Ben Church

@bnchrch

Get notified of user signups and plan changes automatically using Postgres & Phoenix PubSub

A look in to the future 🔮

A lot of time you need to make a system that can send out notifications to yourself or other users based on events, specifically events related to changes in your database. Postgres and Phoenix make this ridiculously easy.

By the end of this short post you will have:

  1. Setup a Phoenix 1.3 project
  2. Setup a broadcast function and trigger in Postgres
  3. Used a GenServer to listen for broadcasts from Postgres
  4. Used Bamboo to send emails to yourself when users signs up or changes their payment plan.

Getting Setup

Before you start

Please make sure you have the following installed and/or ready to go:

  1. Elixir
  2. Phoenix
  3. PostgreSQL
  4. A valid SendGrid account

Initialize the project

Lets create a new Phoenix 1.3 project called pub_sub_demo

mix phx.new pub_sub_demo
cd pub_sub_demo
mix ecto.create

Add Two Additional Dependencies

Next we want to add HTTPoison to help with decoding strings sent from the database and Bamboo to help us send emails.

To do this in your mix.exs file add the following to your deps:

{:httpoison, "~> 1.0"},
{:bamboo, "~> 0.8"}

Also update the extra_applications section to include :bamboo

extra_applications: [:logger, :runtime_tools, :bamboo]

Create a model

Finally lets create the User model we will be using to show off all of this broadcasting goodness.

mix phx.gen.context Accounts User users name:string payment_plan:string

Note: We created the User in a new context called Accounts. If you are unfamiliar with contexts they are nothing to be afraid of just a convention phoenix uses to group functionality. They are not special, just a way to bag functions together.

Kick it off!

Migrate the database, grab your dependencies and lets go!

mix ecto.migrate
mix deps.get
mix phx.server

Note: You should now be able to navigate to localhost:4000 and see your application running. Though for the purposes of this demo you wont really be viewing any screens.

Broadcast Changes with Postgres

The whole idea of this is for Postgres to let US know when things have changed. To do so we need to set up two things:

  1. A function that takes an action (INSERT, UPDATE, DELETE) performed on a row for the purpose of broadcasting it outside of the database.
  2. A trigger that calls this function when an action has occurred on a specific table.

Create the Ecto Migration

mix ecto.gen.migration broadcast_users_table_changes

Add the Function and Trigger

Update the migration file you created above to include the following

If you want to better understand what is occurring here check out this blog post by @kaisersly which largely is the inspiration for what you are reading here:

https://medium.com/@kaisersly/postgrex-notifications-759574f5796e

Listen for Changes

Now that we have our database broadcasting changes that occur on our users table we need to be able to listen and act on the same channel. Thankfully Postgrex provides this by default using Postgrex.Notifications.

To use this we must create a GenServer responsible for listening to Postgrex and acting on the messages broadcast.

Creating our GenServer

Create the file lib/pub_sub_demo/pub_sub/listener.ex and populate it with the following:

For now all that this does is listen on a channel provided and log the messages broadcast to that channel.

Next lets configure this as a Worker and give it the channel it will listen to.

Hook up the listener

Update the application.ex file so that it starts the Listener we just defined with the appropriate users_changes channel.

See it in Action

You should now be able to start your application again by running:

mix ecto.migrate
mix phx.server

Then any change you make to the users table via Postgres’s CLI should be output to our Applications stdout.

Listening for Specific Actions

While it’s nice to be able to see everything that occurs in the database often we only care about a small subset of these changes. Specifically for this demo:

  1. User created
  2. User subscription updated

Luckily Elixir’s pattern matching provides a wonderfully simple way to cut down on all the noise and focus exactly on the shape of data you’re looking for.

What we want to do is update handle_info to send the payload to a function that will match on the events that matter.

The above only acts on only the exact events we are looking for and outputs a specific message to our log. Anything else is ignored.

Link it up to an email service

Standard out is nice but in a running application you don’t want to be combing logs for events that your business cares about, you want to be notified in realtime.

Lets hook our messages up to an email provider so that we get a notification right in our inbox when a user signs up or upgrades their plan.

The Bamboo Elixir library and Send Grid service make this far too easy.

Setup Bamboo

Add the following to your config.exs file

Note: Bamboo offers many more adapters than just Send Grid if you have a different preference. https://github.com/thoughtbot/bamboo#adapters

Create a mailer.ex file in your pub_sub_demo folder

Sending emails from our Listener

Update handle_user_changes in listener.ex to send our log messages to our own email address.

Run it for a Final Time

Now if you create or update a user in your users table you should see emails appear in your SendGrid logs.

Thats it!

Conclusion

The concept of broadcasting events directly from your database isn’t something new and as a result this type of functionality is available to you in any language and framework.

But! The core features and functionality of Elixir and Phoenix make using database events much easier and more reasonable than I have found in any other system. Pattern Matching and OTP principles are both simple and powerful and I encourage anyone to take a deep dive into them when possible.

🧞‍ This is open source! you can find it here on Github
❤️ I only write about programming and remote work. If you follow me on Twitter I won’t waste your time.

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