### Add service to supervision tree

Source: https://hexdocs.pm/external_service

Include the service module in the application's supervision tree to initialize circuit breakers and rate limiters.

```elixir
def start(_type, _args) do
  children = [
    MyApp.Stripe
    # ... the rest of your children
  ]

  Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
end
```

--------------------------------

### Define a service module

Source: https://hexdocs.pm/external_service

Use the ExternalService macro to configure circuit breaker and retry policies, then wrap API calls within the call function.

```elixir
defmodule MyApp.Stripe do
  use ExternalService,
    circuit_breaker: [tolerate: 5, within: :timer.seconds(1), reset: :timer.seconds(5)],
    retry: [max_attempts: 5, backoff: :exponential, jitter: true]

  def charge(params) do
    call fn ->
      case Stripe.charge(params) do
        {:ok, result} -> {:ok, result}
        {:error, %{status: status}} when status in 500..599 -> :retry
        other -> other
      end
    end
  end
end
```

--------------------------------

### Add ExternalService dependency

Source: https://hexdocs.pm/external_service

Include the library in your mix.exs file to manage project dependencies.

```elixir
def deps do
  [
    {:external_service, "~> 2.0"}
  ]
end
```

--------------------------------

### Handle ExternalService errors with pattern matching

Source: https://hexdocs.pm/external_service

Use pattern matching to handle specific error types like RetriesExhausted and CircuitBreakerOpen when calling external services.

```elixir
case MyApp.Stripe.charge(params) do
  {:ok, result} ->
    handle_success(result)

  {:error, %ExternalService.RetriesExhausted{}} ->
    # transient failure outlasted our retry budget
    {:error, :payment_unavailable}

  {:error, %ExternalService.CircuitBreakerOpen{}} ->
    # the breaker is open; fail fast
    {:error, :payment_unavailable}

  {:error, reason} ->
    # an error your own function returned
    {:error, reason}
end
```

--------------------------------

### Call external service

Source: https://hexdocs.pm/external_service

Invoke the service method to automatically apply configured retry, circuit-breaker, and rate-limit logic.

```elixir
MyApp.Stripe.charge(%{amount: 1000, currency: "usd", source: token})
```

--------------------------------

### Implement custom retry logic

Source: https://hexdocs.pm/external_service

Define retriable failures within the call block by returning :retry or {:retry, reason} tuples.

```elixir
def fetch(id) do
  call fn ->
    case HTTP.get("/widgets/#{id}") do
      {:ok, %{status: 200, body: body}} -> {:ok, body}
      {:ok, %{status: status}} when status in 500..599 -> {:retry, status}
      {:ok, %{status: 404}} -> {:error, :not_found}   # not retried — your error
      {:error, %HTTPError{}} = err -> err             # not retried — your error
    end
  end
end
```

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