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Better File Uploads with Shrine: Motivation

By Janko Marohnić on shrine

It’s been over a year since I started working on Shrine, and during that time Shrine has acquired a lot of cool features, the ecosystem has grown significantly, and many people are already using Shrine in production.

Before I go in depth explaining all the cool things that you can do with Shrine, I thought it would be good to take a step back, and first explain what was my motivation to create Shrine in the first place.

Specifically, I want to talk about limitations of existing file attachment libraries. I think it’s important to be aware of these limitations, so that you can make a choice which best meets your requirements.


My requirements were the following:

  1. files should be uploaded directly to Amazon S3
  2. processing and deleting files should happen in a background job
  3. processing can happen on upload or on-the-fly
  4. has to integrate with the Sequel ORM
  5. has to work in a web framework which isn’t Rails

In my opinion the first two requirements should be very common, because that way you achieve the best performance and user experience, but the last two also shouldn’t be unusual:

1. Uploading files directly to Amazon S3 or another storage service frees your application from accepting file uploads. This has several benefits: your server uses less resources, works with multiple servers which don’t share the filesystem, works with Heroku and other hosting services which don’t allow saving to disk or have a 30-second request timeout.

2. Offloading processing and deleting files to background jobs enables managing file attachments to be completely asynchronous, regardless of whether you’re storing on the local filesystem or to an external storage service like Amazon S3, which significantly improves the user experience because there is no waiting. Using background jobs is also necessary for maintaining high throughput of your application, because your request instances won’t be tied waiting for slow tasks.

3. Doing processing on-the-fly can really work great for small files like images, especially if you want to generate many sizes for different pages or devices. On the other hand, processing on upload is still necessary for videos or other large files. Therefore, I want my file attachment library to allow both, so that I can use it for any type of files.

4. Using the Sequel ORM also makes a lot of sense, because Sequel is objectively a better ORM than ActiveRecord (see why Sequel ≻ ActiveRecord, and why ActiveRecord ⊁ Sequel). If a Sequel integration for the file attachment library doesn’t exist, it should at least be straightforward to write one.

5. Finally, there are many reasons for using other web frameworks than Rails. For example, Roda has really advanced routing, and routing is coincidentally the most important part of a web framework (all other parts are interchangable). So I need the file attachment library to be easy to use in any web framework.

Now I want walk through existing file attachment libraries, and explain what were the major limitations for me, with a focus on the above requirements.


Easy file attachment management for ActiveRecord

Ok, it looks like we can immediately say goodbye to using Paperclip with Sequel. But let’s continue going through other requirements, since the majority of you are using ActiveRecord anyway.

Direct uploads

Paperclip doesn’t have a solution for handling direct S3 uploads. We could still use aws-sdk to generate URL and parameters for direct upload to S3, and then update Paperclip columns directly, in the same way that Paperclip would update them if we uploaded the file through Paperclip.

However, since Paperclip has only one main storage, for this to work the direct uploads should go directly to your main S3 storage. And that’s a security problem, because an attacker can upload many files without attaching them, and then you have many orphan files on S3 which will be difficult to find and delete if your app receives a lot of traffic. It would be much easier if you could just have S3 do it for you.


For processing in backgrounding there is delayed_paperclip. However, delayed_paperclip will spawn a background job only after the file is uploaded. This means that if you don’t want or can’t do direct S3 uploads, your users have to wait for the file to be uploaded twice (first to the app, then to the storage), before any backgrounding even takes place. And that is really slow.

Furthermore, delayed_paperclip doesn’t support deleting files in the background. This is a big disadvantage if you have multiple versions stored on S3, because that’s one HTTP request per version. No, wait, two HTTP requests per version, because Paperclip also checks whether each version exists before deleting. Sure, you could disable file deletions, but then you have the same problem with orphan files.

Finally delayed_paperclip is now tied to ActiveJob, which means that you cannot use it with a backgrounding libary directly, and that’s exactly what I want to do in non-Rails projects.

MIME type spoofing

Paperclip has the feature to detect whether someone is trying to spoof the MIME type of the file, e.g. if someone tries to upload a PHP file with a .jpg extension. However, this feature is known for many false positives, meaning it can raise a validation error even when file extension matches the content of the file. This is a dealbreaker for me, because in that case there is no way for the user to correct their input.

I could disable this feature, but I don’t want to leave my app vulnerable to basic file upload attacks.


