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Diffstat (limited to 'actioncable/lib/action_cable/channel/streams.rb')
| -rw-r--r-- | actioncable/lib/action_cable/channel/streams.rb | 174 |
1 files changed, 174 insertions, 0 deletions
diff --git a/actioncable/lib/action_cable/channel/streams.rb b/actioncable/lib/action_cable/channel/streams.rb new file mode 100644 index 0000000000..0a0a95f453 --- /dev/null +++ b/actioncable/lib/action_cable/channel/streams.rb @@ -0,0 +1,174 @@ +module ActionCable + module Channel + # Streams allow channels to route broadcastings to the subscriber. A broadcasting is, as discussed elsewhere, a pubsub queue where any data + # placed into it is automatically sent to the clients that are connected at that time. It's purely an online queue, though. If you're not + # streaming a broadcasting at the very moment it sends out an update, you will not get that update, even if you connect after it has been sent. + # + # Most commonly, the streamed broadcast is sent straight to the subscriber on the client-side. The channel just acts as a connector between + # the two parties (the broadcaster and the channel subscriber). Here's an example of a channel that allows subscribers to get all new + # comments on a given page: + # + # class CommentsChannel < ApplicationCable::Channel + # def follow(data) + # stream_from "comments_for_#{data['recording_id']}" + # end + # + # def unfollow + # stop_all_streams + # end + # end + # + # Based on the above example, the subscribers of this channel will get whatever data is put into the, + # let's say, <tt>comments_for_45</tt> broadcasting as soon as it's put there. + # + # An example broadcasting for this channel looks like so: + # + # ActionCable.server.broadcast "comments_for_45", author: 'DHH', content: 'Rails is just swell' + # + # If you have a stream that is related to a model, then the broadcasting used can be generated from the model and channel. + # The following example would subscribe to a broadcasting like <tt>comments:Z2lkOi8vVGVzdEFwcC9Qb3N0LzE</tt>. + # + # class CommentsChannel < ApplicationCable::Channel + # def subscribed + # post = Post.find(params[:id]) + # stream_for post + # end + # end + # + # You can then broadcast to this channel using: + # + # CommentsChannel.broadcast_to(@post, @comment) + # + # If you don't just want to parlay the broadcast unfiltered to the subscriber, you can also supply a callback that lets you alter what is sent out. + # The below example shows how you can use this to provide performance introspection in the process: + # + # class ChatChannel < ApplicationCable::Channel + # def subscribed + # @room = Chat::Room[params[:room_number]] + # + # stream_for @room, coder: ActiveSupport::JSON do |message| + # if message['originated_at'].present? + # elapsed_time = (Time.now.to_f - message['originated_at']).round(2) + # + # ActiveSupport::Notifications.instrument :performance, measurement: 'Chat.message_delay', value: elapsed_time, action: :timing + # logger.info "Message took #{elapsed_time}s to arrive" + # end + # + # transmit message + # end + # end + # end + # + # You can stop streaming from all broadcasts by calling #stop_all_streams. + module Streams + extend ActiveSupport::Concern + + included do + on_unsubscribe :stop_all_streams + end + + # Start streaming from the named <tt>broadcasting</tt> pubsub queue. Optionally, you can pass a <tt>callback</tt> that'll be used + # instead of the default of just transmitting the updates straight to the subscriber. + # Pass `coder: ActiveSupport::JSON` to decode messages as JSON before passing to the callback. + # Defaults to `coder: nil` which does no decoding, passes raw messages. + def stream_from(broadcasting, callback = nil, coder: nil, &block) + broadcasting = String(broadcasting) + + # Don't send the confirmation until pubsub#subscribe is successful + defer_subscription_confirmation! + + # Build a stream handler by wrapping the user-provided callback with + # a decoder or defaulting to a JSON-decoding retransmitter. + handler = worker_pool_stream_handler(broadcasting, callback || block, coder: coder) + streams << [ broadcasting, handler ] + + connection.server.event_loop.post do + pubsub.subscribe(broadcasting, handler, lambda do + transmit_subscription_confirmation + logger.info "#{self.class.name} is streaming from #{broadcasting}" + end) + end + end + + # Start streaming the pubsub queue for the <tt>model</tt> in this channel. Optionally, you can pass a + # <tt>callback</tt> that'll be used instead of the default of just transmitting the updates straight + # to the subscriber. + # + # Pass `coder: ActiveSupport::JSON` to decode messages as JSON before passing to the callback. + # Defaults to `coder: nil` which does no decoding, passes raw messages. + def stream_for(model, callback = nil, coder: nil, &block) + stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder) + end + + # Unsubscribes all streams associated with this channel from the pubsub queue. + def stop_all_streams + streams.each do |broadcasting, callback| + pubsub.unsubscribe broadcasting, callback + logger.info "#{self.class.name} stopped streaming from #{broadcasting}" + end.clear + end + + private + delegate :pubsub, to: :connection + + def streams + @_streams ||= [] + end + + # Always wrap the outermost handler to invoke the user handler on the + # worker pool rather than blocking the event loop. + def worker_pool_stream_handler(broadcasting, user_handler, coder: nil) + handler = stream_handler(broadcasting, user_handler, coder: coder) + + -> message do + connection.worker_pool.async_invoke handler, :call, message, connection: connection + end + end + + # May be overridden to add instrumentation, logging, specialized error + # handling, or other forms of handler decoration. + # + # TODO: Tests demonstrating this. + def stream_handler(broadcasting, user_handler, coder: nil) + if user_handler + stream_decoder user_handler, coder: coder + else + default_stream_handler broadcasting, coder: coder + end + end + + # May be overridden to change the default stream handling behavior + # which decodes JSON and transmits to client. + # + # TODO: Tests demonstrating this. + # + # TODO: Room for optimization. Update transmit API to be coder-aware + # so we can no-op when pubsub and connection are both JSON-encoded. + # Then we can skip decode+encode if we're just proxying messages. + def default_stream_handler(broadcasting, coder:) + coder ||= ActiveSupport::JSON + stream_transmitter stream_decoder(coder: coder), broadcasting: broadcasting + end + + def stream_decoder(handler = identity_handler, coder:) + if coder + -> message { handler.(coder.decode(message)) } + else + handler + end + end + + def stream_transmitter(handler = identity_handler, broadcasting:) + via = "streamed from #{broadcasting}" + + -> (message) do + transmit handler.(message), via: via + end + end + + def identity_handler + -> message { message } + end + end + end +end |