Last updated May 27, 2020

Amazon Simple Storage Service (S3) is a “highly durable and available store” that is ideal for storing application content such as media files, static assets, and user uploads.

S3 allows you to offload the storage of static files from your app. This is crucial on Heroku, because your app’s dynos have an ephemeral filesystem. This means that all files that aren’t part of your application’s slug are lost whenever a dyno restarts or is replaced (this happens at least once daily).

Overview

All files in S3 are stored in buckets. Buckets act as a top-level container, much like a directory. All files sent to S3 belong to a bucket, and a bucket’s name must be unique across all of S3.

Access to the S3 API is governed by an Access Key ID and a Secret Access Key. The access key identifies your S3 user account, and the secret key is a password-like credential that should be stored securely.

S3 setup

Enabling an application to use S3 requires that the application have access to the AWS credentials as well as the name of the bucket to store files.

Credentials

Your S3 credentials can be found on the Security Credentials section of the AWS “My Account/Console” menu

Because of the sensitive nature of your S3 credentials, you should never commit them to version control. Instead, set them as the values of config vars for the Heroku apps that will use them.

Use the heroku config:set to set both keys:

$ heroku config:set AWS_ACCESS_KEY_ID=xxx AWS_SECRET_ACCESS_KEY=yyy
Adding config vars and restarting app... done, v21
  AWS_ACCESS_KEY_ID     => xxx
  AWS_SECRET_ACCESS_KEY => yyy

Bucket

A single bucket typically stores the files, assets, and uploads for an application. To create a bucket, access the S3 section of the AWS Management Console and create a new bucket in the US Standard region:

Although you have a lot of freedom in choosing a bucket name, take care in choosing a name to ensure maximum interoperability.

Store the bucket name in a config var to give your application access to its value:

$ heroku config:set S3_BUCKET_NAME=appname-assets
Adding config vars and restarting app... done, v22
  S3_BUCKET_NAME     => appname-assets

Naming buckets

Your S3 bucket’s name can have unintended effects on which features of AWS you can use, including basic features like encryption. To ensure maximum compatibility, the ideal bucket name has:

• 32 total characters or fewer
• No capital letters (A-Z)
• No periods (.)
• No underscores (_)
• No hyphens (-) as the first or last character.

The reasons why these naming conventions matter are as follows:

• Period (.) characters cause certificate failures when using SSL or TLS.
• Capital letters, underscores (_), and beginning/terminating hyphens (-) disable the use of the preferred “Virtual Hosted” API format for S3, and they are incompatible with some S3 regions.
• Bucket names that are greater than 32 characters in length cannot use AWS STS policies, which provide flexible, temporary credentials.

Static assets

Static assets such as videos, PDFs, Javascript, CSS, and image files can be manually uploaded to your S3 account using the command line or a graphical browser like the Amazon S3 console.

After assets are uploaded, you can refer to their public URLs (such as http://s3.amazonaws.com/bucketname/filename) in your application’s code. These files will now be served directly from S3, freeing up your application to serve only dynamic requests.

File uploads

There are two approaches to processing and storing file uploads from a Heroku app to S3: direct and pass-through.

Direct upload

In a direct upload, a file is uploaded to your S3 bucket from a user’s browser, without first passing through your app. This method is recommended for user uploads that might exceed 4MB in size.

Although this method reduces the amount of processing your application needs to perform, it can be more complex to implement. It also limits the ability to modify (transform, filter, resize, etc.) files before storing them in S3.

Most of the articles listed below demonstrate how to perform direct uploads.

Pass-through uploads

In a pass-through upload, a file is uploaded to your app, which in turn uploads it to S3. This method enables you to perform preprocessing on user uploads before you push them to S3. However, depending on your chosen language and framework, this method might cause latency issues for other requests that are initiated while the upload is taking place.

Language-specific guides

Here are language-specific guides to handling uploads to S3: