email 💌

---

2023 Update: Project Retired

This project was working perfectly well for us:

Sadly, a lot has changed in the 2 years since we built & deployed it to Heroku. Heroku decided to eliminate their "Free Tier" so these ultra-low overhead / volume apps (less than 2h per month) are no longer viable on Heroku. We would gladly have paid the $16/month (minimum) to keep it running, but they deleted our database (that only had a couple of MBs):

and deprecated our stack:

So they are forcing us to update everything. This is super lame in software that was working fine.

So, for the time being we are pausing development on this project. If we find a use for it again in the future, we will re-build it.

Thanks for your interest! ❤️

---

Why? 🤷‍

We needed a way to send and keep track of email in our App.
We want to know precise stats for deliverability, click-through and bounce rates for the emails we send in real-time.
This allows us to monitor the "health" of our feedback loop and be more data-driven in our communications.

What? 💭

An email API and analytics dashboard for our App.

The main App does not do any Email as that is is not it's core function. It delegates all email sending and monitoring activity to the email service. This means we have an independently tested/maintained/documented function for sending and tracking email that we never have to think about or setup again.

Edit this diagram

The Email app provides a simplified interface for sending emails that ensures our main App can focus on it's core functionality.

Who? 👤

We built the email App for our own (internal) use @dwyl. It handles all email related functionality so that our main App does not have to. It can be considered a "microservice" with a REST API. We have not built this as a reusable module as it is very specific to our needs. However, as with everything we do, it's Open Source and extensively documented/tested so others can learn from it.

If you find this interesting or useful, please ⭐️the repository on GitHub!
If you have any feedback/questions, please open an issue

How?

To run the email App, follow these instructions:

Get the Code

git clone this project from GitHub:

git clone [email protected]:dwyl/email.git && cd email

Dependencies

Install the dependencies:

mix deps.get
cd assets && npm install && cd ..

Environment Variables

Ensure you have the environment variables defined for the Phoenix App. All the required environment variables are listed in the .env_sample file.

In our case we are reusing the SECRET_KEY_BASE to verify JWTs. That means that the SECRET_KEY_BASE of the Phoenix App needs to be exported as the JWT_SECRET in the Lambda function.

Deploy the Lambda Function

In our case the aws-ses-lambda function is deployed automatically by Travis-CI (continuous delivery). For anyone else following along, please read the instructions in https://github.com/dwyl/aws-ses-lambda to deploy the Lambda function; there are quite a few steps but they work!

Provided you have:
a. created the SNS Topic,
b. subscribed to SES notifications on the topic
c. made it the trigger for Lambda function,
d. defined all the necessary environment varialbes for the Lambda,
you should be all set. These steps are all described in detail in: SETUP.md

If you get stuck getting this running or have any questions/suggestions, please open an issue.

Sending Email!

In order to send an email - e.g: from the Auth app - use the POST /api/send where the "authorization" header is a JWT signed using the shared JWT_SECRET.

The JWT should contain the keys email, name and template e.g:

{
  "email": "[email protected]",
  "name": "Alex",
  "template": "welcome"
}

Please see the test for clarity: sent_controller_test.exs#L126-L143

Want to Understand How we Made This? 🤷‍

If you want to recreate the email app from scratch, follow all the steps outlined here.

If you are adding the email functionality to an existing App, you can skip to step 2.
If you are creating an email functionality and dashboard from scratch, follow steps 0 and 1.

0. Create a New Phoenix App 🆕

In your terminal, run the following mix command:

mix phx.new app

That will create a few files. e.g: github.com/dwyl/email/commit/1c999be

Follow the instructions in the terminal to download all the dependencies.

At this point the email App is just a basic "hello world" Phoenix App.
It should be familiar to you if you have followed any of the Phoenix tutorials,
e.g: https://github.com/dwyl/phoenix-chat-exa mple or https://github.com/dwyl/phoenix-liveview-counter-tutorial

1. Copy the Migration Files from the MVP 📋

In order to speed up our development of the email App, we are only going to create one schema/table; sent (see: step 2). Since our app will refer to email addresses, we need a people schema which in turn refers to both tags and status.

