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A HAL client customized for Shaf APIs
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Shaf Client

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ShafClient is a hypermedia client using the HAL mediatype. It supports some mediatype profiles and customizations used in APIs built with Shaf.


gem install shaf_client

Or put gem 'shaf_client' in your Gemfile and run bundle install


Create an instance of ShafClient with a uri to the API entry point. Then call get_root on the returned client to get back a ShafClient::Resource and start interacting with the API.

client = ShafClient.new('https://my.hal_api.com/')
root = client.get_root

Instances of ShafClient::Resource respond to the following methods:

  • #attributes - Returns a hash of all attributes
  • #links - Returns a hash of all links
  • #curies - Returns a hash of all curies
  • #embedded_resources - Returns a hash of all embedded resources
  • #attribute(key) - Returns the value for the attribute with the given key
  • #link(rel) - Returns a ShafClient::Link for the given rel
  • #curie(rel) - Returns a ShafClient::Curie for the given rel
  • #embedded(rel) - Returns a ShafClient::BaseResource for the given rel
  • #[](key) - Alias for attribute(key)
  • #actions - Returns a list of all links relations
  • #to_s - Returns a String representation
  • #inspect - Returns a detailed String representation
  • #get(rel, **options) - Performs a GET request to the href of the link with rel rel
  • #put(rel, payload: nil, **options) - Performs a PUT request to the href of the link with rel rel
  • #post(rel, payload: nil, **options) - Performs a POST request to the href of the link with rel rel
  • #delete(rel, payload: nil, **options) - Performs a DELETE request to the href of the link with rel rel
  • #patch(rel, payload: nil, **options) - Performs a PATCH request to the href of the link with rel rel
  • #get_doc(rel) - Retrieves the documentation for a rel by looking up its curie
  • #get_hal_form(rel) - Retrieves a form by performing a GET request on the value of rel.
  • #rel?(rel) - Returns true if the resource has a link with rel rel
  • #reload! - Refresh itself by fetching the self link (by-passing cache)
  • #destroy! - Performs a DELETE request to the href of the link with rel delete
  • #http_status - The response HTTP status returned by the server
  • #headers - The response HTTP headers returned by the server

They will also respond to each attribute key that they contain (i.e like #[](key) or #attribute(key))

Instances of ShafClient respond to the following methods:

  • #get_root(**options) - Performs a GET request to the root_uri (first arg to #initialize)
  • #head(uri, **options) - Performs a HEAD request to the given uri
  • #get(uri, **options) - Performs a GET request to the given uri
  • #put(uri, payload, **options) - Performs a PUT request to the given uri
  • #post(uri, payload, **options) - Performs a POST request to the given uri
  • #delete(uri, payload, **options) - Performs a DELETE request to the given uri
  • #patch(uri, payload, **options) - Performs a PATCH request to the given uri


require 'shaf_client'
client = ShafClient.new('http://localhost:3000')
root = client.get_root      # Equivalent to client.get('http://localhost:3000')
root.headers                # => {"content-type"=>"application/hal+json", "cache-control"=>"private, max-age=20"…
root = client.get_root      # Returns same response as above, except this time no network request is performed. A cached                                 # response is returned instead
root.actions                # => [:self, :posts, :comments]

posts = root.get(:posts)
posts.embedded_resources    # => {:posts=>[#<ShafClient::Resource:0x00005615723cad10 @payload…
posts.embedded(:posts)      # Returns an array of `ShafClient::Resource` instances
posts.actions               # => [:self, :up, :"doc:create-form"]
form = posts.get("doc:create-form") # this assumes that Content-Type contains the profile 'shaf-form'.
                                    # it's also possible to type: posts.get("create-form") or posts.get(:create_form)
form.headers                # => {"content-type"=>"application/hal+json;profile=shaf-form", "cache-control"=>"private, max-age=3600", "etag"=>"W/\"83ef6d28f4b81f8f9ceae17f5f8a42d6dedfff73\"…"}
form.class                  # => ShafClient::ShafForm
form.values                 # => {:title=>nil, :message=>nil}
form.valid?                 # => false
form[:title] = "hello"
form[:message] = "world"
created_post = form.submit  # Returns a new `ShafClient::Resource`

