ExWaiter

Helper for waiting on asynchronous conditions to be met.

Hexdocs found at https://hexdocs.pm/ex_waiter.

Installation

Add the latest release to your mix.exs file:

defp deps do
  [
    {:ex_waiter, "~> 0.3.3"}
  ]
end

Then run mix deps.get in your shell to fetch the dependencies.

Why This Exists?

In some testing scenarios there is no obvious way to ensure that asynchronous side effects have taken place without continuously checking for successful completion. For example, perhaps an assertion is needed on click data being asynchronously persisted to the database. It is not difficult to write a recursive function to handle this one-off, but there is a bit of ceremony involved.

Additionally, perhaps it is desirable to configure the amount of delay prior to each check, the total number of attempts, a convention for handling exhausted retries, an easy way to inject callbacks, and a record of the history of each attempt.

This simple package provides all that and more! Well, actually just that.

A Walkthrough

The package provides await/2 and await!/2 functions. Each requires an anonymous function that may return {:ok, value}, :ok, or true for a successful attempt or {:error, value}, :error, or false for a failed attempt. The tagged tuples must be used if you need a return value or want to track the history of value changes. Additional options are available for setting the desired number of attempts and custom delay between attempts.

Let's use await!/2 to check the database for the most recently persisted click.

click = ExWaiter.await!(fn ->
  case Clicks.most_recent() do
    %Click{} = click ->
      {:ok, click}

    value ->
      # This is a good place for a callback you might want to run each
      # time the condition is unmet (e.g. flushing jobs).
      {:error, value}

  end
end)

By default, this will check the database up to 5 times spaced out over 150ms. If, at some point, the condition is met, the %Click{} will be returned. If retries are exhausted, an exception will be raised that looks something like:

 ** (ExWaiter.Exceptions.RetriesExhausted) Tried 5 times over 150ms, but condition was never met.

 %ExWaiter.Waiter{
   attempt_num: 5,
   attempts: [
     %ExWaiter.Attempt{attempt_num: 1, delay_before: 10, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 2, delay_before: 20, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 3, delay_before: 30, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 4, delay_before: 40, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 5, delay_before: 50, fulfilled?: false, value: nil},
   ],
   attempts_left: 0,
   delay_before: #Function<...>,
   fulfilled?: false,
   checker_fn: #Function<...>,
   num_attempts: 5,
   total_delay: 150,
   value: nil
 }

This displays a Waiter struct, which includes a recording of everything that happened during attempts.

The await/2 function would return either {:ok, %Click{}, %Waiter{}} or {:error, nil, %Waiter}. It can be helpful to inspect this Waiter struct for debugging and optics into timing. The anonymous function to check if the condition has been met can take 0 or 1 arguments, with the argument being the %Waiter{}.

Additional Options

• :delay_before - takes either an integer or a function that receives the %Waiter{} struct at that moment and returns a number of milliseconds to delay prior to performing the next attempt. The default is fn waiter -> waiter.attempt_num * 10 end.
• :num_attempts - The number of attempts before retries are exhausted. (default: 5)

Thanks

Thanks to my friends at Enbala, especially @itsgreggreg and @s3cur3, for providing helpful feedback to polish the API.