In order to prevent downtime during an email service provider outage, you’re tasked with creating a service that provides an abstraction between two different email service providers. This way, if one of the services goes down, you can quickly failover to a different provider without affecting your customers.

This application is written in Python. It uses Flask as the framework. I chose Python/Flask because it's fairly light-weight and scalable framework and also it is easy to implement the required service with JSON data.

The following is Prerequisites for this application.

• Python 2.7 (Tested with 2.7.6, Don't use 3.x.)
• Flask 0.10 (Tested with 0.10.1) (Note: Flask has the prerequisites such as Jinja2. You will need them in your environment.)
• requests 2.2 or above (Tested with 2.2.1)

To install the application, please follow the below steps.

• Download and install Python 2.7 from https://www.python.org/downloads/.
• Install pip for easy installation for the prerequisites. (Note: you can still install the prerequisites without pip.) To install pip, please refer to https://pip.pypa.io/en/stable/installing/.
• Install Flask 0.10 by following the instruction at http://flask.pocoo.org.
• Install requests if it's not available after installing Flask and its prerequisites.
• Get the application from https://github.com/recto/email_providers.git.

Once all prerequisites and the application is checked out into your environment, you can start the application and also run its test cases as below.

The application supports all basic features given in the instruction. In addition to that, it supports the following feature mentioned in "Bonus Points".

• Instead of relying on a configuration change for choosing which email provider to use, dynamically select a provider based on their error responses. For instance, if Mailgun started to timeout or was returning errors, automatically switch to Mandrill.

You will find the following at your workspace where you checked out the application.

• README.md - this file.
• src/application.py - application main.
• src/providers.py - providers module used by application.py.
• src/resources/providers.json - provider configuration file.
• test/simple_app.py - simple client application to test the service.
• test/test_applicatoin.py - Test case for Flask application.
• test/test_providers.py - Test case for providers module.

• This application requires Mailgun and/or Mandrill services. Please create your account with them and have the necessary information such as service URL and API key.
• From your file manager, go to the workspace where you checked out the application. Please find "src/resources/providers.json".
• Open "src/resources/providers.json" with text editor and modify as needed. Here is a sample.

{ "default" : "mailgun_email",  <-- Default mail provider.
  "providers": [
    { "name": "mailgun_email", <-- You can name the mail provider as you like.
      "type": "mailgun", <-- "type" should be set to either "mailgun" or "mandrill".
      "url": "https://api.mailgun.net/v3/<your sandbox>.mailgun.org/messages",
      "api_key": "<your API key given by Mailgun>"
    },
    { "name": "mandrill_email",
      "type": "mandrill",
      "url": "https://mandrillapp.com/api/1.0/messages/send.json",
      "api_key": "<your API key given by Mandrill>"
    }
  ]
}

• Save your changes.
• Start the terminal or command prompt.
• Go to the workspace where you checked out the application in the terminal.
• Change directory to "src"
• Perform "python application.py"
• Application will be running at http://localhost:8000/email.
• If you have your own client application, stop here and your client application. Otherwise, you can use /test/simple_app.py.
• Open /test/simple_app.py and edit payload part with your email information.

def test_email():
    payload = {
        "to": "fake@example.com",
        "to_name": "Ms. Fake",
        "from": "noreply@uber.com",
        "from_name": "Uber",
        "subject": "A message from Uber.",
        "body": "<h1>Your billing amount:</h1><p>$10</p>"
    }
    return requests.post(
        "http://localhost:8000/email",
        data=json.dumps(payload),
        headers={"Content-Type" : "application/json"}
    )

• Save your changes.
• Start the terminal or command prompt.
• Go to the workspace where you checked out the application in the terminal.
• Change directory to "test".
• Perform "python simple_app.py".
• Check your email inbox.

• Test cases require both Mailgun and Mandrill services. Please create your account with them and have the necessary information such as service URL and API key.
• From your file manager, go to the workspace where you checked out the application. Please find "test/resources/providers.json". (Note: This is the different JSON file from the one for application.)
• Open "test/resources/providers.json" with text editor and modify only placeholders. Here is a sample.

{ "default" : "mailgun",
  "providers": [
    { "name": "mailgun",
      "type": "mailgun",
      "url": "https://api.mailgun.net/v3/<your sandbox>.mailgun.org/messages",
      "api_key": "<your API key given by Mailgun>"
    },
    { "name": "mandrill",
      "type": "mandrill",
      "url": "https://mandrillapp.com/api/1.0/messages/send.json",
      "api_key": "<your API key given by Mandrill>"
    },
    { "name": "mailgun_with_wrong_key",
      "type": "mailgun",
      "url": "https://api.mailgun.net/v3/sandbox.mailgun.org/messages",
      "api_key": "key-wrong"
    },
    { "name": "missing_url",
      "type": "mailgun",
      "api_key": "any-key"
    }
  ]
}

• Save your changes.
• Open "test/resources/payloads.json" with your text editor and modify only placeholders. Here is a sample.

{ "payloads" : [
    {
      "testcase" : "1",
      "to": "<your target address>",
      "to_name": "Ms. Fake",
      "from": "<your email address>",
      "from_name": "Uber",
      "subject": "Billing Information",
      "body": "<h1>Your billing amount:</h1><p>$10</p>"
    },
    {
      "testcase" : "2",   <-- this test case 2 is missing body on purpose.
      "to": "<your target address>",
      "to_name": "Ms. Fake",
      "from": "<your email address>",
      "from_name": "Uber",
      "subject": "Billing Information",
    }
  ]
}

• Save your changes.
• Start the terminal or command prompt.
• Go to the workspace where you checked out the application in the terminal.
• Change directory to "test".
• Perform "python test_providers.py".
• Perform "python test_application.py".

If I had more time, I could probably do better in switching between mail services. It currently checks the response from mail service and switches to other service at global level. So, the following requests will try to use the service set by other request first. This might be what's expected but I am not sure. It should better check more details of response and decides whether it's caused by the outage of the service or the problem caused by payload.

I was in the middle of implementing the feature to keep a record of emails in database. But I ran out time and stopped.

One of the reasons why I ran out time was Mandrill setup. While Mailgun setup was easy and straight forward, Mandrill requires domain verification these days. Refer to the bottom of [About Domain Verification] (https://mandrill.zendesk.com/hc/en-us/articles/205582247-About-Domain-Verification) for details. It seems this is only applicable for users, who sign up after Dec 1, 2015.

Since I have the domain (I actually struggled more with GoDaddy DNS control panel than Mandrill because their new control panel no longer allow us to modify SPF setting.), I could do this but it may not be the case for other candidates. You may want to look for some other mail service for this exercise.