See: github.com/dwyl/email/commit/bcafb2f

1.b Copy the person.ex and status.ex Schemas

In order to have the schema for the person and status, which is required to insert a sent record because sent has fields for person_id and status_id,

In my case given that I had the app-mvp-phoenix on my localhost, I just ran the following commands:

cp ../app-mvp-phoenix/lib/app/ctx/person.ex ./lib/app/ctx/
cp ../app-mvp-phoenix/lib/app/ctx/status.ex ./lib/app/ctx/

Commit adding these two files and the Fields dependency: email/commit/95f9ade

But we are not done yet. person.ex depends on a couple of functions contained in app-mvp-phoenix/lib/app/ctx.ex specifically App.Ctx.get_status_verified/0. Open ../app-mvp-phoenix/lib/app/ctx.ex in your editor window, or web browser: app-mvp-phoenix/lib/app/ctx.ex

Locate the get_status_verified/0 function:

def get_status_verified() do
  Repo.get_by(Status, text: "verified")
end

Copy it and paste it into /lib/app/ctx/person.ex.

We also need to add the following aliases to the top of the person.ex file:

alias App.Ctx.Status
alias App.Repo

The code for these changes is contained in dwyl/email/commit/81fa2a9

Why reuse migrations?

Our objective is to be able to run the email App in several ways:

1. Independently from any "main" App. So the email dashboard can be 100% anonymised and we just display aggregate stats for all email being sent/received.

2. Inside the "main" App. If we don't want to have to deploy separate Apps, we can simply include the email functionality within a "main" App.

3. Umbrella App where the email App is run as a "child" to the "main" app.

By reusing the migration files from our "main" App, (the files need to have the exact same name and contents), we maintain full flexibility to run our email App in any way. This is because if we run the migrations against the "main" PostgreSQL DB, the migrations with those timestamps will already exist in the migrations table; so no change will be required. However

2. Create the sent Schema/Table 📤

In order to store the data on the emails that have been sent, we need to create the sent schema:

mix phx.gen.html Ctx Sent sent message_id:string person_id:references:people request_id:string status_id:references:status template:string

When you run this command in your terminal, you should see the following output showing all the files that were created:

* creating lib/app_web/controllers/sent_controller.ex
* creating lib/app_web/templates/sent/edit.html.eex
* creating lib/app_web/templates/sent/form.html.eex
* creating lib/app_web/templates/sent/index.html.eex
* creating lib/app_web/templates/sent/new.html.eex
* creating lib/app_web/templates/sent/show.html.eex
* creating lib/app_web/views/sent_view.ex
* creating test/app_web/controllers/sent_controller_test.exs
* creating lib/app/ctx/sent.ex
* creating priv/repo/migrations/20200224224024_create_sent.exs
* creating lib/app/ctx.ex
* injecting lib/app/ctx.ex
* creating test/app/ctx_test.exs
* injecting test/app/ctx_test.exs

Add the resource to your browser scope in lib/app_web/router.ex:

    resources "/sent", SentController

Remember to update your repository by running migrations:

$ mix ecto.migrate

We will follow these instructions in the next steps!

Why So Many Files?

When using mix phx.gen.html to create a set of phoenix resources, the files for the migration, context, controller, views, templates and tests are generated. This is a good thing because Phoenix does all the work for us and we don't have to think about any of the "boilerplate" code. It can feel like a lot of code especially if you are new to Phoenix, but don't get hung up on it. Right now we are only interested in the migration file: /priv/repo/migrations/20200224224024_create_sent.exs

Feel free to read through the other files created in step 2: github.com/dwyl/email/commit/b8d4b06 The code is fairly straightforward, but if there is anything you don't understand, please ask!

We are not doing much with these files in the next few steps, but we will return to them later when work on the dashboard!

What are the message_id and request_id fields for?

In case you are wondering what the message_id and request_id fields in the sent schema are for. The message_id is, as you would expect, the Globally Unique ID (GUID) for the message in the AWS SES system. We need to keep track of this ID because all SNS notifications will reference it. So if we receive a "delivered" or "bounce" SNS notification, we need to match it up to the original message_id so that our data reflects the status of the message.

The aws-ses-lambda function returns a response in the following form:

{
  MessageId: '010201703dd218c7-ae82fd07-9c08-4215-a4a9-4b723b98d8f3-000000',
  ResponseMetadata: {
    RequestId: 'def1b013-331e-4d10-848e-6f0dbd709434'
  }
}

Or when invoked from Elixir see: github.com/dwyl/elixir-invoke-lambda-example the response is:

{:ok,
 %{
   "MessageId" => "010201703dd218c7-ae82fd07-9c08-4215-a4a9-4b723b98d8f3-000000",
   "ResponseMetadata" => %{
     "RequestId" => "def1b013-331e-4d10-848e-6f0dbd709434"
   }
 }}

We are storing MessageId as message_id and RequestId as request_id.

