Configuring Elixir's ExAws with Req and Storj

I pulled ExAws into an Elixir project recently. The app is Relinker. I intend to use cloud object storage for OpenGraph preview images for websites.

I’ve used ExAws before in other projects at work, but I wasn’t the one who had to do the setup. Along the way, I found that configuring ExAws was not easy to get right at first.

Motivation

I wanted two unrelated things:

• I wanted to configure the HTTP client to use Req instead of the default Hackney, since I already had Req listed as a dependency.
• Instead of S3, I wanted to connect to Storj which boasts an S3-compatible API because the free tier for Storj (25gb storage, 25gb/month egress) is really attractive for my pet project.

Config

Here’s where I landed after much trial:

config :ex_aws,
  http_client: ExAws.Request.Req,
  access_key_id: [{:system, "STORJ_ACCESS_KEY"}, :instance_role],
  secret_access_key: [{:system, "STORJ_SECRET_KEY"}, :instance_role]

config :ex_aws, :s3,
  scheme: "https://",
  host: "gateway.storjshare.io"

As a library consumer…

I think that documentation for how to configure ex_aws could be improved. In this vein, I submitted a tiny PR to improve the docs for configuring the HTTP client. But more is needed.

Here’s what I learned through trial-and-error:

• The above config cannot be runtime config. It must be build-time. In other words, put it in (e.g.) config.exs, not runtime.exs
• You need to keep those lists of tuples and atoms in the access key fields. They’re actually meaningful.
• Putting those two ideas together: ExAws has a concept of refreshable config, which change at runtime. But that doesn’t mean runtime config; ExAws uses the configuration to understand when and how to access environment variables.

The :system and :instance_role stuff that you see above is deep in the library’s config-parsing and env-var grabbing logic. But, these concepts are currently necessary for developers to interact with, all the way in userland config. In other words, it’s really a public API (at the moment).

This wasn’t apparent to me at all while approaching ExAws for the first time. Now that I’ve done it, it’s making more sense, but I still think that it could be made more clear. Since the docs are open-source, I will take a deeper look in the near future.

Disclaimer

Just to be clear, I’m not raising these as criticisms of community library maintainers, which I know is a totally thankless job.

I am simply documenting my experience, in case it is useful for myself or others.