[bratsche]

Even in these cases Erlang/Elixir processes can be a useful way of designing your app. Off the top of my head, here are a couple obvious use cases I can think of:

Basic state storage in between requests. Maybe you're building a simple shopping cart application, or maybe something more complex like a SurveyMonkey-style form builder. Each time someone makes a web request that modifies this state, you're storing that state somewhere.. but it's usually somewhere outside of Ruby. Maybe postgresql, maybe redis. If you were using Elixir or Erlang that would still be an option, but another option would be to use a process to store this temporary state. When the user adds things to the cart you send a message to the right process and let it know. This can keep all your code in one system (BEAM), so fewer moving pieces, easy to unit-test the entire thing in ExUnit, and potentially more efficient since there's no I/O to some other database. Then you can continue to save your truly long-term data in postgresql, so for example user accounts and sales made.

Another obvious use-case is not needing to have background workers like resque or Sidekiq. When I was doing Rails stuff I found myself doing tons of stuff in Sidekiq workers when I wanted to get the work off the request-response cycle. Sidekiq was great, and it improved things hugely.. but in Elixir I get all that for free with processes. Want to send an email? Have an email-sender process running and just send it a message. Want to do some fancy image processing, or even just thumbnail generation? Just have another process do it asynchronously. And once again, it's all inside the same code base and it's all super easy to unit test. And once again it makes your system simpler because you don't have another system process to deploy and manage.

[joevandyk]

What happens when the process containing the cart data needs to be restarted or the server shuts down?

[brightball]

One of the core ways to model this is to have a process running the cart itself and another with the data. If you wish for that cart data to be persistent, used DETS (writes to disk) just in case.

[emluque]

The data will have to be persisted eventually once the order is finished, keeping that data on process memory would be incredibly irresponsible. And by all means if you ever do an ecommerce app please use an external database. You will want to do all kind of things with the data outside of the usage of the application, like running reports , etc.

[cpeterso]

How do you clean up or time out old "cart" processes that are still in memory?