A presentation on Inter-Application Communication via Secure APIs for MagmaConf 2013
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README.md

Inter-Application Communication

Jon Dean

Lead Software Engineer

Stitch Fix

http://jonathandean.com

@jonathanedean

#MagmaConf

Presentation and sample code

github.com/jonathandean/magmaconf-2013-inter-app-api

Presentation is at jonathandean.github.io/magmaconf-2013-inter-app-api

Run locally by cloning and running grunt serve. See reveal.js for instructions.

Apps are at magma-payments-service/ and magma-client-app/

Purpose of the presentation

  • Explain when and why you might want to make a service application
  • Very simple place to start writing code and show a few utilities
  • You can totally google the rest once you have something started

What this presentation isn't

This is not your production code. Don't stop refactoring! Be sure to write tests.

(And I didn't test it all, so good luck!)

How many know these things already?

  • Rails
  • REST
  • API

What is an API?

Application Programming Interface

What the heck does that mean?

Simply, a clearly defined (and hopefully documented) way for different pieces of software to communicate with one another

Different types of APIs

  • Code interfaces
  • Web APIs

APIs in code

The public methods in a Class define an API for how other code interacts with it

class VeryImportantService
  def self.do_my_bidding
    set_up_some_stuff
    do_some_work
    build_the_output
  end

  private

  def set_up_some_stuff
    # ...
  
  def do_some_work
    # ...
  
  def build_the_output
    # ...
end

The API for VeryImportantService is the do_my_bidding method

The private methods are NOT part of the API but part of the internal implementation

Web APIs

Generally a Server application that exposes some data and/or functionality to be consumed by another Client application

The API (typically) is an HTTP endpoint and the internal implementation is up to you

RESTful Web APIs

REST:

REpresentational State Transfer

REST in Rails

Ruby on Rails encourages RESTful URLs as a best practice and should already feel familiar

Key components of REST

  • Stateless
  • Cacheable
  • Uniform interface
  • Designed for use over HTTP
  • Choice of data format

Stateless

All of the information needed is part of the request. There's no need to store the state of anything between requests.

Cacheable

Just like websites over HTTP, responses can generally be cached by the client to improve performance.

Uniform interface

Clients and Servers have a uniform interface that simplifies architecture and design. Additionally, well-design RESTful APIs look familiar and similar to one another.

Designed for HTTP

Uses familar HTTP verbs GET, POST, PUT, and DELETE

The Word Wide Web itself is RESTful

Choice of data format

Most RESTful APIs use JSON or XML for formatting the data, but you can use whatever you want. (The Web uses HTML)

Examples of public RESTful APIs

Twitter REST API

Tumblr API

What do I need an API for?

A few of the many reasons:

  • Give others an interface to your product (Twitter and Tumblr APIs)
  • Allow systems written in different languages to communicate
  • Provide an abstraction layer for your own systems

Real-world Problems at Stitch Fix

Multiple (3 so far) Rails applications that need to share functionality

...AND

  • A legacy admin application written in Django

Ways to share functionality between apps

  • Create a gem containing a shared code library
  • Set up a RESTful API service that all apps (including future ones) can use

How did we start?

Legacy applications suck, so we made some gems.

(Because who cares about Django, right?)

Seriously, a lot of gems

  • Sharing models
  • Deployment and versioning
  • Logging
  • Emails
  • Payments (charging customers $$)
  • Internal authentication/authorization
  • etc.

Pros of sharing code via a gem

  • Easy
  • Abstract the work via code-level APIs
  • Can cost less money (no additional servers)
  • No network latency
  • No need to implement authentication

Cons of sharing code via a gem

Need to update and deploy all applications when the gem changes

  • Testing
  • Downtime... unless you are really great, like we are ;)
  • Coordination
    • Lots of code churn/branches = really frustrating

Cons of sharing code via a gem

Internal improvements require a change in all applications using it

Cons of sharing code via a gem

PaymentClass.charge_someone

changes to

OtherPaymentThing.charge

Cons of sharing code via a gem

With a service application you are typically sharing data/objects instead with basic instructions (create it, update it, delete it, etc.)

POST /customers/:customer_id/transactions     transactions#create

Cons of sharing code via a gem

Cannot easily add other languages to your systems. (Our legacy app can't take advantage of new code!)

Pros of sharing code via an API service

  • Internal changes have no affect on client applications
  • Can independently scale the resource that deals with payments
  • Can add clients in any language to your systems. (We can update our legacy Django app to take advantage of better payment code)
  • Apps don't have to connect to the same data store

Cons of sharing code via an API service

  • More complex
  • Costs money to run an additional application
  • Network latency
  • Handling timeouts and other service unavailability issues
  • Also need to implement an authentication layer

Which method do I choose?

