Receptacle

NOTE: Version 1.x of this gem won't work on ruby 3+ and is unlikely to be updated. An adapted future version 2+ of this will be maintained at runtastic/receptacle.

About

Provides easy and fast means to use the repository pattern to create separation between your business logic and your data sources.

Battle tested at Runtastic.

Installation

Add this line to your application's Gemfile:

gem 'receptacle'

And then execute:

$ bundle

Or install it yourself as:

$ gem install receptacle

Usage

A repository mediates requests based on it's configuration to a strategy which then itself implements the necessary functions to access the data source.

                                                +--------------------+      +--------+
                                                |                    |      |Database|
                                                |  DatabaseStrategy  +------>        |
                                                |                    |      |        |
+--------------------+     +--------------+     +----------^---------+      |        |
|                    |     |              |                |                +--------+
|   Business Logic   +----->  Repository  +----------------+
|                    |     |              |
+--------------------+     +--------|-----+     +--------------------+
                                    |           |                    |
                                    |           |  InMemoryStrategy  |
                            +-------|-----+     |                    |
                            |Configuration|     +--------------------+
                            +-------------+

Let's look at the pieces:

1. the repository itself - which is a simple module including the Receptacle mixin

module Repository
  module User
    include Receptacle::Repo
    
    mediate :find
  end
end

2. at least one strategy class which are implemented as plain ruby classes

module Strategy
  class Database
    def find(id:)
      # get data from data source and return a business entity
    end
  end
end

Optionally wrapper classes can be defined

module Wrapper
  class Validator
    def before_find(id:)
      raise ArgumentError if id.nil?
      {id: id}
    end
  end
  class ModelMapper
    def after_find(return_value, **_kwargs)
      Model::User.new(return_value)
    end
  end
end

Example

Everything combined a simple example could look like the following:

require "receptacle"

module Repository
  module User
    include Receptacle::Repo
    mediate :find

    module Strategy
      class DB
        def find(id:)
          # code to find from the database
        end
      end
      class InMemory
        def find(id:)
          # code to find from InMemory store
        end
      end
    end

    module Wrapper
      class Validator
        def before_find(id:)
          raise ArgumentError if id.nil?
          {id: id}
        end
      end
      class ModelMapper
        def after_find(return_value, **_kwargs)
          Model::User.new(return_value)
        end
      end
    end
  end
end

For better separation to other repositories the fact that the repository itself is a module can be used to nest both strategies and wrapper underneath.

Somewhere in your application config you now need to setup the strategy and the wrappers for this repository like this:

Repository::User.strategy Repository::User::Strategy::DB
Repository::User.wrappers [Repository::User::Wrapper::Validator,
                           Repository::User::Wrapper::ModelMapper])

With this setup to use the repository method is as simple and straight forward as calling Repository::User.find(id: 123)

Repository Pattern

What is the matter with this repository pattern and why should I care using it?

Motivation

Often the business logic of applications directly accesses a data source like a database. This has several disadvantages such as

• code duplication cased by repeated need to transform raw data into business entities
• no separation between business logic and access to the data source
• harder to add or change global policies like caching
• caused by missing isolation it's harder to test the business logic independent from the data source

Solution

To improve on the disadvantages above and more we can introduce a repository which mediates between the business logic and the data source. The data source can be for example a database, an API(be it internal or external) or other web services.

A repository provides the business logic with a stable interface to interact with the data source. Hereby is the repository mapping the data to business entities. Because the repository is a central place to access the data source caching policies or similar can be applied easily there.

During testing the repository can be switched to a different strategy for example a fast and lightweight in memory data store to ease the process of testing the business logic.

Due to the ability to switch strategies a repository can also help to keep the application architecture flexible as a change in strategy has no impact on the business logic above.

Details

Strategy

A strategy is implemented as simple ruby class which implements the direct access to a data source by implementing the same method (as instance method) which was setup in the repository.