Classier solution for file uploads for Rails, Sinatra and other Ruby web frameworks

CarrierWave was an answer to Paperclip’s hash configuration in the models, and introduced better encapsulation via uploader classes.

CarrierWave does have a Sequel integration, which was a big improvement for me. Unfortunately, carrierwave_backgrounder and carrierwave_direct, the CarrierWave extensions that I wanted to use, both didn’t manage to rely only on CarrierWave’s ORM integration, and needed a lot of additional ActiveRecord-specific code to achieve their functionality. I know ActiveRecord is the most popular ORM, but people do use other ORMs for various reasons, and this pretty-much paints them into a corner.

Direct uploads

As mentioned above, the CarrierWave ecosystem has a solution for direct S3 uploads – carrierwave_direct. It works in a way that it allows you to generate a form for direct S3 upload, and then assign the S3 key of the uploaded file to your uploader.

<!-- Form submits to "https://my-bucket.s3-eu-west-1.amazonaws.com" -->
<%= direct_upload_form_for @photo.image do |f| %>
  <%= f.file_field :image %>
  <%= f.submit %>
<% end %>

However, what if you need to do multiple uploads directly to S3? The README notes that carrierwave_direct is only for single uploads. Also, what about JSON APIs? This form isn’t magical, all it does is generates URL and parameters for the upload to S3, so why doesn’t carrierwave_direct allow retrieving this information in JSON format?

What if carrierwave_direct, instead of reimplementing the whole logic of generating S3 request parameters using fog-aws, simply relied on aws-sdk?

# aws-sdk
bucket  = s3.bucket("my-bucket")
object  = bucket.object(SecureRandom.hex)
presign = object.presigned_post
<!-- HTML version -->
<form action="<%= presign.url %>" method="post" enctype="multipart/form-data">
  <input type="file" name="file">
  <% presign.fields.each do |name, value| %>
    <input type="hidden" name="<%= name %>" value="<%= value %>">
  <% end %>
  <input type="submit" value="Upload">
# JSON version
{ "url": presign.url, "fields": presign.fields }

This way has the following advantages: it’s not Rails-specific, it works with JSON APIs, it supports multiple file uploads (the client can just make a request for this data for each file), and it’s more reliable (since now the presign is generated by a well-maintained gem).


Firstly, it’s worth noting that carrierwave_direct provides instructions how to set up background processing. However, setting up backgrounding reliably is a very complicated task, it makes much more sense to rely on a library that does it for you.

Which brings us to carrierwave_backgrounder. This library supports background processing, but in my experience it has been unreliable (1, then 2). Also, it doesn’t support deleting files in the background, which is a dealbreaker if I have multiple versions stored on S3.

Even if we get past all that, how do you integrate carrierwave_backgrounder with carrierwave_direct? As I mentioned, I want to upload files directly to S3 AND have processing and deleting done in a background job. But it seems like these two libraries aren’t compatible with each other, which means that I cannot achieve the desired performance with CarrierWave for the most common use case.

Closing unresolved issues

I’m aware that being an open source maintainer of a popular library can be an ungrateful task, and that we should always be nice to each other. However, I can’t understand why do CarrierWave maintainers close unresolved issues. It seems that bug reports are likely closed if (a) the bug report isn’t a PR or (b) the maintainers aren’t sure if it’s a bug. Neither of these two are valid reasons to close an issue.

One of these closed issues is about CarrierWave performing processing before validations. This is a huge security issue, because it means that attackers are able to give any file to your image processing tool, since any filesize/MIME/dimensions validations will be performed only after processing. That makes your app wide open to attacks like ImageTragick, image bombs, or just large image uploads.


Ruby file uploads, take 3

Refile was created by Jonas Nicklas, the author of CarrierWave, as a 3rd attempt at solving file uploads in Ruby. Like Dragonfly, Refile is designed for on-the-fly processing. Having had enough of CarrierWave’s complexity, I found Refile’s simple and modern design really promising, so I started contributing to it, and eventually I was invited to the core by Jonas.

Refile.attachment_url(@photo, :image, :fit, 400, 500) # resize to 400x500
#=> "/attachments/15058dc712/store/fit/400/500/ed3153b9cb"

Some of Refile’s awesome new ideas include: temporary and permanent storage as first-class citizens, clean storage abstraction, the IO abstraction, clean internal design (no god objects), and built-in direct uploads (even to S3). Due to Refile’s clean design, creating a Sequel integration was pretty straightforward.

