Yak Serializers

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Yaks is a tool for turning your domain models into Hypermedia resources.

There are at the moment a number of competing media types for building Hypermedia APIs. These all add a layer of semantics on top of a low level serialization format such as JSON or XML. Even though they each have their own design goals, the core features mostly overlap. They typically provide a way to represent resources (entities), and resource collections, consisting of

• Data in key-value format, possibly with composite values
• Embedded resources
• Links to related resources
• Outbound links that have a specific relation to the resource

They might also contain extra control data to specify possible future interactions, not unlike HTML forms.

These different media types for Hypermedia clients and servers base themselves on the same set of internet standards, such as RFC4288 Media types, RFC5988 Web Linking, RFC6906 The "profile" link relation and RFC6570 URI Templates.

Concepts

Yaks is a processing pipeline, you create and configure the pipeline, then feed data through it.

yaks = Yaks.new do
  default_format :hal
  rel_template 'http://api.example.com/rels/{rel}'
  format_options(:hal, plural_links: [:copyright])
  namespace ::MyAPI
  json_serializer do |data|
    MultiJson.dump(data)
  end
end

yaks.call(data) # => JSON

Yaks performs this serialization in three steps

• It maps your data to a Yaks::Resource
• It formats the resource to a syntax tree representation
• It serializes to get the final output

For JSON types, the "syntax tree" is just a combination of Ruby primitives, nested arrays and hashes with strings, numbers, booleans, nils.

A Resource is an abstraction shared by all output formats. It can contain key-value attributes, RFC5988 style links, and embedded sub-resources.

To build an API you create a "mapper" for each type of object you want to represent. Yaks takes care of the rest.

For all configuration options see Yaks::Config::DSL.

See also the API Docs on rdoc.info

Mappers

Say your app has a Post object for blog posts. To serve posts over your API, define a PostMapper

class PostMapper < Yaks::Mapper
  link :self, '/api/posts/{id}'

  attributes :id, :title

  has_one :author
  has_many :comments
end

Configure a Yaks instance and start serializing!

yaks = Yaks.new
yaks.call(post)

or a bit more elaborate

yaks = Yaks.new do
  default_format :json_api
  rel_template 'http://api.example.com/rels/{rel}'
  format_options(:hal, plural_links: [:copyright])
end

yaks.call(post, mapper: PostMapper, format: :hal)

Attributes

Use the attributes DSL method to specify which attributes of your model you want to expose, as in the example above. You can override the load_attribute method to change how attributes are fetched from the model.

For example, if you are representing data that is stored in a Hash, you could do

class PostHashMapper < Yaks::Mapper
  attributes :id, :body

  # @param name [Symbol]
  def load_attribute(name)
    object[name]
  end
end

The default implementation will first try to find a matching method for an attribute on the mapper itself, and will then fall back to calling the actual model. So you can add extra 'virtual' attributes like so :

class CommentMapper < Yaks::Mapper
  attributes :id, :body, :date

  def date
    object.created_at.strftime("at %I:%M%p")
  end
end

Filtering

You can override #attributes, or #associations.

class SongMapper
  attributes :title, :duration, :lyrics

  has_one :artist
  has_one :album

  def minimal?
    env['HTTP_PREFER'] =~ /minimal/
  end

  # @return Array<Yaks::Mapper::Attribute>
  def attributes
    return super.reject {|attr| attr.name.equal? :lyrics } if minimal?
    super
  end

  # @return Array<Yaks::Mapper::Association>
  def associations
    return [] if minimal?
    super
  end
end

Links

You can specify link templates that will be expanded with model attributes. The link relation name should be a registered IANA link relation or a URL. The template syntax follows RFC6570 URI templates.

class FooMapper < Yaks::Mapper
  link :self, '/api/foo/{id}'
  link 'http://api.foo.com/rels/comments', '/api/foo/{id}/comments'
end

To prevent a link to be expanded, add expand: false as an option. Now the actual template will be rendered in the result, so clients can use it to generate links from.

To partially expand the template, pass an array with field names to expand. e.g.

class ProductMapper < Yaks::Mapper
  link 'http://api.foo.com/rels/line_item', '/api/line_items?product_id={product_id}&quantity={quantity}', expand: [:product_id]
end

# "_links": {
#    "http://api.foo.com/rels/line_item": {
#      "href": "/api/line_items?product_id=273&quantity={quantity}",
#      "templated": true
#    }
# }

You can pass a symbol instead of a template, in that case the symbol will be used as a method name on the object to retrieve the link. You can override this behavior just like with attributes.

class FooMapper < Yaks::Mapper
  link 'http://api.foo.com/rels/go_home', :home_url
  # by default calls object.home_url

  def home_url
    object.setting('home_url')
  end
end

Associations

Use has_one for an association that returns a single object, or has_many for embedding a collection.

