Block Repeater

A way to repeat a block of code until either a given condition is met or a timeout has been reached

Installation

Add this line to your application's Gemfile:

gem 'block_repeater'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install block_repeater

Usage

Add the repeater and it's methods into a project

include BlockRepeater

In order to maintain parity with existing Repeater implementations you may also need to expose the repeater class itself

Repeater = BlockRepeater::Repeater

Repeat method

This is the preferred way to use the repeater. It requires two blocks, the one to be executed and the one which sets the exit condition. As these are ordinary ruby blocks they can be defined using either { } or do end syntax.

  repeat { call_database_method }.until{ |result| result.count.positive? }

  repeat do
    call_database_method
  end.until do |result| 
    result.count.positive? 
  end

The repeater also takes two parameters:

  • delay: is how long in seconds the repeater will wait between attempts (defaults to 0.2 seconds)
  • times: is how many attempts the repeater will make before giving up (defaults to 25) ruby repeat (delay: 0.5, times: 10){ call_database_method }.until{ |result| result.count.positive? }

Backoff method

This is a slightly different take on the preferred repeat method. It retries a call whilst exponentially increasing the wait time between each iteration until a timeout is reached.

repeat do
  call_method
end.until do |result|
  result
end.backoff(timeout: 10, initial_wait: 0.5, multiplier: 2)

backoff takes three paramaters:

  • timeout: is how long you want your repeater to run
  • initial_wait: is how long you want to pause before your first retry
  • multiplier: is the rate at which you increase the wait time between each iteration

Exception Handling

Using the catch method you can define how the repeater should respond to specific exception types. To do this you need to provide a list of exceptions to catch, a block of code which will be performed, and an option for how to trigger than block of code.

IMPORTANT: Any catch blocks must be declared before the until block

The following code has custom behaviour for a variety of error types, stopping if it get's an IOError but otherwise continuing without halting the repeater:

  repeat do
    call_database_method
  end.catch(exceptions: IOError, behaviour: :stop) do |e|
    puts "Error in IO operations: #{e}"
  end.catch(behaviour: :continue) do |e|
    puts "Error thrown: #{e}"
  end.until do |result|
    result.count.positive?
  end

exceptions can take either a single exception type or an array. If not provided it will default to StandardError.

There are three supported behaviours:

  • :continue executes the provided block but doesn't stop the repeater
  • :stop executes the provided block and then stops the repeater
  • :defer only executes the provided block if the exception is still occurring on the final attempt. This is the default option.

If no block is provided it will default to attempting to raise the exception.

Default Exception Handling Behaviour

You can also define default exception handling behaviour which all repeaters in a project will use. A catch block on a repeater will override default behaviour for the same exception type. In this following example all repeaters will automatically catch IOErrors and raise them if they're still occurring once the repeater has completed its attempts.

BlockRepeater::Repeater.default_catch(exceptions: [IOError], behaviour: :defer) do |e|
  puts 'An IOError occurred'
  e.raise
end

A common use-case for default exception handling is if using a gem such as RSpec, where you may want to handle failed expectations in a uniform manner. To do so you need define the default behaviour first, in a place such as a env.rb file or similar:

BlockRepeater::Repeater.default_catch(exceptions: [RSpec::Expectations::ExpectationNotMetError], behaviour: :defer) do |e|
  e.raise
end

Then an RSpec expectation can be used in the block for the until method. The expectation will be attempted each try, but the exception will only be raised if it has still failed once the number or attempts has been reached.

  repeat do
    call_database_method
  end.until do |result| 
    expect(result.count).to be_positive, raise 'No result returned from databased'
  end

Non predefined condition methods

Very simple conditions can be utilised without using a block. This expects either one or two method names which will be called against the result of repeating the main block. The required format is until_<method name> or until_<method name>_becomes_<method name>.

  repeat { a_method_which_returns_a_number }.until_positive?
  #Attempts to call the :positive? method on the result of the method call

  repeat { a_method_which_returns_an_array }.until_count_becomes_positive?
  #Attempts to call :count on the result of the method call, then :positive? on that result

This supports up to two consecutive method calls, anything more complex should be written out in full in the standard manner.

Direct class usage

It's also possible to directly access the Repeater class which has been left available as to provide backwards compatibility with older implementations of BlockRepeater. It's not recommended to combine this with the non predefined condition method pattern described above.

  Repeater = BlockRepeater::Repeater

  Repeater.new do
    call_database_method
  end.until do |result| 
    expect(result.count).to be_positive, raise 'No result returned from databased'
  end.repeat(delay: 1, times: 5)

In this case any optional arguments (times or delay) must be sent to the method called repeat, which is called after the second block. The repeat method is mandatory to run the repeater when using it in this manner.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.