Monday, April 19, 2021

Elixir AWS SDK

While AWS doesn't provide an SDK directly for Erlang or Elixir, the AWS for the BEAM project has built a nice solution for this — a code generator that uses the JSON API definitions from the official AWS Go SDK to create native Erlang and Elixir AWS SDK bindings. The result for Elixir is the nifty aws-elixir library.

The aws-elixir library itself doesn't have the automagic functionality from other AWS SDKs of being able to pull AWS credentials from various sources like environment variables, profile files, IAM roles for tasks or EC2, etc. However, the AWS for the BEAM project has another library you can use for that: aws_credentials. Here's how to use aws-elixir in combination with aws_credentials for a standard Mix project:

1. Add aws dependencies

First, add the aws, aws_credentials, and hackney libraries as dependencies to your mix.exs file:

# mix.exs defp deps do [ {:aws, "~> 0.7.0"}, {:aws_credentials, git: "", ref: "0.1.0"}, {:hackney, "~> 1.17"}, ] end

2. Set up AWS.Client struct

Next, set up aws-elixir's AWS.Client struct with the AWS credentials found by the :aws_credentials.get_credentials/0 function. In this example, I'm going to create a simple MyApp.AwsUtils module, with a client/0 function that I can call from anywhere else in my app to initialize the AWS.Client struct:

# lib/my_app/aws_utils.ex defmodule MyApp.AwsUtils do @doc """ Creates a new AWS.Client with default settings. """ @spec client() :: AWS.Client.t() def client, do: :aws_credentials.get_credentials() |> build_client() defp build_client(%{access_key_id: id, secret_access_key: key, token: "", region: region}) do AWS.Client.create(id, key, region) end defp build_client(%{access_key_id: id, secret_access_key: key, token: token, region: region}) do AWS.Client.create(id, key, token, region) end defp build_client(credentials), do: struct(AWS.Client, credentials) end

The aws_credentials library will handle caching for you, so you don't need to separately cache the credentials it returns — just call get_credentials/0 every time you need them. By default, it will first check for the standard AWS environment variables (AWS_ACCESS_KEY_ID etc), then for the standard credentials file (~/.aws/credentials), then for ECS task credentials, and then for credentials from the EC2 metadata service.

So the above example will work if on one system you configure the environment variables for your Elixir program like this:


And on another system you configure the user account running your Elixir program with a ~/.aws/credentials file like this:

# ~/.aws/credentials [default] aws_access_key_id = ABCDEFGHIJKLMNOPQRST aws_secret_access_key = 01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ/+a

And when running the Elixir program in an ECS task or EC2 instance, it will automatically pick up the credentials configured for the ECS task or EC2 instance under which the program is running.

If you do use a credentials file, you can customize the path to the credentials file, or profile within the file, via the :provider_options configuration parameter, like so:

# config/config.exs config :aws_credentials, :provider_options, %{ credential_path: "/home/me/.aws/config", profile: "myprofile" }

Some caveats with the current aws_credentials implementation are:

  1. With environment variables, you can specify the region (via the AWS_DEFAULT_REGION or AWS_REGION variable) only if you also specify the session token (via the AWS_SESSION_TOKEN or AWS_SECURITY_TOKEN variable).
  2. With credential files, the region and aws_session_token settings won't be included.

3. Call AWS.* module functions

Now you can go ahead and call any AWS SDK function. In this example, I'm going to create a get_my_special_file/0 function to get the contents of a file from S3:

# lib/my_app/my_files.ex defmodule MyApp.MyFiles do @doc """ Gets the content of my special file from S3. """ @spec get_my_special_file() :: binary def get_my_special_file do client = MyApp.AwsUtils.client() bucket = "my-bucket" key = "my/special/file.txt" {:ok, %{"Body" => body}, %{status_code: 200}} = AWS.S3.get_object(client, bucket, key) body end

For any AWS SDK function, you can use the Hex docs to guide you as to the Elixir function signature, the Go docs for any structs not explained in the Hex docs, and the AWS docs for more details and examples. For example, here are the docs for the get_object function used above:

  1. Hex docs for AWS.S3.get_object/22
  2. Go docs for S3.GetObject
  3. AWS docs for S3 GetObject

The general response format form each aws-elixir SDK function is this:

# successful response { :ok, map_of_parsed_response_body_with_string_keys, %{body: body_binary, headers: list_of_string_header_tuples, status_code: integer} } # error response { :error, { :unexpected_response, %{body: body_binary, headers: list_of_string_header_tuples, status_code: integer} } }

With the AWS.S3.get_object/22 example above, a successful response will look like this:

iex> AWS.S3.get_object(MyApp.AwsUtils.client(), "my-bucket", "my/special/file.txt") {:ok, %{ "Body" => "my special file content\n", "ContentLength" => "24", "ContentType" => "text/plain", "ETag" => "\"00733c197e5877adf705a2ec6d881d44\"", "LastModified" => "Wed, 14 Apr 2021 19:05:34 GMT" }, %{ body: "my special file content\n", headers: [ {"x-amz-id-2", "ouJJOzsesw0m24Y6SCxtnDquPbo4rg0BwSORyMn3lOJ8PIeptboR8ozKgIwuPGRAtRPyRIPi6Dk="}, {"x-amz-request-id", "P9ZVDJ2L378Q3EGX"}, {"Date", "Wed, 14 Apr 2021 20:40:46 GMT"}, {"Last-Modified", "Wed, 14 Apr 2021 19:05:34 GMT"}, {"ETag", "\"00733c197e59877ad705a2ec6d881d44\""}, {"Accept-Ranges", "bytes"}, {"Content-Type", "text/plain"}, {"Content-Length", "24"}, {"Server", "AmazonS3"} ], status_code: 200 }}

And an error response will look like this:

iex> AWS.S3.get_object(MyApp.AwsUtils.client(), "my-bucket", "not/my/special/file.txt") {:error, {:unexpected_response, %{ body: "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<Error><Code>AccessDenied</Code><Message>Access Denied</Message><RequestId>FJWGFYKL44AB4XZK</RequestId><HostId>G4mzxVPQdjFsHpErTWZhG7djVLks1Vu7RLLYS37XA38c6JsAaJs+QMp3bR3Vm9aKhoWBuS/Mk6Y=</HostId></Error>", headers: [ {"x-amz-request-id", "FJWGFYKL44AB4XZK"}, {"x-amz-id-2", "G4mzxVPQdjFsHpErTWZhG7djVLks1Vu7RLLYS37XA38c6JsAaJs+QMp3bR3Vm9aKhoWBuS/Mk6Y="}, {"Content-Type", "application/xml"}, {"Transfer-Encoding", "chunked"}, {"Date", "Wed, 14 Apr 2021 19:25:01 GMT"}, {"Server", "AmazonS3"} ], status_code: 403 }}}

Temporary aws-elixir workaround

The 0.7.0 version of the aws-elixir library has a few bugs that affect its S3 module. These bugs have already been fixed in the master branch, so if you want to use the S3 module, until its next release, just use the latest version of aws-elixir from the master branch on GitHub — this is the latest version as of 2021-04-16:

# mix.exs defp deps do [ {:aws, "~> 0.7.0"}, {:aws, git: "", ref: "52f67ba"}, {:aws_credentials, git: "", ref: "0.1.0"}, {:hackney, "~> 1.17"}, ] end

Temporary aws_credentials workaround

The 0.1.0 version of the aws_credentials library has bug where it fails to start if you don't explicitly set its :provider_options configuration parameter. Until its next release, the workaround is simply to set that config param to an empty map:

# config/config.exs config :aws_credentials, :provider_options, %{}

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