Action Controller Live

Mix this module into your controller, and all actions in that controller will be able to stream data to the client as it’s written.

class MyController < ActionController::Base
  include ActionController::Live

  def stream
    response.headers['Content-Type'] = 'text/event-stream'
    100.times {
      response.stream.write "hello world\n"
      sleep 1
    }
  ensure
    response.stream.close
  end
end

There are a few caveats with this module. You cannot write headers after the response has been committed (Response#committed? will return truthy). Calling write or close on the response stream will cause the response object to be committed. Make sure all headers are set before calling write or close on your stream.

You must call close on your stream when you’re finished, otherwise the socket may be left open forever.

The final caveat is that your actions are executed in a separate thread than the main thread. Make sure your actions are thread safe, and this shouldn’t be a problem (don’t share state across threads, etc).

Note that Rails includes Rack::ETag by default, which will buffer your response. As a result, streaming responses may not work properly with Rack 2.2.x, and you may need to implement workarounds in your application. You can either set the ETag or Last-Modified response headers or remove Rack::ETag from the middleware stack to address this issue.

Here’s an example of how you can set the Last-Modified header if your Rack version is 2.2.x:

def stream
  response.headers["Content-Type"] = "text/event-stream"
  response.headers["Last-Modified"] = Time.now.httpdate # Add this line if your Rack version is 2.2.x
  ...
end

Namespace

Module

Class

Methods

Instance Public methods

process(name)

📝 Source code 
# File actionpack/lib/action_controller/metal/live.rb, line 273
    def process(name)
      t1 = Thread.current
      locals = t1.keys.map { |key| [key, t1[key]] }

      error = nil
      # This processes the action in a child thread. It lets us return the response
      # code and headers back up the Rack stack, and still process the body in
      # parallel with sending data to the client.
      new_controller_thread {
        ActiveSupport::Dependencies.interlock.running do
          t2 = Thread.current

          # Since we're processing the view in a different thread, copy the thread locals
          # from the main thread to the child thread. :'(
          locals.each { |k, v| t2[k] = v }
          ActiveSupport::IsolatedExecutionState.share_with(t1)

          begin
            super(name)
          rescue => e
            if @_response.committed?
              begin
                @_response.stream.write(ActionView::Base.streaming_completion_on_exception) if request.format == :html
                @_response.stream.call_on_error
              rescue => exception
                log_error(exception)
              ensure
                log_error(e)
                @_response.stream.close
              end
            else
              error = e
            end
          ensure
            @_response.commit!
          end
        end
      }

      ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
        @_response.await_commit
      end

      raise error if error
    end
🔎 See on GitHub

response_body=(body)

📝 Source code 
# File actionpack/lib/action_controller/metal/live.rb, line 319
    def response_body=(body)
      super
      response.close if response
    end
🔎 See on GitHub

send_stream(filename:, disposition: "attachment", type: nil)

Sends a stream to the browser, which is helpful when you’re generating exports or other running data where you don’t want the entire file buffered in memory first. Similar to send_data, but where the data is generated live.

Options: * :filename - suggests a filename for the browser to use. * :type - specifies an HTTP content type. You can specify either a string or a symbol for a registered type with Mime::Type.register, for example :json. If omitted, type will be inferred from the file extension specified in :filename. If no content type is registered for the extension, the default type ‘application/octet-stream’ will be used. * :disposition - specifies whether the file will be shown inline or downloaded. Valid values are ‘inline’ and ‘attachment’ (default).

Example of generating a csv export:

send_stream(filename: "subscribers.csv") do |stream|
  stream.write "email_address,updated_at\n"

  @subscribers.find_each do |subscriber|
    stream.write "#{subscriber.email_address},#{subscriber.updated_at}\n"
  end
end
📝 Source code 
# File actionpack/lib/action_controller/metal/live.rb, line 348
    def send_stream(filename:, disposition: "attachment", type: nil)
      payload = { filename: filename, disposition: disposition, type: type }
      ActiveSupport::Notifications.instrument("send_stream.action_controller", payload) do
        response.headers["Content-Type"] =
          (type.is_a?(Symbol) ? Mime[type].to_s : type) ||
          Mime::Type.lookup_by_extension(File.extname(filename).downcase.delete("."))&.to_s ||
          "application/octet-stream"

        response.headers["Content-Disposition"] =
          ActionDispatch::Http::ContentDisposition.format(disposition: disposition, filename: filename)

        yield response.stream
      end
    ensure
      response.stream.close
    end
🔎 See on GitHub