Grape DSL Extension

[Alejandro Wainzinger]

The Grape DSL on endpoints has great things like "desc" and "params". What would be a simple way to add one's own DSL function?

The use case here is throttling. Let's suppose you want to throttle only some endpoints, and customize it in certain ways:

desc "This endpoint does things"
throttle daily: 3, except: ->(user) { user.is_admin? }
params do
  requires :param1, type: String, desc: "The param"
end

get "/:param1" do
  {result: "You sent #{param1}"}
end

The "throttle" DSL doesn't exist, but I'd like to build something similar to that. In this case it would limit a user (as customly defined by the system elsewhere, not by IP address) to 3 requests per day, but exclude admin users from this. I might not use the "except" part but I just wanted to show the nice kind of things this type of DSL could offer. Having the throttling shown at the level of the endpoint also serves as nice documentation.

The rack-throttle middleware can do the trick with a custom throttle strategy defined, but it needs information on the endpoints, which I think a "throttle" DSL statement from the endpoints could give.

Any good ideas for how to go about this? Thanks!

[Daniel Doubrovkine]

In 0.9.0 we had a hacky way of doing this:

	module Grape
	module Extensions
	module AdminExtension
	[:admin, :user].each do |role|
	define_method role do |value|
	@last_description ||= {}
	@last_description.merge!("role_#{role}".to_sym => value)
	value
	end
	end
	Grape::API.extend self
	end
	end
	end

This would add a role_admin or role_user to the desc which can be examined at runtime:

	module Api
	module Middleware
	class UserRoleMiddleware < Grape::Middleware::Base
	def before
	if route_options[:role_admin]
	authenticated_admin
	elsif route_options[:role_user]
	authenticated_user
	end
	end
	private
	def route_options
	env['api.endpoint'].options[:route_options]
	end
	end
	end
	end

This gets cleaner in 0.10.0 (HEAD) by using route_setting:

	module Grape
	module Extensions
	module AdminExtension
	[:admin, :user].each do |role|
	define_method role do |value|
	route_setting :role, "role_#{role}".to_sym => value
	value
	end
	end
	Grape::API.extend self
	end
	end
	end

And the middleware that uses it via route_setting.

	module Api
	module V2
	module Middleware
	class UserRoleMiddleware < Grape::Middleware::Base
	def before
	if role[:role_admin]
	authenticated_admin
	elsif role[:role_user]
	authenticated_user
	end
	end
	private
	def role
	env['api.endpoint'].route_setting(:role) || {}
	end
	end
	end
	end
	end

I really would love to see a blog post detailing this, I will probably write one when 0.10.0 is released if nobody beats me to it.

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[Daniel Doubrovkine]

Oh and to use my role example you do "admin true" or "user true" after API params, you obviously can define any method (we tried with role :admin instead for example).  

[Alejandro Wainzinger]

Ah, I searched through the Google Group for DSL and found nothing, and didn't notice the UPGRADING had been updated to show this, sorry about that.

So in the example in 0.9.0, Grape::Extensions::AdminExtension would be loaded somewhere before the Grape::API is loaded, in a project's lib/ or so. For Api::Middleware::UserRoleMiddleware, do you have to register the middleware somewhere, or is just reopening Api::Middleware like this sufficient?

...

[Daniel Doubrovkine]

We mount Grape in Rails, so we put the extension in config/initializers/grape/extensions and add the middleware in the API via `use`.

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[Alejandro Wainzinger]

Got it. I tried with 0.10.0 the way you had it laid out. Within the API I did something like:

module Api
  class Users < Grape::API

    use ::Api::Middleware::ThrottleMiddleware

    # ... endpoints

    desc "My method"
    throttle true
    get "/" do
      {"success": true}
    end
  end
end

But it can't find the throttle method. I defined it the same way you defined "admin" and "user" methods. I loaded the extension via config/initializers similar as you (and ensured it's loading by putting a puts in there first). Did I get something wrong?

[Alejandro Wainzinger]

Ok, turned out I had several issues:
* Padrino was outdated (0.11.4, newest is 0.12.4), so wouldn't work with 0.10.0
* Grape mounted in Padrino no longer cascading properly, so I bumped Grape higher up in the mount() calls

Now the extension's throttle method on the endpoint is called, but when I hit the endpoint, it appears not to call what's in the "before" function defined in your example. I suspect the middleware is not being mixed in properly somehow, will keep trying.

...

[Alejandro Wainzinger]

Ok, resolved. Please excuse the large amount of spam. It turns out that the "use" statement must go in the root level Grape API, before mounting. Then it finally loads.

Thanks!

[Daniel Doubrovkine]

If you want to apply some of this to https://github.com/dblock/grape-on-padrino, that'd be great.

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[Alejandro Wainzinger]

Sure, do you have anything specific in mind? This particular one with throttling I made into a gem since it fits well into several projects I work on with Grape (not just within Padrino), but I can add an example of the general "adding a DSL" pattern that you showed me here. What do you think should be applied?

...

[Daniel Doubrovkine]

First, bring that project up-to-date with Grape's recent release. Then add something creative :) I tend to reticulate splines.

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[Alejandro Wainzinger]

Hahahah, ok, will do my best and send out a PR or two.

...

[Henry Tseng]

Is there a new way of extending the Grape DSL now?  

[Daniel D.]

I think it's pretty easy now by extending Grape::API. See https://github.com/ruby-grape/grape/blob/d1b8e865677659dd30e60324120fa72e7ba1a9fa/lib/grape/dsl/api.rb#L16 for how all DSL extensions are added. So Grape::DSL::API.include MyCustomDSL should work.

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[Henry Tseng]

Is it possible to extend specifically Grape::Validations::ParamsScope and adds more functionality?  

[Daniel D.]

There's no new way to do what the OP was trying to do. What's your scenario? Open a feature request and describe that?

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