While we were working on a project we stumbled upon some legacy code that consists of huge ‘if else’ statements. In this post we’ll show you how we refactored it using the Chain of responsibility pattern.
What is the definition of the Chain of responsibility pattern?
Chain the receiving objects and pass the request along the chain until an object handles it.
~Krzysztof Kempiński, Chain of Responsibility Design Pattern in Ruby
It may be useful if we need to store some data that is not related to any model. Essentially, it’s an object oriented version of
if … elsif … elsif … else … end that is much much easier to extend. Because of that we can’t use hashes. Processing objects, aka handlers, contain logic for handling exactly one case.
So where can we use it? For example we could define a chain for handling payments. One handler would take care of payment for one provider such as payU, credit card, bank transfer.
So why did we use it?
We were working with big and messy legacy code. There was a gigant
if else statement in one of the services. In our case it was responsible for choosing shipping provider based on selected carrier and shipment. It was taking into account saved api keys for providers, which provider was enabled in the given store and some other stuff. There were a lot of different cases, every case was handled differently, each one of them consisted of about 50-line method.
Services are usually used for everything in Rails, which may not always be an ideal solution. This is our old, but good post about basic design patterns. Services may be very helpful when we have a lot of processes to accomplish, but in our opinion they are overused. Also services should be responsible for only one thing, as deciding how to handle something and handling it are two completely different things
Check which provider supports chosen carrier, then make sure that selected carrier is active for current provider. If not, continue checking.
We have created main Chain object, which initializes all handlers and creates a “chain”.
class BasicChain class << self def resolve(*args) mapped_handlers[0...-1].each_with_index do |handler, index| mapped_handlers[index + 1].successor = handler end mapped_handlers.last.call(*args) end def handlers  end def mapped_handlers @mapped_handlers ||= handlers.map(&:new) end end end
It’s an interface for all future chains, it initializes handlers and assigns successor to each one of them. As you can see in the code below, successor is just a next handler in the chain.
We’ve also implemented
class BaseHandler attr_accessor :successor def initialize(successor = nil) @successor = successor end def call raise "#call method should be implemented" end end
BaseHandler is an interface for all future handlers. In initializer, we provide a successor, which defines a next handler of the Chain, which will be executed if condition for the current handler is false (in this case, the carrier name is not passing).
BaseHandler provides pattern how every other Chain should look like. So, while creating a new chain, we need an array of handlers as it defines an order in chain and what handlers are involved in the chain.
module ShippingProviderChain class Chain < BasicChain class << self def handlers [ ShippingProviderChain::DefaultProviderHandler, ShippingProviderChain::SecondProviderHandler, ShippingProviderChain::FirstProviderHandler ] end end end end
As you can see in the example below, handler returns active provider when a certain condition is met. In any other case it passes all received data to the next handler in the chain.
module ShippingProviderChain class BaseShippingHandler < BaseHandler UNKNOWN_PROVIDER = "unknown".freeze PROVIDER = "".freeze SUPPORTED_CARRIERS = .freeze def call(carrier_name, shop) return active_provider(carrier_name, shop) if supported_carrier?(carrier_name) return successor.call(carrier_name, shop) if successor.present? UNKNOWN_PROVIDER end def active_provider(carrier_name) PROVIDER end private def supported_carrier?(carrier_name) carrier_name.in?(CARRIERS) end end end
Here’s an example of an implementation of a handler which selects
FirstProvider if direct integration with this provider is enabled. If not, it selects unifaun if indirect integration enabled.
module ShippingProviderChain class FirstProviderHandler < BaseShippingHandler PROVIDER = "first_provider".freeze CARRIERS = ["gls", "dhl", "..."] def active_provider(carrier_name, shop) if shop.first_provider.direct_integration_enabled? # select first provider elsif shop.unifaun_provider.enabled? # select unifaun else BaseShippingHandler::UNKNOWN_PROVIDER end end private def supported_carrier?(carrier_name) carrier_name.in?(CARRIERS) end end end
There is definitely a lot of room for improvement in our implementation but at the same time it’s a huge step forward compared to the original legacy code. Our solution also may not be perfect in this particular application, as there are not that many conditionals but it’s much easier to read and easier to extend in the future (for example when new providers will be added). Adding new Chains is now really easy, thanks to
What other solutions would you suggest for our case and why? We’re always happy to learn new things!
Special thanks to the reviewer of this post: Jakub Flasiński.