3. Add the SentController Resources to router.ex

Open the lib/app_web/router.ex file and locate the section that starts with

scope "/", AppWeb do

Add the following line in that scope:

resources "/sent", SentController

e.g: /lib/app_web/router.ex#L20

4. Run the Migrations

In your terminal run the migrations command:

mix ecto.migrate

You should expect to see outpout similar to the following:

23:15:48.568 [info]  == Running 20200224224024 App.Repo.Migrations.CreateSent.change/0 forward

23:15:48.569 [info]  create table sent

23:15:48.574 [info]  create index sent_person_id_index

23:15:48.575 [info]  create index sent_status_id_index

23:15:48.576 [info]  == Migrated 20200224224024 in 0.0s

Entity Relationship Diagram (ERD)

ERD after creating the sent table:

Checkpoint: Run the App!

Just to get an idea for what the /sent page currently looks like, let's run the Phoenix App and view it.
In your terminal run:

mix phx.server

Then visit: http://localhost:4000/sent in your web browser.
You should expect to see:

Click on the "New sent" link to create a new sent record. You should see a form similar to this:

Input some test data and click "Save".
You will be redirected to: http://localhost:4000/sent/1 with the message "Sent created successfully":

Obviously we are not going to create the sent records manually like this.
(in fact we will be disabling this form later on)
For now we just want to know that record creation is working.

If you return to the http://localhost:4000/sent (index) route, you should see the one "sent" item:

This confirms that our sent schema is working as we expect.

Run the Tests!

For good measure, let's run the tests:

mix test

You should expect to see output similar to the following:

11:23:09.268 [info]  Already up
...................

Finished in 0.2 seconds
19 tests, 0 failures

Randomized with seed 448418

19 tests, 0 failures.

Test Coverage!

Follow the instructions to add code coverage.
Then run:

mix coveralls

You should expect to see:

Finished in 0.2 seconds
19 tests, 0 failures

Randomized with seed 938602
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/app.ex                                      9        0        0
100.0% lib/app/ctx.ex                                104        6        0
100.0% lib/app/ctx/sent.ex                            21        2        0
100.0% lib/app/repo.ex                                 5        0        0
100.0% lib/app_web/channels/user_socket.ex            33        0        0
100.0% lib/app_web/controllers/page_controller.        7        1        0
100.0% lib/app_web/controllers/sent_controller.       62       19        0
100.0% lib/app_web/endpoint.ex                        47        0        0
100.0% lib/app_web/gettext.ex                         24        0        0
100.0% lib/app_web/views/error_view.ex                16        1        0
100.0% lib/app_web/views/layout_view.ex                3        0        0
100.0% lib/app_web/views/page_view.ex                  3        0        0
100.0% lib/app_web/views/sent_view.ex                  3        0        0
[TOTAL] 100.0%
----------------

We think it's awesome that Phoenix creates tests for all the functions generated by the mix gen.html.
This is how software development should work!

With that checkpoint completed, let's move on to the fun part!

5. Insert SNS Notification Data

The magic of our email dashboard is knowing the status of each individual message and the aggregate statistics for all messages. Luckily AWS has already figured out the infrastructure part.

If you are unfamiliar with Amazon Simple Notification Service (SNS), it is a managed service that can send notifications to any other system. In our case the only notifications we are interested in are those that relate to the email messages we have sent using AWS Simple Email Service (SES).

Requirements for upsert_sent/1 Function

We need to create an upsert_sent/1 function in the /lib/app/ctx.ex file that will handle any notification data received from the Lambda function. The point of an UPSERT function is to insert or update a record.
The upsert_sent/1 function needs to do three things:

1. Check if the payload sent by the Lambda function contains an email address.
   a. if the payload includes an email key, we attempt to find that email address in the people table by looking up the email_hash. if the person record does not exist for the given email, create it and retain the person_id. With the person_id, upsert the sent item.

2. If the payload includes a status key, look it up in the status table. if the status exists, use the status.id as status_id for the sent record. if the status does not exist, create it.

3. If the payload does not have an email key, it should have a message_id key which means this is an SNS notification.
   a. Lookup the message_id in the sent table. if there is no record for the message_id, create it!
   if the sent record exists, update it using the revised status.

5.1 Create the Tests for upsert_sent/1

The SNS notification data ingested from aws-ses-lambda will be inserted/updated in the sent table using the upsert_sent/1 function. The function does not currently exist, so let's start by creating the tests according to the spec.