Direct uploads

Refile is the first file attachment library that came with built-in support for direct uploads, allowing you to asynchronously start uploading the attached file the moment the user selects it. You can either upload the file to Refile’s Rack app, or directly to S3 using Refile’s app to generate the S3 request parameters. It even comes with plug-and-play JavaScript which does everything for you.

<%= form.attachment_field :image, presigned: true %>

There is also one cool performance improvement here. When you’re uploading the file directly to S3, you’re uploading to a bucket/directory which you marked as “temporary”. Then when the validations pass and record is saved, the uploaded file is moved to “permanent” storage. However, if both temporary and permanent storage are on S3, instead of downloading and reuploading, Refile will simply issue an S3 COPY request.

Needless to say, my requirement for direct uploads was satisfied. :ok_hand:


One limitation of Refile is that it doesn’t have support for background jobs. You might think that, since Refile performs processing on-the-fly, and it has the S3 COPY optimization, that a background job isn’t needed here.

However, the S3 COPY request is still an HTTP request and impacts the duration of the form submission. Furthermore, the speed of the S3 COPY request depends on the filesize, so the larger the file is, the slower the S3 COPY request will be.

Also, Amazon S3 is just one of the many cloud storage services out there, you might wish to use a different service which better suits your needs, but which doesn’t have this optimization or even support direct uploads.


I think on-the-fly processing works great for images that are stored locally and are fast to process. However, if you storing originals on S3, then Refile’s app will serve the initial request to a version much slower, since it needs to first download the original from S3. In that case you should already think about adding a background job which preprocesses all versions by hitting their URLs after upload.

If you’re uploading larger files like videos, then it’s usually better to process them on upload instead of on-the-fly. But Refile currently doesn’t support that.


A Ruby gem for on-the-fly processing - suitable for image uploading in Rails, Sinatra and much more!

Dragonfly is another solution for on-the-fly processing, which has been on the scene much longer than Refile, and in my opinion has much more advanced and flexible on-the-fly processing abilities.

Dragonfly doesn’t have a Sequel integration, but that was to be expected and I would be prepared to write one, but the generic model-related behaviour seems to be mixed with behaviour specific to ActiveRecord models, so it’s not clear to me how to do that.

There is also no support for background jobs, nor for direct uploads. You could do the latter manually, but it would have the same downsides as for Paperclip.

But I want you to notice something very important. Retrieving files via an image server (Dragonfly’s on-the-fly processing app) is a completely separate responsibility than uploading. What I mean is that you can use another file attachment library which comes with everything (direct uploads, backgrounding, various ORMs etc.) to upload the files to a storage, and still use Dragonfly for serving these files.

map "/attachments" do
  run Dragonfly.app # doesn't care how the files were uploaded


Yet another approach to file upload

Attache is a relatively new file upload library, also for on-the-fly processing. The difference between Dragonfly and Refile is that it was designed to be run as a separate service, so files are both uploaded and served through the Attache server.

Attache includes an ActiveRecord integration for attaching the uploaded files to database records, and has support for direct uploads. But I’m still missing the ability to put backing up and deleting files into a background job. And also I would like to have the flexibility to process files on upload as well.

Note that, as I already explained with Dragonfly, Attache doesn’t need to bring its own model integration – people can just use Shrine for that. This year I went to RedDotRubyConf in Singapore, where I happened to meet the author of Attache, and after a very fun discussion about how complicated file uploads are, we agreed that it would be beneficial to use Shrine for the file attachment logic, and just plug in Attache as a Shrine backend.

That way Attache can still do what it does best – serve files, but leave the complexity of attaching logic to Shrine. So hopefully we’ll come up with an integration soon.

In conclusion

Support for direct uploads, background processing and deleting, processing on upload or on-the-fly, and ability to use with other ORMs is something that I really expect from my file attachment library. However, none of the existing libraries supported all of these requirements.

Therefore I decided to create a new library, Shrine, building on top of the knowledge from existing file upload libraries. The goal of Shrine is not to be opinionated, to provide features and flexibility that allow you to satisfy every use case in an optimal way.

That is a bold goal, but after 1 year of active development and research, I feel quite confident that I achieved it. Or at least that the possibilities of what you can do are greater than in any other file attachment Ruby library. For the rest of this blog post series I will guide you through all of the cool things that you can do with Shrine, so stay tuned!

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