Options

• :mapper : Use a specific for each instance, will be derived from the class name if omitted (see Policy vs Configuration)
• :collection_mapper : For mapping the collection as a whole, this defaults to Yaks::CollectionMapper, but you can subclass it for example to add links or attributes on the collection itself
• :rel : Set the relation (symbol or URI) this association has with the object. Will be derived from the association name and the configured rel_template if ommitted
• :link_if: Conditionally render the association as a link. A :href option is required

class ShowMapper < Yaks::Mapper
  has_many :events, href: '/show/{id}/events', link_if: ->{ events.count > 50 }
end

Calling Yaks

Once you have a Yaks instance, you can call it with call (serialize also works but might be deprecated in the future.) Pass it the data to be serialized, plus options.

• :env a Rack environment, see next section
• :format the format to be used, e.g. :json_api. Note that if the Rack env contains an Accept header which resolves to a recognized format, then the header takes precedence
• :mapper the mapper to be used. Will be inferred if omitted
• :item_mapper When rendering a collection, the mapper to be used for each item in the collection. Will be inferred from the class of the first item in the collection if omitted.

Rack env

When serializing, Yaks lets you pass in an env hash, which will be made available to all mappers.

yaks = Yaks.new
yaks.call(foo, env: my_env)

class FooMapper
  attributes :bar

  def bar
    if env['something']
      #...
    end
  end
end

The env hash will be available to all mappers, so you can use this to pass around context. In particular context related to the current HTTP request, e.g. the current logged in user, which is why the recommended use is to pass in the Rack environment.

If env contains a HTTP_ACCEPT key (Rack's way of representing the Accept header), Yaks will return the format that most closely matches what was requested.

Namespace

Yaks by default will find your mappers for you if they follow the naming convention of appending 'Mapper' to the model class name. This (and all other "conventions") can be easily redefined though, see below. If you have your mappers inside a module, use namespace.

module API
  module Mappers
    class PostMapper < Yaks::Mapper
      #...
    end
  end
end

yaks = Yaks.new do
  namespace API::Mappers
end

If your namespace contains a CollectionMapper, Yaks will use that instead of Yaks::CollectionMapper, e.g.

module API
  module Mappers
    class CollectionMapper < Yaks::CollectionMapper
      link :profile, 'http://api.example.com/profiles/collection'
    end
  end
end

You can also have collection mappers based on the type of members the collection holds, e.g.

module API
  module Mappers
    class LineItemCollectionMapper < Yaks::CollectionMapper
      link :profile, 'http://api.example.com/profiles/line_items'
      attributes :total

      def total
        collection.inject(0) do |memo, line_item|
          memo + line_item.price * line_item.quantity
        end
      end
    end
  end
end

Yaks will automatically detect and use this collection when serializing an array of LineItem objects.

Custom attribute/link/subresource handling

When inheriting from Yaks::Mapper, you can override map_attributes, map_links and map_resources to skip (or augment) above methods, and instead implement your own custom mechanism. These methods take a Yaks::Resource instance, and should return an updated resource. They should not alter the resource instance in-place. For example

class ErrorMapper < Yaks::Mapper
  link :profile, '/api/error'

  def map_attributes(resource)
    attrs = {
      http_code: 500,
      message: object.to_s,
      type: object.class.name.underscore
    }

    case object
    when AllocationException
      attrs[:http_code] = 422
    when ActiveRecord::RecordNotFound
      attrs[:http_code] = 404
      attrs[:type] = "record_not_found"
    end

    resource.update_attributes(attrs)
  end
end

Resources and Serializers

Yaks uses an intermediate "Resource" representation to support multiple output formats. A mapper turns a domain model into a Yaks::Resource. A serializer (e.g. Yaks::Serializer::Hal) takes the resource and outputs the structure of the target format.

Since version 0.4 the recommended API is through Yaks.new {...}.serialize. This will give you back a composite value consisting of primitives that have a mapping to JSON, so you can use your favorite JSON encoder to turn this into a character stream.

my_yaks = Yaks.new
hal = my_yaks.call(model)
puts JSON.dump(hal)

There are at least a handful of JSON libraries and implementations for Ruby out there, with different trade-offs. Yaks does not impose an opinion on which one to use

HAL

This is the default. In HAL one decides when building an API which links can only be singular (e.g. self), and which are always represented as an array. Yaks defaults to singular as I've found it to be the most common case. If you want specific links to be plural, then configure their rel href as such.

hal = Yaks.new do
  format_options :hal, plural_links: ['http://api.example.com/rels/foo']
end

CURIEs are not explicitly supported (yet), but it's possible to use them with some effort, see examples/hal01.rb for an example.

The line between a singular resource and a collection is fuzzy in HAL. To stick close to the spec you're best to create your own singular types that represent collections, rather than rendering a top level CollectionResource.

