Retries

Retries is a gem that provides a single function, with_retries, to evaluate a block with randomized, truncated, exponential backoff.

There are similar projects out there (see retry_block and retry_this, for example) but these will require you to implement the backoff scheme yourself. If you don't need randomized exponential backoff, you should check out those gems.

Installation

You can get the gem with gem install retries or simply add gem "retries" to your Gemfile if you're using bundler.

Usage

Suppose we have some task we are trying to perform: do_the_thing. This might be a call to a third-party API or a flaky service. Here's how you can try it three times before failing:

require "retries"
with_retries(:max_tries => 3) { do_the_thing }

The block is passed a single parameter, attempt_number, which is the number of attempts that have been made (starting at 1):

with_retries(:max_tries => 3) do |attempt_number|
  puts "Trying to do the thing: attempt #{attempt_number}"
  do_the_thing
end

Custom exceptions

By default with_retries rescues instances of StandardError. You'll likely want to make this more specific to your use case. You may provide an exception class or an array of classes:

with_retries(:max_tries => 3, :rescue => RestClient::Exception) { do_the_thing }
with_retries(:max_tries => 3, :rescue => [RestClient::Unauthorized, RestClient::RequestFailed]) do
  do_the_thing
end

Handlers

with_retries allows you to pass a custom handler that will be called each time before the block is retried. The handler will be called with three arguments: exception (the rescued exception), attempt_number (the number of attempts that have been made thus far), and total_delay (the number of seconds since the start of the time the block was first attempted, including all retries).

handler = Proc.new do |exception, attempt_number, total_delay|
  puts "Handler saw a #{exception.class}; retry attempt #{attempt_number}; #{total_delay} seconds have passed."
end
with_retries(:max_tries => 5, :handler => handler, :rescue => [RuntimeError, ZeroDivisionError]) do |attempt|
  (1 / 0) if attempt == 3
  raise "hey!" if attempt < 5
end

This will print something like:

Handler saw a RuntimeError; retry attempt 1; 2.9e-05 seconds have passed.
Handler saw a RuntimeError; retry attempt 2; 0.501176 seconds have passed.
Handler saw a ZeroDivisionError; retry attempt 3; 1.129921 seconds have passed.
Handler saw a RuntimeError; retry attempt 4; 1.886828 seconds have passed.

Delay parameters

By default, with_retries will wait about a half second between the first and second attempts, and then the delay time will increase exponentially between attempts (but stay at no more than 1 second). The delays are perturbed randomly. You can control the parameters via the two options :base_sleep_seconds and :max_sleep_seconds. For instance, you can start the delay at 100ms and go up to a maximum of about 2 seconds:

with_retries(:max_tries => 10, :base_sleep_seconds => 0.1, :max_sleep_seconds => 2.0) { do_the_thing }

Testing

In tests, you may wish to test that retries are being performed without any delay for sleeping:

Retries.sleep_enabled = false
with_retries(:max_tries => 100) { raise "Boo!" } # Now this fails fast

Of course, this will mask any errors to the :base_sleep_seconds and :max_sleep_seconds parameters, so use with caution.

Issues

File tickets here on Github.

Development

To run the tests: first clone the repo, then

$ bundle install
$ bundle exec rake test

Authors

Retries was created by Harry Robertson and Caleb Spare.

Other contributions from:

• Harry Lascelles (hlascelles)
• Michael Mazour (mmazour)

License

Retries is released under the MIT License.