ExWaiter (ex_waiter v1.2.0) View Source

Handy functions for polling and receiving.

Installation

Add the latest release to your mix.exs file:

defp deps do
  [
    {:ex_waiter, "~> 1.2.0"}
  ]
end

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

Link to this section Summary

Functions

Configures an ExWaiter.Polling.Poller struct to be passed into poll/1, poll!/1, or poll_once/1 to keep track of polling status.

Periodically checks that a given condition has been met.

Periodically checks that a given condition has been met. Raises an exception upon exhausted retries.

Checks one time that a given condition has been met.

Returns the next message/s from the mailbox within a timeout.

Returns the next message/s from the mailbox within a timeout. Raises an exception upon timeout.

Link to this section Functions

Link to this function

new_poller(polling_fn, opts \\ [])

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Specs

Configures an ExWaiter.Polling.Poller struct to be passed into poll/1, poll!/1, or poll_once/1 to keep track of polling status.

Usage

Takes a function that checks whether the given condition has been met. This function optionally takes 1 argument, which is the current Poller struct. Returning {:ok, value} or {:error, value} ensures that a "value" is set on the Poller struct that is returned after polling. If the value doesn't matter, :ok and :error may be returned from the function instead.

Create a poller and record the history of each attempt. By default, up to 5 attempts will be made with a backoff delay totaling 100ms.

poller =
  ExWaiter.new_poller(
    fn ->
      case Projects.get(1) do
        %Project{} = project -> {:ok, project}
        _ -> {:error, :nope}
      end
    end,
    record_history: true
  )

Then use poll/1 (or poll!/1) to synchronously poll until the project is successfully found or retries are exhausted. For more complex asynchronous use cases, poll_once/1 will make single attempts.

{:ok, poller} = ExWaiter.poll(poller)

Below is an example of the contents of the poller struct after polling has successfully found the project after 5 attempts. The struct includes information about the number of attempts, the delay between each, total delay, and the eventual value.

%ExWaiter.Polling.Poller{
  attempt_num: 5,
  history: [
    %{value: :nope, next_delay: 10},
    %{value: :nope, next_delay: 20},
    %{value: :nope, next_delay: 30},
    %{value: :nope, next_delay: 40},
    %{value: %Project{}, next_delay: nil}
  ],
  next_delay: nil,
  total_delay: 100,
  value: %Project{}
}

Options

  • :max_attempts - The number of attempts before retries are exhausted. Takes an integer, :infinity, or a function that optionally receives the Poller struct just after the condition has been checked for configuring dynamic retries. The function must return true to retry or false if retries are exhausted. The default is 5.
  • :delay - The delay before retries. Takes either an integer or a function that optionally receives the Poller struct just after the condition has been checked allowing for dynamically configured backoff. The default is fn poller -> poller.attempt_num * 10 end.
  • :record_history - Enabling the recording of attempt history will provide the tracked value and configured delay. The history is disabled by default to avoid growing too large.

See poll/1 for more in-depth usage examples.

Specs

Periodically checks that a given condition has been met.

In some 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, and a record of the history of each attempt.

Usage

Takes an ExWaiter.Polling.Poller struct and checks the condition configured via new_poller/2. If the condition has been met, a tuple with {:ok, %Poller{}} will be returned. If retries are exhausted prior to the condition being met, {:error, :retries_exhausted, %Poller{}} will be returned. Retries will synchronously be attempted until either the condition has been met or max attempts reached. For more complex asynchronous use cases, poll_once/1 will make single attempts; it is what this poll/1 function uses under the hood.

