Arvados

h1. Hacking API server

{{toc}}

h2. Source tree layout

Everything is in @/services/api@.

Key pieces to know about before going much further:

|/|Usual Rails project layout|

|/app/controllers/application_controller.rb|Controller superclass with most of the generic API features like CRUD, authentication|

|/app/controllers/arvados/v1/|API methods other than generic CRUD (users#current, jobs#queue, ...)|

|/app/models/arvados_model.rb|Default Arvados model behavior: permissions, etag, uuid|

h2. Unlike a typical Rails project...

* Most responses are JSON. Very few HTML views. We don't normally talk to browsers, except during authentication.

* We assign UUID strings (see lib/assign_uuid.rb and app/models/arvados_model.rb)

* The @Links@ table emulates a graph database a la "RDF":http://www.rdfabout.com/quickintro.xpd. Much of the interesting information in Arvados is recorded as a Link between two other entities.

* For the most part, relations among objects are not expressed with the usual ActiveRelation features like belongs_to and has_many.

* Permissions: see below.

h2. Running in development mode

Save something like this at @~/bin/apiserver@, make it executable, make sure ~/bin is in your path:

 #!/bin/sh

set -e

cd ~/arvados/services/api

export RAILS_ENV=development

rvm-exec 2.0.0 bundle install

exec rvm-exec 2.0.0 bundle exec rails server --port=3030

h2. Headaches to avoid

If you make a change that affects the discovery document, you need to clear a few caches before your client will see the change.

* Restart API server or: @touch tmp/restart.txt@

* Clear API server disk cache: @rake tmp:cache:clear@

* Clear SDK discovery doc cache on client side: @rm -r ~/.cache/arvados/@

Do not store symbol keys (or values) in serialized attributes.

* Rails supplies @params@ as a HashWithIndifferentAccess so @params['foo']@ and @params[:foo]@ are equivalent. This is usually convenient. However, here we often copy arrays and hashes from @params@ to the database, and from there to API responses. JSON does not have HashWithIndifferentAccess (or symbols) and we want these serialized attributes to behave predictably everywhere.

* API server's policy is that serialized attributes (like @properties@ on a link) always have strings instead of symbols: these attributes look the same in the database, in the API server Rails application, in the JSON response sent to clients, and in the JSON objects received from clients.

* There is no validation (yet!) to check for this.

h2. Features

h3. Authentication

Involves

* UserSessionsController (in app/controllers/, not .../arvados/v1): this is an exceptional case where we actually talk to a browser.

h3. Permissions

Object-level permissions, aka ownership and sharing

* Models have their own idea of create/update permissions. Controllers don't worry about this.

* ArvadosModel updates/enforces modified_by_* and owner_uuid

* Lookups are not (yet) permission-restricted in the default scope, though. Controllers need to use Model.readable_by(user).

* ApplicationController uses an around_filter that verifies the supplied api_token and makes current_user available everywhere. If you need to override create/update permissions, use @act_as_system_user do ... end@.

* Unusual cases: KeepDisks and Collections can be looked up by inactive users (otherwise they wouldn't be able to read & clickthrough user agreements).

Controller-level permissions

* ApplicationController#require_auth_scope_all checks token scopes: currently, unless otherwise specified by a subclass controller, nothing is allowed unless scopes includes "all".

* ApplicationController has an admin_required filter available (not used by default)

h3. Error handling

* "Look up object by uuid, and send 404 if not found" is enabled by default, except for index/create actions.

h3. Routing

* API routes are in the @:arvados@ → @:v1@ namespace.

* Routes like @/jobs/queue@ have to come before @resources :jobs@ (otherwise @/jobs/queue@ will match @jobs#get(id=queue)@ first). (Better, we should rearrange these to use @resources :jobs do ...@ like in Workbench.)

* We use the standard Rails routes like @/jobs/:id@ but then we move params[:id] to params[:uuid] in our before_filters.

h3. Tests

* Run tests with @rvm-exec 2.0.0 bundle exec rake test RAILS_ENV=test@

* Functional tests need to authenticate themselves with @authorize_with :active@ (where @:active@ refers to an ApiClientAuthorization fixture)

* Big deficit of tests, especially unit tests. This is a bug! It doesn't mean we don't want to test things.

h3. Discovery document

* Mostly, but not yet completely, generated by introspection (descendants of ArvadosModel are inspected at run time). But some controllers/actions are skipped, and some actions are renamed (e.g., Rails calls it "show" but everyone else calls it "get").

* Handled by Arvados::V1::SchemaController#index (used to be in #discovery_document before #1750). See @config/routes.rb@

* Must be available to anonymous clients.

* Has no tests! We test it by trying all of our SDKs against it.

h2. Development patterns

h3. Add a model

In shell:

* @rails g model FizzBuzz@

In @app/models/fizzbuzz.rb@:

* Change base class from @ActiveRecord::Base@ to @ArvadosModel@.