JSON-API

default_format :json_api

JSON-API has no concept of outbound links, so these will not be rendered. Instead the key will be inferred from the mapper class name by default. This can be changed per mapper:

class AnimalMapper
  key :pet
end

Or the policy can be overridden:

yaks = Yaks.new do
  derive_type_from_mapper_class do |mapper_class|
    piglatinize(mapper_class.to_s.sub(/Mapper$/, ''))
  end
end

Collection+JSON

default_format :collection_json

Subresources aren't mapped because Collection+JSON doesn't really have that concept, and the other way around templates and queries don't exist (yet) in Yaks.

More formats

Are planned... at the moment HAL is the only format I actually use, so it's the one that's best supported. Adding formats that follow the resource=(attributes, links, subresources) structure or a subset thereof is straightforward. More features, e.g. forms/actions such as used in Mason might be added in the future.

Hooks

It is possible to hook into the Yaks pipeline to perform extra processing steps before, after, or around each step. It also possible to skip a step.

yaks = Yaks.new do
  # Automatically give every resource a self link
  after :map, :add_self_link do |resource|
    resource.add_link(Yaks::Resource::Link.new(:self, "/#{resource.type}/#{resource.attributes[:id]}"))
  end

  # Skip serialization, so Ruby primitives come back instead of JSON
  # This was the default before versions < 0.5.0
  skip :serialize
end

Policy over Configuration

It's an old adage in the Ruby/Rails world to have "Convention over Configuration", mostly to derive values that were not given explicitly. Typically based on things having similar names and a 1-1 derivable relationship.

This saves a lot of typing, but for the uninitiated it can also create confusion, the implicitness makes it hard to follow what's going on.

What's worse, is that often the Configuration part is skipped entirely, making it very hard to deviate from the Golden Standard.

There is another old adage, "Policy vs Mechanism". Implement the mechanisms, but don't dictate the policy.

In Yaks whenever missing values need to be inferred, like finding an unspecified mapper for a relation, this is handled by a policy object. The default is Yaks::DefaultPolicy, you can go there to find all the rules of inference. Single rules of inference can be redefined directly in the Yaks configuration:

yaks = Yaks.new do
  derive_mapper_from_object do |model|
    # ...
  end

  derive_type_from_mapper_class do |mapper_class|
    # ...
  end

  derive_mapper_from_association do |association|
    # ...
  end

  derive_rel_from_association do |mapper, association|
    # ...
  end
end

You can also subclass or create from scratch your own policy class

class MyPolicy < DefaultPolicy
  #...
end

yaks = Yaks.new do
  policy MyPolicy
end

Primitives

For JSON based formats, the "syntax tree" is merely a structure of Ruby primitives that have a JSON equivalent. If your mappers return non-primitive attribute values, you can define how they should be converted. For example, JSON has no notion of dates. If your mappers return these types as attributes, then Yaks needs to know how to turn these into primitives. To add extra types, use map_to_primitive

Yaks.new do
  map_to_primitive Date, Time, DateTime do |date|
    date.iso8601
  end
end

This can also be used to transform alternative data structures, like those from Hamster, into Ruby arrays and hashes. Use call() to recursively turn things into primitives.

Yaks.new do
  map_to_primitive Hamster::Vector, Hamster::List do |list|
    list.map do |item|
      call(item)
    end
  end
end

Yaks by default "primitivizes" symbols (as strings), and classes that include Enumerable (as arrays).

Real World Usage

Yaks is used in production by Ticketsolve. You can find an example API endpoint here.

Get in touch if you like to see your name and API here.

Demo

You can find an example app at Yakports, or browse the HAL api directly using the HAL browser.

Acknowledgment

The mapper syntax is largely borrowed from ActiveModel::Serializers, which in turn closely mimics the syntax of ActiveRecord models. It's a great concise syntax that still offers plenty of flexibility, so to not reinvent the wheel I've stuck to the existing syntax as far as practical, although there are several extensions and deviations.

Lightweight

Yaks is a lean library. It only depends on a few other tiny libraries (inflection, concord, uri_template). It has no core extensions (monkey patches). There is deliberately no built-in "integration" with existing frameworks, since the API is simply enough. You just call it.

If this approach sounds appealing, have a look at microrb.com.

Is it any good

Yes

How to contribute

Run the tests, the examples, try it with your own stuff and leave your impressions in the issues. Or discuss on API-craft.

To fix a bug

1. Fork the repo
2. Fix the bug, add tests for it
3. Push it to a named branch
4. Add a PR

To add a feature

1. Open an issue as soon as possible to gather feedback
2. Same as above, fork, push to named branch, make a pull-request

Yaks uses Mutation Testing. Run rake mutant and look for percentage coverage. In general this should only go up.

License

MIT License (Expat License), see LICENSE