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Wrap External APIs

Mix.install(
  [
    {:ash, "~> 3.0"},
    {:req, "~> 0.4.0"}
  ],
  consolidate_protocols: false
)

Introduction

Wrapping external APIs in Ash resources can allow you to leverage the rich and consistent interface provided by Ash.Resource for interactions with external services.

There are a few approaches to how you might do this, including the still in progress AshJsonApiWrapper. Here we will leverage "manual actions" as this is fully supported by Ash, and is the commonly used approach.

This approach is most appropriate when you are working with an API that exposes some data, like entities, list of entities, etc. For this example, we will be interacting with https://openlibrary.org, which allows for us to search and list books.

This guide covers reading data from the external API, not creating/updating it. This can be implemented using manual actions of a different type, or generic actions.

Wrapping External APIs

  1. Create a resource for interacting with the given API
  2. Create a manual read action
  3. In this manual action, we will:
    1. call the target API
    2. transform the results
    3. apply query operations to simulate capabilities provided by Ash

In the example below, we are calling to a paginated API, and we want to continue fetching results until we have reached the amount of results requested by the Ash.Query. We show this to illustrate that you can do all kinds of creative things when working with external APIs in manual actions.

Example

defmodule Doc do
  use Ash.Resource,
    domain: Domain

  attributes do
    uuid_primary_key :id
    attribute :author_name, :string
    attribute :title, :string
    attribute :type, :string
  end

  actions do
    read :search do
      primary? true
      argument :query, :string, allow_nil?: false
      prepare fn query, _ ->
        # We require that they limit the results to some reasonable set
        # (because this API is huge)
        cond do
          query.limit && query.limit > 250 ->
            Ash.Query.add_error(query, "must supply a limit that is less than or equal to 250")
          query.limit ->
            query
          true ->
            # limit 5 by default
            Ash.Query.limit(query, 5)
        end
      end

      manual Doc.Actions.Read
    end
  end
end

defmodule Domain do
  use Ash.Domain,
    validate_config_inclusion?: false

  resources do
    resource Doc do
      define :search, args: [:query]
    end
  end
end
{:module, Domain, <<70, 79, 82, 49, 0, 0, 250, ...>>,
 [
   Ash.Domain.Dsl.Resources.Resource,
   Ash.Domain.Dsl.Resources.Options,
   Ash.Domain.Dsl,
   %{opts: [], entities: [...]},
   Ash.Domain.Dsl,
   Ash.Domain.Dsl.Resources.Options,
   ...
 ]}

defmodule Doc.Actions.Read do
  use Ash.Resource.ManualRead

  def read(query, _, _opts, _context) do
    # we aren't handling these query options to keep the example simple
    # but you could on your own
    if query.sort != [] || query.offset != 0 do
      {:error, "Cannot sort or offset documents"}
    end

    if query.sort && query.sort != [] do
      raise "Cannot apply a sort to docs read"
    end

    if query.offset && query.offset != 0 do
      raise "Cannot apply a sort to docs read"
    end

    limit = query.limit || :infinity

    query = Ash.Query.unset(query, :limit)

    query_results =
      Stream.resource(
        fn ->
          {limit, 0}
        end,
        fn
          {remaining, page_number} when remaining <= 0 ->
            {:halt, {0, page_number}}

          {remaining, page_number} ->
            api_results =
              query.arguments.query
              |> get!(page_number)
              |> Enum.map(&to_doc/1)

            case Ash.Query.apply_to(query, api_results) do
              {:ok, []} ->
                {:halt, remaining}

              {:ok, results} ->
                count_of_results = Enum.count(results)

                cond do
                  # the api gives us batches of 100
                  remaining == :infinity && count_of_results == 100 ->
                    {results, {:infinity, page_number + 1}}

                  remaining == :infinity ->
                    {results, {0, page_number + 1}}

                  true ->
                    still_remaining = remaining - count_of_results

                    results =
                      if still_remaining <= 0 do
                        Enum.take(results, remaining)
                      else
                        results
                      end

                    {results, {still_remaining, page_number + 1}}
                end

              {:error, error} ->
                raise Ash.Error.to_ash_error(error)
            end
        end,
        fn _ -> :ok end
      )
      |> Enum.to_list()

    {:ok, query_results}
  end

  defp to_doc(api_doc) do
    %Doc{author_name: api_doc["author_name"], type: api_doc["type"], title: api_doc["title"]}
  end

  defp get!(query, page) do
    params =
      if page == 0 do
        [q: query]
      else
        [q: query, page: page]
      end

    Req.get!("https://openlibrary.org/search.json", params: params).body["docs"]
  end
end
{:module, Doc.Actions.Read, <<70, 79, 82, 49, 0, 0, 25, ...>>, {:get!, 2}}

Now we can use this like any other Ash resource

Domain.search!("Lord of the rings")
[
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Lord of the Rings",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Fellowship of the Ring",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Two Towers",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Return of the King",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Lord of the Rings",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >
]

require Ash.Query
query = Doc |> Ash.Query.filter(contains(title, "Two"))
Domain.search!("Lord of the rings", query: query)
[
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Two Towers",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["Alessio Cavatore", "Rick Priestley"],
    title: "The Lord of the Rings - The Two Towers",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "The Two Towers",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["Alessio Cavatore", "Rick Priestley"],
    title: "The Lord of the Rings - The Two Towers",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >,
  #Doc<
    __meta__: #Ecto.Schema.Metadata<:built, "">,
    id: nil,
    author_name: ["J.R.R. Tolkien"],
    title: "Two Towers : The Lord of the Rings",
    type: "work",
    aggregates: %{},
    calculations: %{},
    ...
  >
]