ExpiringMemoize

Memoize a method result, refetching after a time to live has elapsed.

Installation

Add this line to your application's Gemfile:

gem 'expiring_memoize'

And then execute:

$ bundle

Or install it yourself as:

$ gem install expiring_memoize

Usage

class PriceFetcher
  def current_price
    puts 'fetching'
    rand # say it's an expensive operation here
  end

  extend ExpiringMemoize
  memoize :current_price, ttl: Float::INFINITY
end

fetcher = PriceFetcher.new

puts fetcher.current_price # fetched
puts fetcher.current_price # uses cached value
sleep 5
puts fetcher.current_price # fetched again

# works multi-threaded too;
# code below fetches only once

fetcher = PriceFetcher.new
5.times.map do
  Thread.new do
    sleep rand
    puts fetcher.current_price
  end
end.join

Features

• Thread safe. If many threads try to get the value and it's found to be stale, they race to refetch and only one of them does.
• Exception safe. If the fetch raises an exception it'll bubble up to the caller. Since the value will still be stale, next query will retry the fetch.
• Time travel safe. Uses Process.clock_gettime to provide a monotonic clock. Time adjustment events, eg. sleep, ntpdate etc. do not affect the TTL calculation.

Implementation details

Per-object data is stored in @_expiring_memoize_data instance variable as a hash, to avoid polluting the ivar namespace. The memoized method is replaced and the original is held in the closure of the replacement.

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.

Known limitations

• Only nullary instance methods are supported.
• Won't work on platforms where Process.clock_gettime is not supported, or where it does not support CLOCK_MONOTONIC_COARSE.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/cyberark/expiring_memoize.

License

The gem is available as open source under the terms of the MIT License.