WithRetry

with_retry is an additional code block used for writing with statements that have retry logic.

API Reference

Getting started

1. Check requirements

• Elixir 1.7+

2. Install WithRetry

Edit mix.exs and add with_retry to your list of dependencies and applications:

def deps do
  [{:with_retry, "~> 1.0"}]
end

Then run mix deps.get.

3. Use

Configure hosts and queues:

defmodule Example do
  use WithRetry


  def download(url, file) do
    with_retry {:ok, %{body: data}} <- HTTPX.get(url),
               :ok <- File.write(file, data) do
      data
    end
  end
end

Capturing failures

The with_retry captures many possible failures including:

• Pattern mismatch.
• Raise
• Throw
• Exit

All none captured failures will either return in the case of no else or bubble up.

Pattern Mismatch (else)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data) do
  data
else
  _error -> nil
end

Raise (rescue)

with_retry %{body: data} <- HTTPX.get(url),
           :ok <- File.write!(file, data) do
  data
rescue
  _ -> nil
end

Throw (catch)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- might_throw(data),
           :ok <- File.write(file, data) do
  data
catch
  _thrown -> nil
end

Exit (catch)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- might_exit(data),
           :ok <- File.write(file, data) do
  data
catch
  {:exit, _code} -> nil
end

Combined

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- might_throw(data),
           :ok <- might_exit(data),
           :ok <- File.write!(file, data) do
  data
else
  _error -> nil
rescue
  _ -> nil
catch
  {:exit, _code} -> nil
  _thrown -> nil
end

Back Off

The back off (timeouts) can be configured on the last last of the with_retry.

The default back off is 5 total tries with 1s in between attempts. To update the configuration see the following examples.

No Retry

Setting the back_off to false will disable retries and function like a normal with.

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: false do
  data
end

Passing An Enumerable

The back_off accepts any enumerable that returns timeouts in milliseconds.

In this example we wait 250ms after the first attempt, 1_000 after the second, and 5_000 after the third.

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: [250, 1_000, 5_000] do
  data
end

Build In Back Off Strategies

Constant

To retry with a constant timeout use: constant/1 passing the timeout in ms.

Retry endlessly every 5_000ms.

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: constant(5_000) do
  data
end

Linear

To retry with a linearly increasing timeout use: linear/2 passing the base timeout in ms and the increase in ms.

Retry endlessly starting with 1_000ms and increasing the timeout with 1_500 every wait. (e.g. 1_000, 2_500, 4_000, ...)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: linear(1_000, 1_500) do
  data
end

Exponential

To retry with an exponentially increasing timeout use: exponential/2 passing the base timeout in ms and the factor.

Retry endlessly starting with 250ms and doubling after every wait. (e.g. 250, 500, 1_000, 2_000, ...)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: exponential(250, 2) do
  data
end

Build In Back Off Modifiers

Cap

To cap the retry to a maximum timeout one can use cap/2 and give a back_off and a cap in ms.

Retry endlessly starting with 250ms and doubling after every wait, but capping at 1_500. (e.g. 250, 500, 1_000, 1_500, 1_500, ...)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: cap(exponential(250, 2), 1_500) do
  data
end

Max Try

Limit the back off to a given amount of tries using max_try/2 and give a back_off and a count of tries. (Including the first attempt.)

Try 4 times with exponential back off. (e.g. #1, wait 250, #2, wait 500, #3, wait 1_000, #4)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: max_try(exponential(250, 2), 4) do
  data
end

Max Retry

Limit the back off to a given amount of retries using max_retry/2 and give a back_off and a count of retries. (Excluding the first attempt.)

Retry 3 times with exponential back off. (e.g. #1, wait 250, #2, wait 500, #3, wait 1_000, #4)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: max_try(exponential(250, 2), 3) do
  data
end

Limit

Limit execution of the with_retry to a given time limit in ms using limit/2.

This includes the time spend doing processing the actually with. See: limit_wait/2 to limit the time spend waiting, excluding execution time.

The prediction is a best effort limitation and a long execution time might bring the total time spend on executing the with_try over the set limit.

Retry as many times as fit within 4_000ms with exponential back off.

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: limit(exponential(250, 2), 8_000) do
  data
end

Limit Wait

Limit the waiting time of the with_retry to a given time limit in ms using limit_wait/2.

This excludes the time spend doing processing the actually with. See: limit/2 to limit the total time, including execution time.

Retry as many times as fit within 8_000ms with exponential back off. (e.g. 250, 500, 1_000, 2_000, 4_000 for a total of 7_750.)

with_retry {:ok, %{body: data}} <- HTTPX.get(url),
           :ok <- File.write(file, data),
           back_off: limit_wait(exponential(250, 2), 8_000) do
  data
end

Changelog

v1.0

• Fix dialyzer issue.

Copyright and License

Copyright (c) 2018, Ian Luites.

WithRetry code is licensed under the MIT License.