Active Storage makes it simple to upload and reference files in cloud services, like Amazon S3, Google Cloud Storage or Microsoft Azure Storage and attach those files to Active Records. It also provides a disk service for testing or local deployments, but the focus is on cloud storage.
Files can uploaded from the server to the cloud or directly from the client to the cloud.
Image files can further more be transformed using on-demand variants for quality, aspect ratio, size, or any other
MiniMagick supported transformation.
## Compared to other storage solutions
A key difference to how Active Storage works compared to other attachment solutions in Rails is through the use of built-in [Blob](https://github.com/rails/activestorage/blob/master/app/models/active_storage/blob.rb) and [Attachment](https://github.com/rails/activestorage/blob/master/app/models/active_storage/attachment.rb) models (backed by Active Record). This means existing application models do not need to be modified with additional columns to associate with files. Active Storage uses polymorphic associations via the join model of `Attachment`, which then connects to the actual `Blob`.
These `Blob` models are intended to be immutable in spirit. One file, one blob. You can associate the same blob with multiple application models as well. And if you want to do transformations of a given `Blob`, the idea is that you'll simply create a new one, rather than attempt to mutate the existing (though of course you can delete that later if you don't need it).