Examples

By default, this query will be attempted up to 5 times in 100ms. Assuming the condition was successful on the 5th try, the returned Poller struct would include the following polling metadata:

poller =
  ExWaiter.new_poller(fn ->
    case Projects.get(1) do
      %Project{} = project -> {:ok, project}
      _ -> :error
    end
  end)

assert {:ok, poller} = ExWaiter.poll(poller)
assert %{
  attempt_num: 5,
  next_delay: nil,
  total_delay: 100,
  value: %Project{}
} = poller

If we try 5 times without receiving the project, an error tuple will be returned.

poller =
  ExWaiter.new_poller(fn ->
    case Projects.get(1) do
      %Project{} = project -> {:ok, project}
      _ -> :error
    end
  end)

assert {:error, :retries_exhausted, poller} = ExWaiter.poll(poller)
assert %{
  attempt_num: 5,
  next_delay: nil,
  total_delay: 100,
  value: nil
} = poller

The number of attempts and delay between each can be configured. Below we want to make up to 10 attempts with 20ms of delay between each. Both max_attempts and delay can be dynamically configured (more examples below). The max_attempts can also be set to :infinity.

poller =
  ExWaiter.new_poller(
    fn ->
      case Projects.get(1) do
        %Project{} = project -> {:ok, project}
        _ -> :error
      end
    end,
    max_attempts: 10,
    delay: 20
  )

assert {:ok, poller} = ExWaiter.poll(poller)

Enabling the recording of history will provide the tracked value and configured delay for each attempt. History is disabled by default to avoid growing too large.

poller =
  ExWaiter.new_poller(
    fn ->
      case Projects.get(1) do
        %Project{} = project -> {:ok, project}
        _ -> {:error, :nope}
      end
    end,
    record_history: true
  )

assert {:ok, poller} = ExWaiter.poll(poller)
assert %{
  attempt_num: 5,
  history: [
    %{value: :nope, next_delay: 10},
    %{value: :nope, next_delay: 20},
    %{value: :nope, next_delay: 30},
    %{value: :nope, next_delay: 40},
    %{value: %Project{}, next_delay: nil}
  ],
  next_delay: nil,
  total_delay: 100,
  value: %Project{}
} = poller

The delay can be configured via a function that receives the Poller struct immediately after an attempt has been made to configure the delay before the next attempt. Enabling the recording of history allows us to see what was the next configured delay after each attempt.

poller =
  ExWaiter.new_poller(
    fn ->
      case Projects.get(1) do
        %Project{} -> :ok
        _ -> :error
      end
    end,
    record_history: true,
    delay: fn poller -> poller.attempt_num * 2 end
  )

assert {:ok, poller} = ExWaiter.poll(poller)
assert %{
  history: [
    %{next_delay: 2},
    %{next_delay: 4},
    %{next_delay: 6},
    %{next_delay: 8},
    %{next_delay: nil}
  ],
  total_delay: 20,
} = poller

Max attempts can also be configured dynamically. Suppose we wanted to continuously retry on Monday up to 100 attempts, but stop retrying any other day of the week. The function should return true to retry or false to stop retrying. Let's assume it's Monday and the project was returned after 5 attempts.

poller =
  ExWaiter.new_poller(
    fn _poller ->
      case Projects.get(1) do
        %Project{} -> :ok
        _ -> :error
      end
    end,
    max_attempts: fn poller ->
      is_monday? = DateTime.utc_now() |> DateTime.to_date() |> Date.day_of_week() == 1
      is_monday? and poller.attempt_num < 100
    end
  )

assert {:ok, poller} = ExWaiter.poll(poller)
assert %{
  attempt_num: 5
} = poller

The poller function optionally receives the Poller struct. This can be used for customization and logging.

poller = ExWaiter.new_poller(fn poller ->
  case Projects.get(1) do
    %Project{} = project -> {:ok, {project, poller.attempt_num}}
    _ ->
      Logger.info(inspect(poller))
      :error
  end
end)

assert {:ok, {%Project{}, 5}} = ExWaiter.poll(poller)

Specs

Periodically checks that a given condition has been met. Raises an exception upon exhausted retries.

Supports the same options as poll/1. However, if the condition has been met, only the Poller struct will be returned (i.e. not in an :ok tuple). If retries are exhausted prior to the condition being met, an exception will be raised.

Specs

Checks one time that a given condition has been met.

