Scaling phoenix on heroku

Question

I dont have a tonne of experience with heroku, and even less with phoenix, so this may be a stupid question... but want to make sure I am making a good choice on hosting :)

From what I understand, the way you scale phoenix is add another server, launch another node, and connect them, then let BEAM / OTP work its magic to handle work load balancing. On heroku, dynos can't really talk together over a local network, which from what I understand is something that BEAM requires to cluster. So adding dynos will result in a more "traditional" scaling model, where you have an external load balancer balancing connections between unconnected nodes, with the db being shared state.

My question here is how big of an impact will this have? Is it more only an issue when you are hitting serious levels of load / scale, or will it mean spending a lot more money on infrastructure then is necessary?

Answer 1

You'll get the best performance on a host that supports clustering, but Phoenix has a PubSub adapter system exactly for deployments like heroku: https://github.com/phoenixframework/phoenix_pubsub

One line config change and mix.exs deps entry and you'll have multinode channels on heroku via our Redis adapter.

Answer 2

This is very open question, so I am sure my answer won't be comprehensive.

In your situation the most important question is: will I Phoenix use channels?

If you use plain old HTTP, it can be mostly stateless. There are lots of methods to simulate stateful connection like storing sessions in cookies. At the end of the day, it doesn't matter if your backend servers are connected with each other, because each of them is doing independent computations. Your load balancer can randomly select any server and it will always work. This cool feature of http enables this protocol to scale so well. You can definitely use Heroku in that scenario and it will work great.

If you use Phoenix channels, things get complicated. You still want to be able to connect to any of the servers, but you will probably send messages to other users real time and they can be connected to other servers. Phoenix solves this problem for you by clustering using BEAM and this will be hard on Heroku. Or even impossible.

To sum up: it is not a question of small scale/big scale. It is a question of features. Scaling channels will require clustering, scaling plain old HTTP will not.