unable to handle disconnected clients in SSE - ruby-on-rails-4

Question

I'm trying to add SSE functionality to my server application using Redis PubSub, guided by many articles i.e:
how-to-use-actioncontollerlive-along-with-resque-redis.
The server is hosted in Heroku, thus heartbeating is necessary as well.
...
sse = SSE.new(response.stream)
begin
redis = Redis.new(:url => ENV['REDISCLOUD_URL'])
redis.subscribe(<UUID>, HEARTBEAT_CHANNEL) do |on|
on.message do |channel, data|
begin
if channel == HEARTBEAT_CHANNEL
sse.write('', event: "hb")
else
sse.write(data, event: "offer_update")
end
rescue StandardError => e #I'll call this section- "internal rescue"
puts "Internal: #{$!}"
redis.quit
sse.close
puts "SSE was closed
end
end
end
rescue StandardError => e #I'll call this section- "external rescue"
puts "External: #{$!}"
ensure
redis.quit
sse.close
puts "sse was closed"
end
The questions:
I didn't see the "Internal rescue" in any place over the net talking about SSE. but I don't get who can catch an exception if raised by the sse.write? The common scenario is that HB is sent while client isn't connected anymore which makes this section so critical ("Internal: client disconnected" is appeared). Am I right?
In which case will the "external rescue" be triggered? Does a client disconnection cause sse.write to raise an exception in the "inner block" (inside the on.message body)? because it was never caught by the external rescue when I tried to simulate it tens of times.
This code suffers as well. redis.quit in the internal rescue section raises another exception which is caught by the external rescue statement: External: undefined method 'disconnect' for #<Redis::SubscribedClient:0x007fa8cd54b820>. So - how should it done? how can I recognize ASAP a client disconnection in order to free the memory&socket?
How can it be that exceptions raised by the sse.write have NOT been caught by the external rescue (as it should be from beginning) while another error (described in my 3rd question) has been caught? all of this code (outer+inner sections) is running in the same thread, right? I'd be happy for a deep explaination.

Answer 1

You catch the exception inside the subscribe, and so redis doesn't know about it and will not stop it's inner loop properly. redis.quit will cause it to crash and stop since it cannot keep waiting for a message. This is obviously not a good way to do it.
If your code throws an exception inside the subscribe it will cause redis to gracefully unsubscribe and your exception can be rescued outside, as in your "external rescue".
Another point is that you shouldn't catch exceptions without fully handling them, and you should never catch generic exceptions without re-raising them. In this case you can safely let the ClientDisconnected exception bubble to Rails' code.
Here's how your controller's code should look like:
def subscribe
sse = SSE.new(response.stream)
redis = Redis.new(:url => ENV['REDISCLOUD_URL'])
redis.subscribe(<UUID>, HEARTBEAT_CHANNEL) do |on|
on.message do |channel, data|
if channel == HEARTBEAT_CHANNEL
sse.write('', event: "hb")
else
sse.write(data, event: "offer_update")
end
end
end
ensure
redis.quit if reddis # might not have had a chance to initialize yet
sse.close if sse # might not have had a chance to initialize yet
puts "sse was closed"
end