Tests: /test/app/ctx_test.exs#L99

5.2 Implement the upsert_sent/2 function

Implement the function according to the spec: /lib/app/ctx.ex#L108

5.3 Create Tests for Processing API Requests

We are going to create a function called process_jwt/2 that will handle inbound API requests. There are 3 scenarios we want to test:

1. If authorization header is not present, immediately reject the request.
2. If authorization header has invalid JWT, reject as unauthorized.
3. If authorization header has a valid JWT, invoke upsert_sent/1.

Add these 3 tests to the test/app_web/controllers/sent_controller_test.exs file:

describe "process_jwt" do
  test "reject request if no authorization header" do
    conn = build_conn()
       |> AppWeb.SentController.process_jwt(nil)

    assert conn.status == 401
  end

  test "reject request if JWT invalid" do
    jwt = "this.fails"
    conn = build_conn()
       |> put_req_header("authorization", "#{jwt}")
       |> AppWeb.SentController.process_jwt(nil)

    assert conn.status == 401
  end

  test "processes valid jwt upsert_sent data" do
    json = %{
      "message_id" => "1232017092006798-f0456694-ac24-487b-9467-b79b8ce798f2-000000",
      "status" => "Sent",
      "email" => "[email protected]",
      "template" => "welcome"
    }

    jwt = App.Token.generate_and_sign!(json)
    conn = build_conn()
       |> put_req_header("authorization", "#{jwt}")
       |> AppWeb.SentController.process_jwt(nil)

    assert conn.status == 200
    {:ok, resp} = Jason.decode(conn.resp_body)
    assert Map.get(resp, "id") > 0 # id increases each time test is run
  end
end

For the complete test code, see: test/app_web/controllers/sent_controller_test.exs

To run one of these tests, execute the following command in your terminal:

mix test test/app_web/controllers/sent_controller_test.exs:97

The tests will fail until you implement the function below.

e.g: /lib/app_web/router.ex#L28

5.4 Implement the process_jwt Function

Open the /lib/app_web/controllers/sent_controller.ex file and add the following code:

@doc """
`unauthorized/2` reusable unauthorized response handler used in process_jwt/2
"""
def unauthorized(conn, _params) do
  conn
  |> send_resp(401, "unauthorized")
  |> halt()
end

@doc """
`process_jwt/2` processes an API request with a JWT in authorization header.
"""
def process_jwt(conn, _params) do
  jwt = List.first(Plug.Conn.get_req_header(conn, "authorization"))
  if is_nil(jwt) do
    unauthorized(conn, nil)
  else # fast check for JWT format validity before slower verify:
    case Enum.count(String.split(jwt, ".")) == 3 do
      true -> # valid JWT proceed to verifying it
        {:ok, claims} = App.Token.verify_and_validate(jwt)
        sent = App.Ctx.upsert_sent(claims)
        data = %{"id" => sent.id}
        conn
        |> put_resp_header("content-type", "application/json;")
        |> send_resp(200, Jason.encode!(data, pretty: true))

      false -> # invalid JWT return 401
        unauthorized(conn, nil)
    end
  end
end

5.5 Create /api Endpoint

Open the /lib/app_web/router.ex file and add the following route to the scope "/api", AppWeb do block:

post "/", SentController, :process_jwt

Before:

# Other scopes may use custom stacks.
scope "/api", AppWeb do
  pipe_through :api
end

After:

# Other scopes may use custom stacks.
scope "/api", AppWeb do
  pipe_through :api
  post "/sns", SentController, :process_jwt
end

See: /lib/app_web/router.ex#L25-L29

5.6 Test /api/sns Endpoint Using curl

Test the /api endpoint in terminal using curl!!

On localhost run the app::

mix phx.server

Execute the following curl command:

curl -X POST "http://localhost:4000/api/sns"\
  -H "Content-Type: application/json"\
  -H "authorization: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJKb2tlbiIsImVtYWlsIjoiYW1hemVAZ21haWwuY29tIiwiZXhwIjoxNTgzMjgzMzEyLCJpYXQiOjE1ODMyNzYxMTIsImlzcyI6Ikpva2VuIiwianRpIjoiMm5zZXFmMzhzcWVqMDk3bjVrMDAwMHQ0IiwibWVzc2FnZV9pZCI6IjEyMzIwMTcwOTIwMDY3OTgtZjA0NTY2OTQtYWMyNC00ODdiLTk0NjctYjc5YjhjZTc5OGYyLTAwMDAwMCIsIm5iZiI6MTU4MzI3NjExMiwic3RhdHVzIjoiU2VudCIsInRlbXBsYXRlIjoid2VsY29tZSJ9.-T-8BdGlbOGacVSja5EXfWhbRaUBon1HUocdJbPaf1Q"