* Add some more standard behavior.

<pre><code class="ruby">

include AssignUuid

include KindAndEtag

include CommonApiTemplate

</code></pre>

In @db/migrate/{timestamp}_create_fizzbuzzes.rb@:

* Add the generic attribute columns.

* Run @t.timestamps@ and add (at least!) a @:uuid@ index.

<pre><code class="ruby">

class CreateFizzBuzz < ActiveRecord::Migration

  def change

    create_table :fizzbuzzes do |t|

      t.string :uuid, :null => false

      t.string :owner_uuid, :null => false

      t.string :modified_by_client_uuid

      t.string :modified_by_user_uuid

      t.datetime :modified_at

      t.text :properties

      t.timestamps

    end

    add_index :humans, :uuid, :unique => true

  end

end

</code></pre>

Apply the migration:

* @rake db:migrate@

* @RAILS_ENV=test rake db:migrate@ (to migrate your test database too)

* Inspect the resulting @db/schema.rb@ and include it in your commit.

* Don't forget to @git add@ the new migration and model files.

h3. Add an attribute to a model

* Generate migration as usual

<pre>

rails g migration AddBazQuxToFooBar baz_qux:column_type_goes_here

</pre>

* Consider adding null constraints and a default value to the @add_column@ statement in the migration in @db/migrate/timestamp_add_baz_qux_to_foo_bar.rb@:

<pre><code class="ruby">, null: false, default: false</code></pre>

* Consider adding an index

* You probably want to add it to the API response template so clients can see it: @app/models/model_name.rb@ → @api_accessible :user ...@

* Sometimes it's only visible to privileged users; see @ping_secret@ in @app/models/keep_disk.rb@

* If it's a serialized attribute, add @serialize :the_attribute_name, Hash@ to the model. Always specify Hash or Array!

* Run @rake db:migrate@ and inspect your @db/schema.rb@ and include the new @schema.rb@ in the *same commit* as your @db/migrate/*.rb@ migration script.

* Run @rake tmp:cache:clear@ and @touch tmp/restart.txt@ in your dev apiserver, to force it to generate a new REST discovery document.

h3. Add a controller

* @rails g controller Arvados::V1::FizzBuzzesController@

* Avoid adding top-level controllers like @app/controllers/fizz_buzzes_controller.rb@.

* Avoid adding top-level routes. Everything should be in @namespace :arvados@ → @namespace :v1@ except oddballs like login/logout actions.

h3. Add a controller action

Add a route in @config/routes.rb@.

* Choose an appropriate HTTP method: GET has no side effects. POST creates something. PUT replaces/updates something.

* Use the block form:

<pre><code class="ruby">

resources :fizz_buzzes do

  # If the action operates on an object, i.e., a uuid is required,

  # this generates a route /arvados/v1/fizz_buzzes/{uuid}/blurfl

  post 'blurfl', on: :member

  # If not, this generates a route /arvados/v1/fizz_buzzes/flurbl

  get 'flurbl', on: :collection

end

</code></pre>

In @app/controllers/arvados/v1/fizz_buzzes_controller.rb@:

* Add a method to the controller class.

* Skip the "find_object" before_filters if it's a collection action.

* Specify required/optional parameters using a class method @_action_requires_parameters@.

<pre><code class="ruby">

skip_before_filter :find_object_by_uuid, only: [:flurbl]

skip_before_filter :render_404_if_no_object, only: [:flurbl]

def blurfl

  @object.do_whatever_blurfl_does!

  show

end

def self._flurbl_requires_parameters

  {

    qux: { type: 'integer', required: true, description: 'First flurbl qux must match this qux.' }

  }

end

def flurbl

  @object = model_class.where('qux = ?', params[:qux]).first

  show

end

</code></pre>

h3. Add a configuration parameter

* Add it to @config/application.default.yml@ with a sensible default value. (Don't fall back to default values at time of use, or define defaults in other places!)

* If there is no sensible default value, like @secret_token@: specify @~@ (i.e., nil) in @application.default.yml@ *and* put a default value in the @test@ section of @config/application.yml.example@ that will make tests pass.

* If there is a sensible default value for development/test but not for production, like return address for notification email messages, specify the test/dev default in the @common@ section @application.default.yml@ but specify @~@ (nil) in the @production@ section. This prevents someone from installing or *updating a production server* with defaults that don't make sense in production!

* Use @Rails.configuration.config_setting_name@ to retrieve the configured value. There is no need to check whether it is nil or missing: in those cases, "rake config:check" would have failed and the application would have refused to start.

h3. Add a test fixture

Generate last part of uuid from command line:

<pre><code class="ruby">ruby -e 'puts rand(2**512).to_s(36)[0..14]'

j0wqrlny07k1u12</code></pre>

Generate uuid from @rails console@:

<pre><code class="ruby">Group.generate_uuid

=> "xyzzy-j7d0g-8nw4r6gnnkixw1i"

</code></pre>