You tell me.

Building a service

Simple application that is the starting point for handling payments in our systems

Shared functionality

  • Create a Customer in the Payment Gateway (Braintree)
  • Charge the customer's credit card
  • List previous transactions for a customer
  • Log the response from the Payment Gateway
  • Handle cases of payment failure
  • etc.

Payments Service Models

  • Customer
  • Transaction

Payments Service Actions/Routes

  • Create a Customer
  • Update Customer information
  • Create a Transaction
  • Show all Transactions for a Customer
  • Show details of a Specific Transaction

RESTful Routes in Payments Service

config/routes.rb

MagmaPaymentsService::Application.routes.draw do
  resources :customers, only: [:create, :update] do
    resources :transactions, only: [:create, :index, :show]
  end
end

RESTful Routes in Payments Service

rake routes

GET  /customers/:customer_id/transactions     transactions#index
POST /customers/:customer_id/transactions     transactions#create
GET  /customers/:customer_id/transactions/:id transactions#show
POST /customers                               customers#create
PUT  /customers/:id                           customers#update

Start with creating a customer

app/controllers/customers_controller.rb

class CustomersController < ApplicationController
  def create
    # Need to send a hash of :id, :first_name, :last_name, :email
    result = Braintree::Customer.create({
      :id => params[:id],
      :first_name => params[:last_name],
      :last_name => params[:last_name],
      :email => params[:email]
    })
    # Build a hash we can send as JSON in the response
    resp = { success: result.success?, message: (result.message rescue '') }
    # Render JSON as the response
    respond_to do |format|
      format.json { render json: resp }
    end
  end
end

Quick setup for Braintree

config/initializers/braintree.rb

Braintree::Configuration.environment = ENV['BRAINTREE_ENV'].to_sym
Braintree::Configuration.merchant_id = ENV['BRAINTREE_MERCHANT_ID']
Braintree::Configuration.public_key = ENV['BRAINTREE_PUBLIC_KEY']
Braintree::Configuration.private_key = ENV['BRAINTREE_PRIVATE_KEY']

Then go to https://www.braintreepayments.com/get-started to sign up and set some environment variables like

export BRAINTREE_ENV=sandbox
export BRAINTREE_MERCHANT_ID=[get your own!]
export BRAINTREE_PUBLIC_KEY=[get your own!]
export BRAINTREE_PRIVATE_KEY=[get your own!]

Let's try

rails server
curl --data "id=1&first_name=Jon&last_name=Dean&email=jon@example.com" \
       http://localhost:3000/customers

outputs

{"success":true,"message":""}

Let's try again

curl --data "id=1&first_name=Jon&last_name=Dean&email=jon@example.com" \
       http://localhost:3000/customers

outputs

{"success":false,"message":"Customer ID has already been taken."}

Tip

If you ever get something like WARNING: Can't verify CSRF token authenticity then remove protect_from_forgery from ApplicationController. We aren't submitting forms from our application to itself and so it will complain.

Models in our Client App

  • User
  • Address
  • etc.

Actions/Routes in our Client App

  • Create a User
  • Update a User
  • Charge the User some money
  • Show the times we charged a User
  • Show details about a time we charged a User
  • etc.

How can our Client App call the API?

httparty is the curl of Ruby

Create the customer from Client App

We want to create a customer in Braintree as soon as the User is created

app/models/user.rb

class User < ActiveRecord::Base

  attr_accessible :name, :email
  after_create :create_payments_customer

  def create_payments_customer
    params = {
      id:         self.id,
      first_name: self.name.split(' ').first,
      last_name:  self.name.split(' ').last,
      email:      self.email
    }
    response = HTTParty.post('http://localhost:3000/customers.json', { body: params })
    answer = response.parsed_response
    puts "response success: #{answer['success']}"
    puts "response message: #{answer['message']}"
  end

end

Let's try it!

rails console
1.9.3p194 :004 > user = User.create(name: "Jon Dean", email: "jon@example.com")
response success: true
response message: 

Clean it up

Each environment will have a different URL

config/environments/development.rb

MagmaClientApp::Application.configure do
  ...
  config.payments_base_uri = 'http://localhost:3000'
end

app/services/payments_service.rb

class PaymentsService
  include HTTParty
  base_uri MagmaClientApp::Application.config.payments_base_uri
end

app/models/user.rb

response = PaymentsService.post('/customers.json', { body: params })

We can do better still

app/services/payments_service.rb

class PaymentsService
  include HTTParty
  base_uri MagmaClientApp::Application.config.payments_base_uri

  def self.create_customer(user)
    params = {
      id:         user.id,
      first_name: user.name.split(' ').first,
      last_name:  user.name.split(' ').last,
      email:      user.email
    }
    response = self.post('/customers.json', { body: params })
    response.parsed_response
  end
end

app/models/user.rb

def create_payments_customer
  answer = PaymentsService.create_customer(self)
  puts "response success: #{answer['success']}"
  puts "response message: #{answer['message']}"
end

What about the Django app?