created_post.attributes     # => {:title=>"hello", :message=>"world"}
created_post.title          # => "hello"
created_post.actions        # => [:"collection", :self, :"edit-form", :"doc:delete"]
puts created_post.to_s      # => {
                            #      "title": "hello",
                            #      "message": "world",
                            #      "_links": {
                            #        "collection": {
                            #          "href": "http://localhost:3000/posts",
                            #          "title": "up"
                            #        },
                            #        "self": {
                            #          "href": "http://localhost:3000/posts/1"
                            #        },
                            #        "edit-form": {
                            #          "href": "http://localhost:3000/posts/1/edit",
                            #          "title": "edit"
                            #        },
                            #        "doc:delete": {
                            #          "href": "http://localhost:3000/posts/1",
                            #          "title": "delete"
                            #        },
                            #        "curies": [
                            #          {
                            #            "name": "doc",
                            #            "href": "http://localhost:3000/doc/post/rels/{rel}",
                            #            "templated": true
                            #          }
                            #        ]
                            #      }
                            #    }

created_post.link(:self).href      # => "http://localhost:3000/posts/1"

delete_doc = created_post.get_doc("doc:delete")
puts delete_doc.actions     # => [:self, :up]
puts delete_doc.attribute(:delete) # => Link to delete this post.
                                   #    Method: DELETE
                                   #    Example:
                                   #    curl -H "Accept: application/hal+json" \
                                   #         -H "Authorization: abcdef \"
                                   #         -X DELETE \
                                   #         /posts/5

# Request headers can be given to #get, #put, etc throught the headers keyword argument
problem_json = client.get('http://localhost:3000/idonotexist', headers: {'Accept' => 'application/problem+json'})
puts problem_json.content_type    # => application/problem+json
puts problem_json
                                  # => {
                                  #      "status": 404,
                                  #      "type": "about:blank",
                                  #      "title": "Not Found",
                                  #      "detail": "Resource \"/idonotexist\" does not exist"
                                  #    }

Adding semantic meaning to resources

Note the form in the example above. form is an instance of ShafClient::ShafForm (which is a subclass of ShafClient::Form which in turn is a subclass of ShafClient::Resource). Forms have a few extra methods that makes it easy to fill in values and submitting them. The reason that we received an instance of ShafClient::ShafForm rather than ShafClient::Resource is that the server responded with the Content-Type application/hal+json;profile=shaf-form. The shaf-form profile describes the semantic meaning of this representation and luckily ShafClient knowns about this profile.
Adding support for other profiles is as simple as creating a subclass of ShafClient::Resource and call the class method profile with the name of your profile. To be correct the profile should actually be a URI and it can be specified as a media type parameter (as above), using a Link header or using a link in the payload. (Note: to be able to parse the link from the payload, a resource class that matches the Content-Type header must be registered.)
So say that you have a server that returns a response with Content-Type: application/hal+json;profile="https://example.com/foobar". Then you could do something like this:

class CustomResource < ShafClient::Resource
  profile 'https://example.com/foobar'

  def attr_string

foobar = client.get_root.get(:some_rel_returning_foobar)
foobar.class            # => CustomResource
foobar.attr_string      # => "key1_key2_key3"

Note: This only serves the purpose of understanding how this works :)
Instances of ShafClient::Form respond to the following methods:

  • #values - Returns a hash of the form inputs
  • #[](key) - Returns the value of a given input
  • #[]=(key, value) - Sets the value of a given input
  • #title - Returns the title of the form
  • #target - Returns the target href (where the form will be submitted to)
  • #http_method - Returns the HTTP method to be used when submitting the form
  • #content_type - Returns the content type used when the form is submitted
  • #submit - Submit the form
  • #valid? - Returns true if client side validations pass. Otherwise false

If the profile URI is dereferencable and the returned payload is presented as application/alps+json, then ShafClient will parse the ALPS profile to understand more about the resource. Each link relation that is described with an http_method extension (look here for more info) will get a method for activating the corresponding link relation. For example if a payload contains a link with rel publish, by default ShafClient wont know how to activate that link. Should it use GET, PUT, POST etc? But if we get a profile that happens to resolve to something like this:

  "alps": {
    "version": "1.0",
    "descriptor": [
        "id": "publish",
        "type": "idempotent",
        "doc": {
          "value": "The link relation 'publish' means that the corresponding post resource\nmay be requested to be published. To activate this link relation, perform\nan HTTP PUT request to the href of this link relation.\n"
        "name": "publish",
        "ext": [
            "id": "http_method",
            "href": "https://gist.github.com/sammyhenningsson/2103d839eb79a7baf8854bfb96bda7ae",
            "value": [

This would generate a publish! method which will use HTTP PUT. Note: the bang version.


