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Dimitris Zorbas

Code spells, smells and sourcery

sidekiq-dry

I published a new gem, sidekiq-dry aiming to tackle a variety of common frustrations when it comes to Sidekiq jobs and their arguments.

Rationale

Sidekiq is among the most popular background job solutions. It’s my first choice for Ruby apps. The dry-rb family of gems is also indispensable in non-trivial applications. What if we combined the two..

With sidekiq-dry you may pass instances of Dry::Struct as arguments to your Sidekiq jobs. But why?

Prevent Type Ambiguity

Numerous times I’ve had to debug jobs which where failing due to being enqueued with invalid arguments.

Example:

class SendInvitationEmailJob
  include Sidekiq::Worker

  def perform(user_id, invitee_email)
    # code
  end
end
SendInvitationEmailJob.perform_async(user.id, params[:invitee_email])

The problem with the above code is that if the user_id is not an Integer id or the invitee_email is not a valid email String then there’s absolutely no chance that the enqueued job will complete successfully. Of course Dry::Struct is not to be used for validations, there’s dry-validate for that, or ActiveModel / ActiveRecord validations if you prefer. Giving more structure to your background job arguments improves the system’s robustness. Your objects in transport through Redis, as long as the job is enqueued, they are guaranteed to have the expected structure when the job is performed.

The above example would be refactored to:

class SendInvitationEmailJob
  include Sidekiq::Worker

  def perform(params)
    # code
  end
end
class SendInvitationEmailJob::Params < Dry::Struct
  attribute :user_id, Types::Strict::Integer
  attribute :invitee_email, Types::Strict::String.constrained(format: /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z/i)
end
job_params = SendInvitationEmailJob::Params.new(user_id: user.id, invitee_email: params[:invitee_email])

SendInvitationEmailJob.perform_async(job_params)

At this point you might ask, what if we passed in a Hash, instead of a Dry::Struct? Well, Hash arguments are deserialised with String keys which can lead to surprises.

Eliminate Positional Arguments

When your background job takes two or more positional arguments, it’s better to refactor it to take a single struct object with a comprehensible name.

In the Rails world it’s common to enqueue jobs with a record’s id. There’s nothing wrong with this pattern. However, in some cases, developers may define a model blindly following the convention.

Documentation

By using Dry::Struct arguments you’ll be able to express constraints straight in your code. Instead of documenting the types of each job argument, which can easily become outdated, you can refer to the types of the attributes of the struct.

class Post < Dry::Struct
  attribute :title,  Types::Strict::String
  attribute :tags,   Types::Array.of(Types::Coercible::String).optional
  attribute :status, Types::String.enum('draft', 'published', 'archived')
  attribute :body,   Types::String.constrained(min_size: 10, max_size: 10_000)
end

Arguably, in the example above, both types and constraints improve readability.

Versioning

Adding this gem does not break any existing jobs in your app. It only works on jobs enqueued with Dry::Struct objects.

Adding a new attribute to a parameter struct won’t break already enqueued jobs.

It’s trivial to version your structs using either a version attribute:

class Coupons::ApplyCouponJob::Params < Dry::Struct
  attribute :user_id,     Types::Strict::Integer
  attribute :coupon_code, Types::Strict::String
  attribute :version,     Types::Strict::String.default('1')
end

or versioned classes:

class Coupons::ApplyCouponJob::Params::V1 < Dry::Struct
  attribute :user_id,     Types::Strict::Integer
  attribute :coupon_code, Types::Strict::String
end

Caveats

Job processing libraries compatible with Sidekiq, for example exq, won’t deserialise your Dry::Struct arguments. This is most likely an acceptable tradeoff.

The Gem

The gem is hosted on rubygems (link). It provides two Sidekiq middlewares which serialise and deserialise instances of Dry::Struct arguments in your jobs.

Installation

Add the gem in your Gemfile:

gem 'sidekiq-dry'

Configure Sidekiq to use the middlewares of the gem:

# File: config/initializers/sidekiq.rb

Sidekiq.configure_client do |config|
  config.client_middleware do |chain|
    chain.add Sidekiq::Dry::Client::SerializationMiddleware
  end
end

Sidekiq.configure_server do |config|
  config.client_middleware do |chain|
    chain.add Sidekiq::Dry::Client::SerializationMiddleware
  end
  config.server_middleware do |chain|
    chain.add Sidekiq::Dry::Server::DeserializationMiddleware
  end
end

Rubocop

Finally, you may set up a custom Rubocop rule like the following to nudge developers use Dry::Struct arguments.

module RuboCop
  module Cops
    module Jobs
      class Arguments < RuboCop::Cop::Cop
        MAX_JOB_ARGUMENTS = 3
        MSG = 'Replace %<args_count>d arguments with a single Dry::Struct'.freeze

        def on_args(node)
          return unless perform_method?(node.parent)

          args_count = node.children.size

          return if args_count <= MAX_JOB_ARGUMENTS

          add_offense(node.parent, message: MSG % { args_count: args_count })
        end

        private

        def_node_matcher :perform_method?, <<~PATTERN
          (def :perform (args ...) ...)
        PATTERN
      end
    end
  end
end

Further Reading

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