MainProxy - Run multiple Phoenix apps on one port

Note: previously the name of this library was MasterProxy

I just released version 0.2.0 of MainProxy. If you ever want to run multiple Phoenix Endpoints or Plugs from a single HTTP(S) port without manually writing a cowboy handler than MainProxy is for you!

To define your own proxy you add {:main_proxy, "~> 0.2"} to your deps, then create a proxy module:

defmodule MyApp.Proxy do
  use MainProxy.Proxy

  @impl MainProxy.Proxy
  def backends do
        domain: "",
        phoenix_endpoint: MyCoolAppWeb.Endpoint
        domain: "",
        phoenix_endpoint: MyAppMembersWeb.Endpoint

And then add MyApp.Proxy to your supervision tree in (usually in application.ex) and you can now serve multiple domains from one BEAM instance!

I’m interested in MainProxy because I run multiple applications from one BEAM instance with it:

And I’d like to give a big thanks to @jesse who created the initial version of the library :heart:


First off, thank you for the project. It is a default dep for me on all my umbrella projects.

Will this get updated to work with Phoenix 1.7 when it is released?


Supporting the Phoenix 1.7 update is proving fairly tricky because there’s a large update to how we sockets work under the hood. I have started the work to update but it isn’t complete yet (and hasn’t been pushed up to GitHub). MainProxy will be updated to support Phoenix 1.7 but I can’t make any promises as to when.

Here’s the issue to follow if you’re interested: Update `main_proxy` to support Phoenix 1.7 · Issue #28 · Main-Proxy/main_proxy · GitHub


Shouldn’t the new abstractions being put in to allow bandit ws support make it more trivial?

/cc @mtrudel


In theory: everything that comes into Phoenix now does so via either Plug or WebSock. Specifically, WebSocket upgrades are entirely mediated through Plug, and the resultant connections are managed via WebSock. This is why Phoenix.Endpoint.Cowboy2Handler has gone away; its only purpose was to shim Cowboy’s WebSocket support (both the process of upgrading & also managing the subsequent connections). Phoenix no longer exposes a Cowboy Handler implementation because it doesn’t need to.

The problem here is that MainProxy is (to my understanding) fundamentally concerned with routing to Cowboy handlers. To the extent that it seems to support ‘raw plugs’, this should be enough to do everything that Phoenix 1.7 needs. I can’t back that up with anything beyond a fairly broad architectural wave of the hand, however; I haven’t looked at MainProxy’s code in any really depth (though axelson and I have talked about this before).

All of this is also to say that the functionality of MainProxy (at least insofar as it proxies to Phoenix / Plug apps) could probably be subsumed by Plug.Router, since everything is just a Plug now. This would come with the side benefit of being functional on all servers, not just Cowboy (again, architectural handwaving abounds in this statement).


Okay, a PR to update MainProxy to support Phoenix 1.7.0-rc.2 is now available! Please check it out at Update to support Phoenix 1.7-rc.2 by axelson · Pull Request #31 · Main-Proxy/main_proxy · GitHub and let me know how it goes. I hope to cut a new release soon, although I’ll probably wait until Phoenix 1.7 is released since this update is not backwards compatible with earlier Phoenix versions.

@mtrudel Thanks for all the improvements you made to Phoenix 1.7 (e.g. websock)! :tada: It definitely makes MainProxy pretty much unnecessary (which I plan to note in its readme in the future). Although it does still seem a bit easier because MainProxy takes care of running a base plug for you since calling another Phoenix Endpoint from within a Phoenix Endpoint doesn’t work for websockets in my testing.

Edit: And also thanks for giving me pointers on the upgrade @mtrudel :heart:


MainProxy 0.3.0 was just released today! :tada:

It adds support for Phoenix 1.7 (and drops support for earlier Phoenix versions). Thanks again to @mtrudel for helping on the upgrade. Here’s the full release notes:


Thanks for this @axelson! Any idea how to run this with Bandit?

MainProxy is currently very specific to Cowboy. I haven’t looked deeply at what would be required to add support for Bandit.

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With the websocket handling abstracted in phoenix 1.7, is there still a need to interact with cowboy directly?

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Thanks, me neither! :sweat_smile: If I get some time I shall.

Looking at the source it’s a very thin layer that calls Plug.Cowboy but I haven’t delved too deep in what would be required to change this to Bandit. Hopefully not too difficult?

Now that WebSockets are plug-mediated, there’s no need for any Cowboy specific code to make any of this work; MainProxy could be rewritten entirely in Plug and ‘just work’ everywhere.

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Around the time that Phoenix 1.7 came out I spent a couple hours trying to create a plug-only version of main_proxy that works with Phoenix, it worked fine on HTTP but didn’t work for websockets (I forget the details). But I didn’t investigate too much further because I was focused on other projects. It’s likely that if you’re using Bandit then the websockets will work just fine via plug.