The beauty is that none of this Ruby-specific. Clients just need to know HTTP and JSON, so they can be written in any language.

If we just made a Ruby gem, the Django app would be out of luck.

Securing the Service

We can't have just anyone creating customers!

The Payments Service App needs a way to know that the client is accepted.

Simple solution

A shared secret token!

> rake secret
0224651fc98de3a615243ebf75188ff430bdb2c1c983ab87614b3db2f4c7a167455354d6c0d2e7e788651bbead373bf0a9a166b12c63d47b48f060cdf759e16e

Setting the token requirement in Payments Service App

config/application.rb

module MagmaPaymentsService
  class Application < Rails::Application
    # NOTE: config.secret_token is used for cookie session data
    config.api_secret = '0224651fc98de3a615243ebf75188ff430bdb2c1c983ab87614b3db2f4c7a167455354d6c0d2e7e788651bbead373bf0a9a166b12c63d47b48f060cdf759e16e'
  end
end

app/controllers/application_controller.rb

class ApplicationController < ActionController::Base

  before_filter :authenticate

  private

  def authenticate
    authenticate_or_request_with_http_token do |token, options|
      token == MagmaPaymentsService::Application.config.api_secret
    end
  end
end

Verify we secured the Service

Restart the Payments Service app and now you should get this if you try the API call again:

Started POST "/customers.json" for 127.0.0.1 at 2013-06-01 15:48:19 -0400
Processing by CustomersController#create as JSON
  Parameters: {"id"=>"38", "first_name"=>"Jon", "last_name"=>"Dean", "email"=>"jon@example.com"}
  Rendered text template (0.0ms)
Filter chain halted as :authenticate rendered or redirected
Completed 401 Unauthorized in 8ms (Views: 7.3ms | ActiveRecord: 0.0ms)

Sending the token with the client request

config/application.rb

module MagmaClientApp
  class Application < Rails::Application
    # NOTE: for both apps you are better off settings these as ENV vars
    config.payments_api_secret = '0224651fc98de3a615243ebf75188ff430bdb2c1c983ab87614b3db2f4c7a167455354d6c0d2e7e788651bbead373bf0a9a166b12c63d47b48f060cdf759e16e'
  end
end

Sending the token (cont.)

app/services/payments_service.rb

class PaymentsService
  include HTTParty
  base_uri MagmaClientApp::Application.config.payments_base_uri

  def self.create_customer(user)
    params = { ... }
    options = { body: params, headers: { "Authorization" => authorization_credentials }}
    response = self.post('/customers.json', options)
    response.parsed_response
  end

  private

  def self.authorization_credentials
    token = MagmaClientApp::Application.config.payments_api_secret
    ActionController::HttpAuthentication::Token.encode_credentials(token)
  end
end

Secure your secret token!

  • Always use SSL in production
  • Use a different token in each environment
  • Move the token to ENV vars and out of source control

Versioning

Sometimes (hopefully not often) the external API of your service will have to change.

Many of these changes don't have to force an immediate upgrade of all clients.

A Simple change

We realized that our client apps just store one field for name for users and each of them is splitting the name for the benefit of Braintree. So we want to move this logic into the service.

So our API will now accept just name for creating a customer instead of first_name and last_name.

Tip on versioning

Do this up front. Don't wait until you realize you need it.

What if I don't have deprecated versions?

What if the new version is always mandatory?

You still want versioning.

Why?

Because it's better to serve a message that the service refused the request rather than trying to run your code and things breaking unexpectedly. In both cases it's broken, but only one of them will be easy to handle.

(Without versioning you may not even realize things are going wrong!)

OK, so how?

Common approach: Modules and URL namespacing

URL Namespacing

config/routes.rb

MagmaPaymentsService::Application.routes.draw do
  namespace :v1 do
    resources :customers, only: [:create, :update] do
      resources :transactions, only: [:create, :index, :show]
    end
  do
end

New URLs

rake routes
GET  /v1/customers/:customer_id/transactions     v1/transactions#index
POST /v1/customers/:customer_id/transactions     v1/transactions#create
GET  /v1/customers/:customer_id/transactions/:id v1/transactions#show
POST /v1/customers                               v1/customers#create
PUT  /v1/customers/:id                           v1/customers#update

Move controllers into v1 directory

app/controllers/customers_controller.rb -> app/controllers/v1/customers_controller.rb
app/controllers/transactions_controller.rb -> app/controllers/v1/transactions_controller.rb

