soapy metadata

big balls of mud killer, in 27 March 2013

My internet radio station datafruits.fm needed metadata in its audio stream. I wanted to make a cool hack to set metadata using liquidsoap. This would solve 2 problems:

dj’s are unable to set metadata with their clients, so I’ll do it for them when they authenticate.
the <audio> tag is unable to read the metadata from the stream. I’ll have to get it some other way.

I’ll have liquidsoap call a ruby script when the metadata in the stream changes, using on_meta. We’ll store the metadata in redis, using pub-sub. Then I’ll set up a controller to relay this metadata.

As a bonus I now effectively have an api for getting the current song that I can use in my mobile apps for this radio.

pub_metadata.rb

# adopted from https://github.com/gorsuch/sinatra-streaming-example/blob/master/worker.rb

require 'redis'

redis_url = ENV["REDISTOGO_URL"] || "redis://localhost:6379"
uri = URI.parse(redis_url)
r = Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)

meta = ARGV[0]

puts "setting metadata..."
r.publish "metadata", meta

radio.liq

source = on_metadata(pub_metadata, source)

def pub_metadata(m) =
  log("metadata changed: #{m}")
  title = m["title"]
  result = get_process_lines("./pub_metadata.rb #{m}")
  log("pub_metadata: #{result}")
end

I wanted to use streaming in Sinatra to update the metadata quickly, instead of polling.

sinatra_app.rb

require 'redis'
require 'sinatra'

configure do
redis_url = ENV["REDISTOGO_URL"] || "redis://localhost:6379"
uri = URI.parse(redis_url)
set :redis, Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)
end

get '/metadata' do
  puts "connection made"

  stream do |out|
    settings.redis.subscribe 'metadata' do |on|
      on.message do |channel, message|
        out << "#{message}\n"
      end
    end
  end
end

Then we can do something like this in the javascript:

var source = new EventSource('/metadata');
source.addEventListener('refresh', function(e){
  console.log("got sse");
  console.log(e.data);
  $('#nowplaying').html(e.data);
});

This was a little bit more difficult to manage than I expected. It was working sporadically, and it turns out you need to manage all the connections by hand, you can’t just code one connection and expect it to work.

So I decidedly to go with the certainly not as cool solution of using polling. However I think its a perfectly fine solution for this situation. Polling is simple, fairly cheap, and as far as this situation goes, truly ‘live’ updating of the metadata is not really necessary.

So I ended up just using a regular redis key.

pub_metadata.rb

# adopted from https://github.com/gorsuch/sinatra-streaming-example/blob/master/worker.rb

require 'redis'

redis_url = ENV["REDISTOGO_URL"] || "redis://localhost:6379"
uri = URI.parse(redis_url)
r = Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)

meta = ARGV[0]

puts "setting metadata..."
puts r.set "currentsong", meta

We can just set up a normal get route to get the key.

sinatra_app.rb

  redis_url = ENV["REDISTOGO_URL"] || "redis://localhost:6379"
  uri = URI.parse(redis_url)
  set :redis, Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)

  get '/metadata' do
    settings.redis.get("current-song").to_s
  end

A simple set setInterval will work for polling and updating the html.

  setInterval(function(){
    $.get("/metadata",function(data){
      console.log("got data: "+data);
      $('#nowplaying').html(data);
    });
  },5000);

While not as cool, this is a lot easier to work with at the moment. Perhaps I can think of other radio data I could store in redis and attach an API to from my app.

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