On each call to the repository a new instance of this class is created on which then the mediated method is called.

module Strategy
  class Database
    def find(id:)
      # get data from data source and return a business entity
    end
  end
end

Due to the nature of creating a new instance on each method call persistent connections to the data source like a connection pool should be maintained outside the strategy itself. For example in a singleton class.

Wrapper

In addition to create a separation between data access and business logic often there is the need to perform certain actions in the context of a data source access. For example there can be the need to send message on a message bus whenever a resource was created - independent of the strategy.

This gem allow one to add such actions without adding them to all strategies or applying them in the business logic by using wrappers.

One or multiple wrappers sit logically between the repository and the strategies. Based on the repository configuration it knows when and in which order they should be applied. Right now there is support for 2 1/2 types of actions.

1. a before method action: This action is called before the final strategy method is executed. It has access to the method parameter and can even modify them.
2. a after method action: This action is called after the strategy method was executed and has access to the method parameters passed to the strategy method and the return value. The return value could be modified here too.

The extra 1/2 action type is born by the fact that if a single wrapper class implements both an before and after action for the same method the same wrapper instance is used to execute both. Although this doesn't cover the all use cases an around method action would but many which need state before and after the data source is accessed are covered.

Implementation

Wrapper actions are implemented as plain ruby classes which provide instance methods named like before_<method_name> or after_<method_name> where <method_name> is the repository/strategy method this action should be applied to.

module Wrapper
  class Validator
    def before_find(id:)
      raise ArgumentError if id.nil?
      {id: id}
    end
  end
end

This wrapper class would provide a before action for the find method. The return value of this wrapper will be used as parameters for the strategy method (or the next wrapper in line). Keyword arguments can simply be returned as hash.

If multiple wrapper classes are defined the before wrapper actions are executed in the order the wrapper classes are defined while the after actions are applied in reverse order.

Memory Strategy

Although currently not part of the gem a simple memory strategy can be implemented in this way:

class MemoryStore
  class << self
    def store
      @store || clear
    end
    def clear
      @store = {}
    end
  end
  
  def clear
    self.class.clear
  end
  
  private def store
    self.class.store
  end
end

How does it compare to other repository pattern implementations

Compared to other gem implementing the repository pattern this gem makes no assumptions regarding the interface of your repository or what kind of data source is used. Some alternative have some interesting features nevertheless:

• Hanami::Repository is for one closely tied to the the Hanami entities and does not separate the repository interface from the implementing strategies. For straight forward mapping of entity to data source this might be enough though. Another caveat is that it currently only supports SQL data sources.
• ROM::Repository similarly is tied to other facilities of ROM like the ROM containers. It also appears to take a similar approach as Hanami to custom queries which should not leak to the outside application. There is predefined interface for manipulating resources through. The addition of ROM::Changeset brings an interesting addition to the mix which might make it an interesting alternative if using ROM fits into the applications structure.

This gem on the other hand makes absolutely no assumptions about your data source or general structure of your code. It can be simply plugged in between your business logic and data source to abstract the two. Of course like the other repository pattern implementations strategy details should be hidden from the interface. The data source can essentially be anything. A SQL database, a no-SQL database, a JSON API or even a gem. Placing a gem behind a repository can be useful if you're not yet sure this is the correct or best possible gem, the faraday gem is essentially doing this by giving all the different http libraries a common interface).

Testing

A module called TestSupport can be found here. Right now it provides 2 helper methods with_strategy to easily toggle temporarily to another strategy and ensure_method_delegators to solve issues caused by Rspec when attempting to stub a repository method. Both methods and how to use them is described in more detail in the inline documentation.

Goals of this implementation

• small core codebase
• minimal processing overhead - fast method dispatching
• flexible - all kind of methods should possible to be mediated
• basic but powerful callbacks/hooks/observer possibilities

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test 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.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/andreaseger/receptacle. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

Attribution

Runtastic is using the repository pattern extensively in its backend services and inspired the creation of this library. Nevertheless no code developed at Runtastic was used in this library.

License

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