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# The ReactiveCaching concern is used to fetch some data in the background and
# store it in the Rails cache, keeping it up-to-date for as long as it is being
# requested. If the data hasn't been requested for +reactive_cache_lifetime+,
# it stop being refreshed, and then be removed.
#
# Example of use:
#
# class Foo < ActiveRecord::Base
# include ReactiveCaching
#
# self.reactive_cache_key = ->(thing) { ["foo", thing.id] }
#
# after_save :clear_reactive_cache!
#
# def calculate_reactive_cache
# # Expensive operation here. The return value of this method is cached
# end
#
# def result
# with_reactive_cache do |data|
# # ...
# end
# end
# end
#
# In this example, the first time `#result` is called, it will return `nil`.
# However, it will enqueue a background worker to call `#calculate_reactive_cache`
# and set an initial cache lifetime of ten minutes.
#
# Each time the background job completes, it stores the return value of
# `#calculate_reactive_cache`. It is also re-enqueued to run again after
# `reactive_cache_refresh_interval`, so keeping the stored value up to date.
# Calculations are never run concurrently.
#
# Calling `#result` while a value is in the cache will call the block given to
# `#with_reactive_cache`, yielding the cached value. It will also extend the
# lifetime by `reactive_cache_lifetime`.
#
# Once the lifetime has expired, no more background jobs will be enqueued and
# calling `#result` will again return `nil` - starting the process all over
# again
module ReactiveCaching
extend ActiveSupport::Concern
included do
class_attribute :reactive_cache_lease_timeout
class_attribute :reactive_cache_key
class_attribute :reactive_cache_lifetime
class_attribute :reactive_cache_refresh_interval
# defaults
self.reactive_cache_lease_timeout = 2.minutes
self.reactive_cache_refresh_interval = 1.minute
self.reactive_cache_lifetime = 10.minutes
def calculate_reactive_cache
raise NotImplementedError
end
def with_reactive_cache(&blk)
within_reactive_cache_lifetime do
data = Rails.cache.read(full_reactive_cache_key)
yield data if data.present?
end
ensure
Rails.cache.write(full_reactive_cache_key('alive'), true, expires_in: self.class.reactive_cache_lifetime)
ReactiveCachingWorker.perform_async(self.class, id)
end
def clear_reactive_cache!
Rails.cache.delete(full_reactive_cache_key)
end
def exclusively_update_reactive_cache!
locking_reactive_cache do
within_reactive_cache_lifetime do
enqueuing_update do
value = calculate_reactive_cache
Rails.cache.write(full_reactive_cache_key, value)
end
end
end
end
private
def full_reactive_cache_key(*qualifiers)
prefix = self.class.reactive_cache_key
prefix = prefix.call(self) if prefix.respond_to?(:call)
([prefix].flatten + qualifiers).join(':')
end
def locking_reactive_cache
lease = Gitlab::ExclusiveLease.new(full_reactive_cache_key, timeout: reactive_cache_lease_timeout)
uuid = lease.try_obtain
yield if uuid
ensure
Gitlab::ExclusiveLease.cancel(full_reactive_cache_key, uuid)
end
def within_reactive_cache_lifetime
yield if Rails.cache.read(full_reactive_cache_key('alive'))
end
def enqueuing_update
yield
ensure
ReactiveCachingWorker.perform_in(self.class.reactive_cache_refresh_interval, self.class, id)
end
end
end
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