Usage

Takes an ExWaiter.Polling.Poller struct and checks the condition configured via new_poller/2. If the condition has been met, a tuple with {:ok, %Poller{}} will be returned. If the condition is unmet and retries have exhausted, {:error, :retries_exhausted, %Poller{}} will be returned. If additional retries are available, {:error, :attempt_failed, %Poller{}} will be returned. Subsequent retries via poll_once/1 should supply the returned Poller struct from the previous failed attempt as it will maintain the current attempt number, next and total delay, and value. The next_delay should be used to schedule the attempt at the desired later time (e.g. via Process.send_after).

See poll/1 for additional configuration examples as it uses this function under the hood running it recursively until either success or exhausted retries.

Examples

Below is a contrived example of scheduling retries. In practice, you might use a GenServer with handle_info and send to self or a different process that notifies the caller when finished.

poller =
  ExWaiter.new_poller(fn ->
    case Projects.get(1) do
      %Project{} = project -> {:ok, project}
      _ -> :error
    end
  end)

Suppose the first attempts fails...

assert {:error, :attempt_failed, poller} = ExWaiter.poll_once(poller)

The returned Poller struct includes the default delay for the first retry of 10 milliseconds. This can be used to schedule a later retry.

assert poller.next_delay == 10
Process.send_after(self(), {:retry, poller}, poller.next_delay)

Using the receive_next!/2 function built into this package we receive the {:retry, poller} message sent via Process.send_after.

assert {:retry, poller} = ExWaiter.receive_next!()

We try another attempt that fails, but there are still retries available.

assert {:error, :attempt_failed, poller} = ExWaiter.poll_once(poller)

The default delay for a second retry is 20 milliseconds and we use that to schedule another retry.

assert poller.next_delay == 20
Process.send_after(self(), {:retry, poller}, poller.next_delay)

We again receive our scheduled message and kickoff another poll attempt. This time our project is there and we can get it on the returned Poller struct in the value attribute.

assert {:retry, poller} = ExWaiter.receive_next!()
assert {:ok, poller} = ExWaiter.poll_once(poller)
assert %{
  attempt_num: 3,
  next_delay: nil,
  total_delay: 30,
  value: %Project{}
} = poller
Link to this function

receive_next(num_messages \\ 1, opts \\ [])

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Specs

receive_next(pos_integer(), ExWaiter.Receiving.Receiver.options()) ::
  {:ok, any()} | {:error, any()}

Returns the next message/s from the mailbox within a timeout.

Especially in testing scenarios, it can be useful to be able to assert that a number of messages are received in a mailbox in a specific order and that all of those messages are received within a timeout. It is not difficult to use receive to grab the messages, but there is a bit of ceremony/verbosity involved especially if requiring that all messages are received in a specific total amount of time.

Usage

By default, the next single message in the mailbox will be returned if it appears within 100ms. The number of messages to return and timeout are configurable. If the message/s are received within the timeout window, {:ok, message} will be returned for a single message or {:ok, [messages]} for multiple. If the configured timeout is reached prior to returning a single requested message, :error will be returned. If multiple messages were requested, {:error, [messages]} will be returned containing any messages that were received.

Options

  • :timeout - The time to wait for the number of messages requested from the mailbox. Takes either an integer (ms) or :infinity. (default: 100)

Examples

By default, the next message in the mailbox is returned if it appears within 100ms.

send(self(), :hello)

assert {:ok, :hello} = ExWaiter.receive_next()

Multiple messages may be returned.

send(self(), :hello)
send(self(), :hi)
send(self(), :yo)

assert {:ok, [:hello, :hi]} = ExWaiter.receive_next(2)

A timeout (in ms) can be set. If the timeout occurs prior to receiving all requested messages, the messages that were received will be returned in the error tuple.

send(self(), :hello)
send(self(), :hi)
Process.send_after(self(), :yo, 80)

assert {:error, [:hello, :hi]} = ExWaiter.receive_next(3, timeout: 50)
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receive_next!(num_messages \\ 1, opts \\ [])

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Specs

Returns the next message/s from the mailbox within a timeout. Raises an exception upon timeout.

Supports the same options as receive_next/2. However, if the mailbox has the right number of messages, only the message/s will be returned (i.e. not in an :ok tuple). If the messages are not received prior to the timeout, an exception will be raised.