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

Streaming and Execution State¶ ↑

When streaming, the action is executed in a separate thread. By default, this thread shares execution state from the parent thread.

You can configure which execution state keys should be excluded from being shared using the config.action_controller.live_streaming_excluded_keys configuration:

# config/application.rb config.action_controller.live_streaming_excluded_keys = [:active_record_connected_to_stack]

This is useful when using ActionController::Live inside a connected_to block. For example, if the parent request is reading from a replica using connected_to(role: :reading), you may want the streaming thread to use its own connection context instead of inheriting the read-only context:

# Without configuration, streaming thread inherits read-only connection ActiveRecord::Base.connected_to(role: :reading) do @posts = Post.all render stream: true # Streaming thread cannot write to database end

# With configuration, streaming thread gets fresh connection context # config.action_controller.live_streaming_excluded_keys = [:active_record_connected_to_stack] ActiveRecord::Base.connected_to(role: :reading) do @posts = Post.all render stream: true # Streaming thread can write to database if needed end

Common keys you might want to exclude: - :active_record_connected_to_stack - Database connection routing and roles - :active_record_prohibit_shard_swapping - Shard swapping restrictions

By default, no keys are excluded to maintain backward compatibility.