This repo illustrates how to set up a simple email integration for the Dream web framework. I wrote it in July of 2021, while I was at the Recurse Center, in order to learn more about web programming in OCaml.
Sending email can be quite slow. Most real web servers can't afford to do this task in a blocking fashion because they need to respond to requests quickly. To avoid latency, it's common for the server to add email tasks to a queue for a background worker to process.
This project illustrates how to implement this design. The app exposes
a simple form with address/subject/text fields and allows the user to
send test emails. The project creates a single executable,
email.exe, that can run either as a web server or a worker. The two
processes interact using a RabbitMQ container to track tasks. The
worker process uses Mailgun's REST API (via cohttp) to send email.
You'll need to sign up for a mailgun account
here. You should be able to get a free
"sandbox" account without having to pay. Once you've signed in, open
the "sending domains" tab on the main menu. You should see a sandbox
domain named something like: sandbox<some hex id string>.mailgun.org. Selecting that domain should take you to a menu
with the following data:
- Your mailgun domain.
- The API key: A hexadecimal string with some dashes.
- An API base URL. Outside the EU, this looks like
https://api.mailgun.net/v3/<Your Domain>.
Open run.sh and populate the environment variables
accordingly. Sandbox domains aren't allowed to send mail to arbitrary
addresses, so add yours as an "authorized recipient" in the mailgun
console.
Next, you'll need to make sure you have the dependencies to build the
project. I used version 4.11.1 of the compiler as well as dream,
emile, amqp-client-lwt, cohttp, cohttp-lwt-unix, and
ocaml-lsp-server. More details are in dream-email-example.opam.
Finally, you'll need to install docker and docker-compose. Run
docker-compose up to download the RabbitMQ container and view its
logs.
Start the web server by running ./run.sh server. In a separate
shell, run ./run.sh worker to start the worker process, which should
print a simple log:
Starting Queue Worker
No new tasks, waiting.
No new tasks, waiting.
Finally, visit http://localhost:8080. Fill
out a message (using your "authorized recipient" email address), and
hit send. Shortly afterward in the worker shell, you should see:
Initiating email send to <your email here>
API Response Status: 200.
API Response body {
"id": "<some mailgun email address here>"
"message": "Queued. Thank you."
}.
Time to send an email: 0.67s
Later, you should see the message in your inbox. Notice how the worker process helps the server avoid the latency of a request to Mailgun API (0.67s in the example above).
- Mailgun also supports SMTP. Update the worker process to use SMTP instead of Mailgun's API. Consider using the library letters for this.
- In the current system, if a request to Mailgun fails then the message gets dropped. Update the worker process to be more fault tolerant.
If you looked through the source code for this server, let me know how what you thought! I'm interested in adding more tutorial resources to the OCaml ecosystem, so feel free to post a PR or issue if you have ideas about how to make these resources better.