streaming datafruits radio metadata with action controller::live and redis pub/sub

Right now the metadata for the currently streaming song/live dj on datafruits is updated via simple polling and ajax requests. The metadata is stored in a simple redis key, and a sinatra app with an endpoint /currentsong will return this key in JSON format. The ajax request simply polls this app every few seconds.

I’d like to use EventSource instead of polling via setInterval. I thought this would be a good opportunity to try out the new ActionController::Live feature in Rails. Luckily Aaron Patterson has a good write-up on his blog.

I ran into a few caveats along the way. First off is some good news, an implementation of Aaron’s SSE emitter class has been merged into Rails, so you no longer need to write your own.

sse = SSE.new(response.stream)

Also one thing is that it seems if there is most any kind of error in your controller, nothing happens. The Rails developers seem to have decided this is correct behaviour for the most part. https://github.com/rails/rails/pull/9604 There is an on_error callback, although I couldn’t find any documentation on how to use it.

The final caveat is that you are probably going to new a different server than you are used to. I tried out Puma, mostly since that’s what Aaron used in his guide.

I thought of the implications of every datafruits visitor keeping a connection open to my site. Am I going to require a thread for each connection? Is each connection going to additionally require a database connection in the activerecord pool?

I think I can come up with a simpler solution. I could extract this functionality to a smaller service, perhaps running on faye or sinatra. All I really require is the redis connection, this doesn’t actually have anything to do with the rest of my rails application anyway.