Add them to the V1 module

app/controllers/v1/customers_controller.rb

module V1
  class CustomersController < ApplicationController
    ...
  end
end

Update the client to request V1

app/services/payments_service.rb

class PaymentsService
  include HTTParty
  base_uri MagmaClientApp::Application.config.payments_base_uri
  VERSION = 'v1'

  def self.create_customer(user)
    params = { ... }
    options = { body: params, headers: { "Authorization" => authorization_credentials }}
    response = self.post("/#{VERSION}/customers.json", options)
    response.parsed_response
  end

  private

  def self.authorization_credentials
    token = MagmaClientApp::Application.config.payments_api_secret
    ActionController::HttpAuthentication::Token.encode_credentials(token)
  end
end

Creating a new version

  • Add the new routes
  • Add a new module
  • Copy files from previous module to new one
  • Make your changes in the new module

Version 2 of the Payments Service API

app/controllers/v2/customers_controller.rb

module V2
  class CustomersController < ApplicationController
    def create
      # Need to send a hash of :id, :name, :email
      result = Braintree::Customer.create({
        :id => params[:id],
        :first_name => params[:name].split(' ').first,
        :last_name => params[:name].split(' ').last,
        :email => params[:email]
      })
      resp = { success: result.success?, message: (result.message rescue '') }
      respond_to do |format|
        format.json { render json: resp }
      end
    end
  end
end

Two available versions

Now clients can either send a first_name and last_name to /v1/customers or just a name to /v2/customers. They have time to upgrade!

Don't like having the version in the URL?

Another common approach is to use an Accept header that specifies the version.

Read about it on RailsCasts later... there's other good info there as well :)

What about the duplicated code?

Some stuff may never change in your service. In this case, the non-version part of our routes and the TransactionsController stayed the same

Less duplication in routes

You can cleverly avoid duplication in routes.rb

MagmaPaymentsService::Application.routes.draw do
  (1..2).each do |version|
    namespace :"v#{version}" do
      resources :customers, only: [:create, :update] do
        resources :transactions, only: [:create, :index, :show]
      end
    end
  end
end

Using base classes to avoid duplication

  • Create a Base module when you create V1 and put the code there
  • When you make a new version you can move just the code you change out of Base and into the new version

What it looks like

app/controllers/base/customers_controller.rb

module Base
  class CustomersController < ApplicationController
    # Stuff that rarely or never will change
  end
end

app/controllers/v2/customers_controller.rb

module V2
  class CustomersController < Base::CustomersController
    # Stuff that overrides or is new functionality from Base
  end
end

Why I'm not crazy about it

You make a change in V5. Now you have to do one of the following:

  • Move the code from Base to each of V1 to V4
  • Forever override that method from V5 on, but if you forget you'll get old functionality

Additional Thought on Removing Duplication

You may realize this adds more complication than anything, so decide if it's worth it. If you're building an internal service, it is very likely the previous versions won't live long anyway because you control the clients.

If you do it, be sure to test the behavior of this refactoring as well and keep tests for all active versions.

Client handling of versions

Decide and document from the beginning how depcrecation and removal of versions will work

Remember, the point is to not break clients! Don't make them implement version handling starting with V2

Helpful tools

RocketPants in the Service

config/routes.rb

MagmaPaymentsService::Application.routes.draw do
  api :version => 1 do
    resources :customers, only: [:create, :update] do
      resources :transactions, only: [:create, :index, :show]
    end
  end
end

app/controllers/customers_controller.rb

class CustomersController < RocketPants::Base
  version '1'
  def create
    result = Braintree::Customer.create( ... )
    expose({ success: result.success?, message: (result.message rescue '') })
  end
end

RocketPants in the Client

app/clients/payments_client.rb

class PaymentsClient < RocketPants::Client
  version '1'
  base_uri MagmaClientApp::Application.config.payments_base_uri

  class Result < APISmith::Smash
    property :success
    property :message
  end

  def create_customer(user)
    post 'customers', payload: { name: user.name, email: user.email }, transformer: Result
  end
end

Call the new RocketPants Client

Now just call this is the after_create

app/models/user.rb

PaymentsClient.new.create_customer(self)

Other goodies of RocketPants

Register and Handle errors

  • Error key: :unauthenticated
  • Exception: RocketPants::Unauthenticated
  • HTTP status: 401 Unauthorized

Example

In controller you can do

error! :not_found

or raise the exception class

raise RocketPants::NotFound

and it will do the 404 Not Found HTTP response for you!

Final Thoughts/Summary

Create an API service application

  • to keep concerns separate and organize code
  • when you will have many applications (you probably will)
  • when you may use multiple languages (there's a decent change some day)
  • before you have to (by then it's really hard!)