[WIP] Tram::Middleware

Simple DSL for building configurable middleware

Installation

Add this line to your application's Gemfile:

gem 'tram-middleware'

And then execute:

$ bundle

Or install it yourself as:

$ gem install tram-middleware

Example

Let's build a tiny translator that translates a text via Google Translate, but skips translation of texts, surrounded by double ##.

We're going to provide the following API:

translator.call text: "The ##Ruby## is awesome!", from: :en, into: :ru
# => "Ruby потрясающий!"

We will build it as a stack of 3 middleware:

• check if the translation is necessary
• transform texts surrounded by ## into <span class='notranslate'>...</span>, and restore it back to the original chunk after translation
• send strings to the Google API via [GoogleTranslateDiff][google-translate-diff]

The Design

Let's start from defining a translator.

translator = Tram::Middleware.new do
  desc "Translate the text from one locale into another"

  # This is an input contract for every layer
  option :text, proc(&:to_s), desc: "The text to be translated"
  option :from, proc(&:to_s), desc: "The source locale"
  option :into, proc(&:to_s), desc: "The target locale"

  # This is an output contract
  output proc(&:to_s)

  # Build a stack in the natural order from outer to inner layers
  use CheckNecessity
  use SanitizeNotranslate, as: :sanitize
  use GoogleTranslate do |options|
    # Define one of the options, expected by this layer at a load time
    options[:api_key] = ENV["GOOGLE_API_KEY"]
  end
end

This allows to extend an already configured middleware

Layers

Earlier we used some classes as a layers. Let's declare them (in fact, they must be defined first).

When defining a layer we should:

• provide the layer's description
• define options expected by the layer
• specify additional rules be satisfied by options

class CheckNecessity < Tram::Middleware
  desc "Skip translation to the same language"

  # These are options used by a layer. All the rest of options are ignored
  option :text, desc: "The text to be translated"
  option :from, desc: "The source locale"
  option :into, desc: "The target locale"

  def call
    return text if from == into

    # Call the next layer and return its results
    # Through the options you can access all input, including
    # options that weren't declared by the layer, but
    # declared by a middleware
    yield(options)
  end
end

That was simple. The next layer uses a local state shared by handlers of input and output:

class SanitizeNotranslate < Tram::Middleware
  desc "Prevent translation of texts inside double ##"

  # This is the only option this layer is interested in
  # Nethertheless, the `options` would also contain keys
  # `:from` and `:into` because they are declared by a middleware
  option :text, proc(&:to_s), desc: "The text to be sanitized"

  def call
    # Remember the local state which is necessary for the outtput
    input, state = prepare(text)

    # Call the next layer
    output = yield(**options, text: input)

    # Use the local state
    restore(output, state)
  end

  private

  # Extracts chunks wrapped in '##' like: 'The ##OS#2## system' -> 'OS#2'
  CHUNK = /##((?:#(?!#)|[^#])*)##/.freeze

  def prepare(text)
    state = text.scan(CHUNK).uniq
    input = state.with_index.reduce(text) do |text, (chunk, num)|
      text.gsub "###{chunk}##", "<span class='notranslate'>#{num}</span>"
    end

    [input, state]
  end

  def restore(text, state)
    state.with_index.reduce(text) do |text, (chunk, num)|
      text.gsub "<span class='notranslate'>#{num}</span>", chunk
    end
  end
end

In the innest layer we show how to add a configuration.

class GoogleTranslate < Tram::Middleware
  desc "Send text for translation by GoogleTranslateDiff"

  # This option is not defined at the middleware layer,
  # and it wan't be included into the `#options` hash.
  # You should provide it when adding a layer in the method `use`
  option :api_key, proc(&:to_s), desc: "Google auth key"
  option :text,    proc(&:to_s), desc: "The text to be translated"

  def call
    # We don't yield here because this is the innest layer.
    # If we still yielded, this would raise NotImplementedError
    # because there's no more layers to call.
    client.translate(text, options.slice(:from, :into))
  end

  private

  # Use pre-configured `api_key`
  def client
    @client ||= GoogleTranlsateDiff.new(api_key: api_key)
  end
end

That's that. We defined both the stack and inter-layer interfaces, and can use the translator:

translator = Tram::Middleware do
  # ...
end

translator.call text: "The ##Ruby## is awesome!", from: :en, into: :ja
# => "Rubyは素晴らしいです！"

Inspection

With all the definitions above you can inspect the resulting middleware (that's what descriptions were for):

> puts translator.inspect
Tram::Middleware: Translate text from one language into another
  Input options:
    text: The text to be translated (required)
    from: The source locale (required)
    into: The target locale (required)
  Stack layers:
    CheckNecessity: Skip translation to the same language
    sanitize: Prevent translation of texts inside double ##
    GoogleTranslate: Send text for translation by GoogleTranslateDiff
      api_key: "foobar" (The authentication key to the Google Translate API)
  Output: The translated text
=> nil

Extencion of the Existing Stack

Before we composed the stack with the only method use, which appends layers to the bottom.

You can also extend the existing stack by adding a layer to the arbitrary place, and removing layers from a stack.

Suppose we have a stack:

translator = Tram::Middleware.new do
  # ...
  use CheckNecessity
  use SanitizeNotranslate, as: :sanitize
  use GoogleTranslate do |options|
    # Define one of the options, expected by this layer at a load time
    options[:api_key] = ENV["GOOGLE_API_KEY"]
  end
end

Now we can modify it:

translator.drop :sanitize
translator.use DoSomethingElse, before: :GoogleTranslate, as: :new_layer

Now the stack differs:

> puts translator.inspect
Tram::Middleware: Trrnslate text from one language into another
  # ...
  Stack layers:
    CheckNecessity: Skip translation to the same language
    new_layer: Do something else
    GoogleTranslate: Send text for translation by GoogleTranslateDiff
      api_key: "foobar" (The authentication key to the Google Translate API)

That's how you can do your middleware extendable.

Development

After checking out the repo, run bundle to install dependencies. Then, run bundle exec rake to run the tests and linters.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/tram-rb/tram-middleware.

License

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