ShafClient also support forms presented using the HAL-FORMS mediatype. The workflow using HAL-FORMS differs a bit from shaf-forms and requires the client to intentionally request a form. For this, the method #get_hal_form(rel) is used. The returned object is an instance of ShafClient::HalForm (which is a subclass of ShafClient::Form). So submitting the form follows the same flow as shown above. The rel given to #get_hal_form(rel) may be compacted with a curie. In that case it will be "expanded" before the GET request is performed.

Non HAL responses

Of course, not all responses will be formatted as HAL. Whenever the response body is empty an instance of ShafClient::EmptyResource is returned. If the Content-Type cannot be understood an instance of ShafClient::UnknownResource is returned. These two classes also inherit from ShafClient::Resource so all the usual methods are still available (though most of them return nil, "", {} or []). Instances of ShafClient::UnknownResource also has a#body method. #body, #http_status and #headers are basically the only usefull methods for those instances. Problem Json responses will return instances of ShafClient::ProblemJson (see example above).


ShafClient supports basic auth and token based authentication.
For Basic Auth, pass keyword arguments :user and password when instantiating the client.

client = ShafClient.new('https://my.hal_api.com/', user: "alice", password: "ecila")

For Token based authentication, pass keyword argument :auth_token when instantiating the client. This will send the token in the X-Auth-Token header. To use another header, set it with the keyword argument :auth_header.

client = ShafClient.new('https://my.hal_api.com/', auth_token: "Ohd2quet")
# or
client = ShafClient.new('https://my.hal_api.com/', auth_token: "Ohd2quet", auth_header: "Authorization")


ShafClient wraps the faraday gem. By default it uses the Net::HTTP adapter. To use another adapter pass in the corresponding symbol in the :faraday_adapter when instantiating the client. (Note: make sure to install and require corresponding dependencies.)

client = ShafClient.new('https://my.hal_api.com/', faraday_adapter: :net_http_persistent)

HTTP cache

ShafClient supports HTTP caching by using the faraday-http-cache, Faraday middleware. This means that if the server returns responses with caching directives (e.g. Cache-Control, Etag etc), those responses are properly cached. And no unnecessary request will be made when a valid cache entry exist. To pass down options to faraday-http-cache (e.g a cache store) pass them to ShafClient as options under the :faraday_http_cache key.

store = ActiveSupport::Cache.lookup_store(:mem_cache_store, ['localhost:11211'])
client = ShafClient.new('https://my.hal_api.com/', faraday_http_cache: {store: store})

Hypertext Cache Pattern

Servers may preload resources (by embedded them) in hope of increasing the api performance. See Hypertext Cache Pattern for more info. An application using ShafClient, might have a "hard coded" workflow where it always fetches posts and then their authors. E.g.

post = client.get(some_post_uri)
author = post.get(:author)

Normally this would result in two requests to the server. However if the API server decides that the client probably also want the corresponding author resource (which normally is just linked to but not embedded), it may embedded the author resource in the post response. In this case ShafClient will only make one request and simply return the embedded author on the second line above. This adds great flexibility for servers to dynamically change the responds to increase performance.
However this may cause problems if the embedded resource should not be interpreted as application/hal+json (i.e. plain HAL without any extensions). ShafClient has three settings for how to handle this:

  1. Ignore embedded resources and refetch them through the link relation.
  2. Return the embedded resource (as regular HAL resource).
  3. Perform a HEAD request and return the resource with correct headers.

This can be configured globally by calling ShafClient.default_hypertext_cache_strategy=(strategy). Where strategy is one of (Note: same order as above):

  1. :no_cache
  2. :use_embedded
  3. :fetch_headers

It can also be configured per request, by using the hypertext_cache_strategy option. E.g.

post.get(:author, hypertext_cache_strategy: :fetch_headers)

Example API

If you would like to try out ShafClient but don't yet have a HAL API, then an example api, created with Shaf, can be found here.

client = ShafClient.new("https://shaf-blog-demo.herokuapp.com/")


ShafClient will automatically follow redirects.


If you find a bug or have suggestions for improvements, please create a new issue on Github. Pull request are welcome! As usual: Fork, commit changes to a new branch, open a pull request!


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