Once the app is deployed to Heroku:

curl -X POST "https://phemail.herokuapp.com/api/sns"\
  -H "Content-Type: application/json"\
  -H "authorization: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJtZXNzYWdlX2lkIjoiMDEwMjAxNzA5MjAwNjc5OC1mMDQ1NjY5NC1hYzI0LTQ4N2ItOTQ2Ny1iNzliOGNlNzk4ZjItMDAwMDAwIiwic3RhdHVzIjoiQm91bmNlIFBlcm1hbmVudCIsImlhdCI6MTU4MzM0NTgyOX0.oSp0gOTcoV-YN7yk-tUtni-HHHuP58cg6AjIEJ0-tDk"

Expect to see the following result:

{
  "id": 2
}

Optional

We created a test endpoint /api/hello in order to have a basic URL we can test on Heroku.

see:

• lib/app_web/router.ex#L27
• /lib/app_web/controllers/sent_controller.ex#L63-L69

On localhost:

curl "http://localhost:4000/api/hello"\
  -H "Content-Type: application/json"

You should expect to see the following response:

{
  "hello": "world"
}

On Heroku the result should be identical:

curl "https://phemail.herokuapp.com/api/hello"\
  -H "Content-Type: application/json"

6. Realtime Email Status Dashboard

In this section we are going to use Phoenix LiveView to create a realtime dynamic Email status/stats dashboard.

The setup steps for Phoenix LiveView are covered in: github.com/dwyl/phoenix-liveview-counter-tutorial

The dashboard is available on localhost:4000/ or

Made a quick video of the dashboard and sending a test email: https://youtu.be/yflPSotYd9Y

Download Data from Heroku

If you want to demo the dashboard on your localhost with real data, followe these instructions: dev-guide.md#using-real-data

7. Track Email Read Status

Keeping track of email read status is nothing new. All email newsletter platforms have this feature e.g. mailchimp https://mailchimp.com/help/about-open-tracking

We could use one of the 3rd party email services, but we really don't want them tracking the users of our App and selling that data to others!

We simply include a 1x1px image in the email template. Which makes an HTTP GET request to the /read/:jwt endpoint when the email is viewed.

The code is very simple.

@doc """
`render_pixel/2` extracts the id of a sent item from a JWT in the URL
and if the JWT is valid, updates the status to "Opened" and returns the pixel.
"""
def render_pixel(conn, params) do
  case check_jwt_url_params(params) do
    {:error, _} ->
      unauthorized(conn, nil)

    {:ok, claims} ->
      App.Ctx.email_opened(Map.get(claims, "id"))

      conn # instruct browser not to cache the image
      |> put_resp_header("cache-control", "no-store, private")
      |> put_resp_header("pragma", "no-cache")
      |> put_resp_content_type("image/gif")
      |> send_resp(200, @image)
  end
end

See: sent_controller.ex#L105-L127

Test it on Localhost with the following curl command:

curl "http://localhost:4000/read/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6MSwiaWF0IjoxNTg0NzEzOTk1fQ.OzgxrvzrRmVas0yJcKGIeLOSznNisenC0zSQ80knX60"

The effect is we can see when people open an email message.

Why Not Subscribe to the SNS/SES Notifications in Phoenix?

We could configure AWS SNS to send all SES related notifications directly to our email (Phoenix) App, however that has a potential downside: DDOS When we create an API endpoint that allows inbound POST HTTP requests, we need to consider how it can (will) be abused.

In order to check that an SNS payload is genuine we need to retrieve a signing certificate from AWS and cryptographically check if the Signature is valid. This requires a GET HTTP Request to fetch the certificate which takes around 200ms for the round trip.

So rather than subscribing directly to the notifications in our email (Phoenix) App, which would open us to DDOS attacks, because of the additional HTTP Request, we are doing the SNS parsing in our Lambda function and securely sending the parsed data back to the Phoenix app.

Relevant Reading

• JWT intro: https://jwt.io/introduction
• Learn JWT: https://github.com/dwyl/learn-json-web-tokens
• Joken (Elixir JWT library): https://github.com/joken-elixir/joken
  • Usage: https://hexdocs.pm/joken/introduction.html#usage
• JWT with Joken: https://elixirschool.com/blog/jwt-auth-with-joken