
require "image_processing/mini_magick"

class ImageUploader < Shrine
  plugin :derivation_endpoint, prefix: "derivations/image" # matches mount point

  derivation :thumbnail do |file, width, height|
    ImageProcessing::MiniMagick
      .source(file)
      .resize_to_limit!(width.to_i, height.to_i)
  end
end
# config/routes.rb (Rails)
Rails.application.routes.draw do
  # ...
  mount ImageUploader.derivation_endpoint => "/derivations/image"
end

Now you can generate thumbnail URLs from attached files, and the actual thumbnail will be generated when the URL is requested:

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

The plugin is highly customizable, be sure to check out the documentation, especially the performance section.

Dynamic derivation

If you have multiple types of transformations and don't want to have a derivation for each one, you can set up a single derivation that applies any series of transformations:

class ImageUploader < Shrine
  derivation :transform do |original, transformations|
    transformations = Shrine.urlsafe_deserialize(transformations)

    vips = ImageProcessing::Vips.source(original)
    vips.apply!(transformations)
  end
end
photo.image.derivation_url :transform, Shrine.urlsafe_serialize(
  crop:          [10, 10, 500, 500],
  resize_to_fit: [300, 300],
  gaussblur:     1,
)

You can create a helper method for convenience:

def derivation_url(file, transformations)
  file.derivation_url(:transform, Shrine.urlsafe_serialize(transformations))
end
derivation_url photo.image,
  crop:          [10, 10, 500, 500],
  resize_to_fit: [300, 300],
  gaussblur:     1

Processing other filetypes

So far we've only been talking about processing images. However, there is nothing image-specific in Shrine's processing API, you can just as well process any other types of files. The processing tool doesn't need to have any special Shrine integration, the ImageProcessing gem that we saw earlier is a completely generic gem.

To demonstrate, here is an example of transcoding videos using streamio-ffmpeg:

# Gemfile
gem "streamio-ffmpeg"
require "streamio-ffmpeg"

class VideoUploader < Shrine
  Attacher.derivatives do |original|
    transcoded = Tempfile.new ["transcoded", ".mp4"]
    screenshot = Tempfile.new ["screenshot", ".jpg"]

    movie = FFMPEG::Movie.new(original.path)
    movie.transcode(transcoded.path)
    movie.screenshot(screenshot.path)

    { transcoded: transcoded, screenshot: screenshot }
  end
end

Polymorphic uploader

Sometimes you might want an attachment attribute to accept multiple types of files, and apply different processing depending on the type. Since Shrine's processing blocks are evaluated dynamically, you can use conditional logic:

class PolymorphicUploader < Shrine
  IMAGE_TYPES = %w[image/jpeg image/png image/webp]
  VIDEO_TYPES = %w[video/mp4 video/quicktime]
  PDF_TYPES   = %w[application/pdf]

  Attacher.validate do
    validate_mime_type IMAGE_TYPES + VIDEO_TYPES + PDF_TYPES
    # ...
  end

  Attacher.derivatives do |original|
    case file.mime_type
    when *IMAGE_TYPES then process_derivatives(:image, original)
    when *VIDEO_TYPES then process_derivatives(:video, original)
    when *PDF_TYPES   then process_derivatives(:pdf,   original)
    end
  end

  Attacher.derivatives :image do |original|
    # ...
  end

  Attacher.derivatives :video do |original|
    # ...
  end

  Attacher.derivatives :pdf do |original|
    # ...
  end
end

Extras

Automatic derivatives

If you would like derivatives to be automatically created with promotion, you can override Attacher#promote for call Attacher#create_derivatives before promotion:

class Shrine::Attacher
  def promote(*)
    create_derivatives
    super
  end
end

This shouldn't be needed if you're processing in the background, as in that case you have a background worker that will be called for each attachment, so you can call Attacher#create_derivatives there.

libvips

As mentioned, ImageProcessing gem also has an alternative backend for processing images with libvips. libvips is a full-featured image processing library like ImageMagick, with impressive performance characteristics – it's often multiple times faster than ImageMagick and has low memory usage (see Why is libvips quick).

Using libvips is as easy as installing it and switching to the ImageProcessing::Vips backend:

$ brew install vips
# Gemfile
gem "image_processing", "~> 1.8"
require "image_processing/vips"

# all we did was replace `ImageProcessing::MiniMagick` with `ImageProcessing::Vips`
thumbnail = ImageProcessing::Vips
  .source(image)
  .resize_to_limit!(600, 400)

thumbnail #=> #<Tempfile:...> (a 600x400 thumbnail of the source image)

Parallelize uploads

If you're generating derivatives, you can parallelize the uploads using the concurrent-ruby gem:

# Gemfile
gem "concurrent-ruby"
require "concurrent"

derivatives = attacher.process_derivatives

tasks = derivatives.map do |name, file|
  Concurrent::Promises.future(name, file) do |name, file|
    attacher.add_derivative(name, file)
  end
end

Concurrent::Promises.zip(*tasks).wait!

External processing

You can also integrate Shrine with 3rd-party processing services such as Cloudinary and Imgix. In the most common case, you'd serve images directly from these services, see the corresponding plugin docs for more details (shrine-cloudinary, shrine-imgix and others)

You can also choose to use these services as an implementation detail of your application, by downloading the processed images and saving them to your storage. Here is how you might store files processed by Imgix as derivatives:

# Gemfile
gem "down", "~> 5.0"
gem "http", "~> 4.0"
gem "shrine-imgix", "~> 0.5"
Shrine.plugin :derivatives
Shrine.plugin :imgix, client: { host: "my-app.imgix.net", secure_url_token: "secret" }
require "down/http"

class ImageUploader < Shrine
  IMGIX_THUMBNAIL = -> (file, width, height) do
    Down::Http.download(file.imgix_url(w: width, h: height))
  end

  Attacher.derivatives do
    {
      large:  IMGIX_THUMBNAIL[file, 800, 800],
      medium: IMGIX_THUMBNAIL[file, 500, 500],
      small:  IMGIX_THUMBNAIL[file, 300, 300],
    }
  end
end
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