# frozen_string_literal: true
require "rack/chunked"
module ActionController #:nodoc:
# Allows views to be streamed back to the client as they are rendered.
#
# By default, Rails renders views by first rendering the template
# and then the layout. The response is sent to the client after the whole
# template is rendered, all queries are made, and the layout is processed.
#
# Streaming inverts the rendering flow by rendering the layout first and
# streaming each part of the layout as they are processed. This allows the
# header of the HTML (which is usually in the layout) to be streamed back
# to client very quickly, allowing JavaScripts and stylesheets to be loaded
# earlier than usual.
#
# This approach was introduced in Rails 3.1 and is still improving. Several
# Rack middlewares may not work and you need to be careful when streaming.
# Those points are going to be addressed soon.
#
# In order to use streaming, you will need to use a Ruby version that
# supports fibers (fibers are supported since version 1.9.2 of the main
# Ruby implementation).
#
# Streaming can be added to a given template easily, all you need to do is
# to pass the :stream option.
#
# class PostsController
# def index
# @posts = Post.all
# render stream: true
# end
# end
#
# == When to use streaming
#
# Streaming may be considered to be overkill for lightweight actions like
# +new+ or +edit+. The real benefit of streaming is on expensive actions
# that, for example, do a lot of queries on the database.
#
# In such actions, you want to delay queries execution as much as you can.
# For example, imagine the following +dashboard+ action:
#
# def dashboard
# @posts = Post.all
# @pages = Page.all
# @articles = Article.all
# end
#
# Most of the queries here are happening in the controller. In order to benefit
# from streaming you would want to rewrite it as:
#
# def dashboard
# # Allow lazy execution of the queries
# @posts = Post.all
# @pages = Page.all
# @articles = Article.all
# render stream: true
# end
#
# Notice that :stream only works with templates. Rendering :json
# or :xml with :stream won't work.
#
# == Communication between layout and template
#
# When streaming, rendering happens top-down instead of inside-out.
# Rails starts with the layout, and the template is rendered later,
# when its +yield+ is reached.
#
# This means that, if your application currently relies on instance
# variables set in the template to be used in the layout, they won't
# work once you move to streaming. The proper way to communicate
# between layout and template, regardless of whether you use streaming
# or not, is by using +content_for+, +provide+ and +yield+.
#
# Take a simple example where the layout expects the template to tell
# which title to use:
#
# <html>
# <head><title><%= yield :title %></title></head>
# <body><%= yield %></body>
# </html>
#
# You would use +content_for+ in your template to specify the title:
#
# <%= content_for :title, "Main" %>
# Hello
#
# And the final result would be:
#
# <html>
# <head><title>Main</title></head>
# <body>Hello</body>
# </html>
#
# However, if +content_for+ is called several times, the final result
# would have all calls concatenated. For instance, if we have the following
# template:
#
# <%= content_for :title, "Main" %>
# Hello
# <%= content_for :title, " page" %>
#
# The final result would be:
#
# <html>
# <head><title>Main page</title></head>
# <body>Hello</body>
# </html>
#
# This means that, if you have <code>yield :title</code> in your layout
# and you want to use streaming, you would have to render the whole template
# (and eventually trigger all queries) before streaming the title and all
# assets, which kills the purpose of streaming. For this purpose, you can use
# a helper called +provide+ that does the same as +content_for+ but tells the
# layout to stop searching for other entries and continue rendering.
#
# For instance, the template above using +provide+ would be:
#
# <%= provide :title, "Main" %>
# Hello
# <%= content_for :title, " page" %>
#
# Giving:
#
# <html>
# <head><title>Main</title></head>
# <body>Hello</body>
# </html>
#
# That said, when streaming, you need to properly check your templates
# and choose when to use +provide+ and +content_for+.
#
# == Headers, cookies, session and flash
#
# When streaming, the HTTP headers are sent to the client right before
# it renders the first line. This means that, modifying headers, cookies,
# session or flash after the template starts rendering will not propagate
# to the client.
#
# == Middlewares
#
# Middlewares that need to manipulate the body won't work with streaming.
# You should disable those middlewares whenever streaming in development
# or production. For instance, <tt>Rack::Bug</tt> won't work when streaming as it
# needs to inject contents in the HTML body.
#
# Also <tt>Rack::Cache</tt> won't work with streaming as it does not support
# streaming bodies yet. Whenever streaming Cache-Control is automatically
# set to "no-cache".
#
# == Errors
#
# When it comes to streaming, exceptions get a bit more complicated. This
# happens because part of the template was already rendered and streamed to
# the client, making it impossible to render a whole exception page.
#
# Currently, when an exception happens in development or production, Rails
# will automatically stream to the client:
#
# "><script>window.location = "/500.html"</script></html>
#
# The first two characters (">) are required in case the exception happens
# while rendering attributes for a given tag. You can check the real cause
# for the exception in your logger.
#
# == Web server support
#
# Not all web servers support streaming out-of-the-box. You need to check
# the instructions for each of them.
#
# ==== Unicorn
#
# Unicorn supports streaming but it needs to be configured. For this, you
# need to create a config file as follow:
#
# # unicorn.config.rb
# listen 3000, tcp_nopush: false
#
# And use it on initialization:
#
# unicorn_rails --config-file unicorn.config.rb
#
# You may also want to configure other parameters like <tt>:tcp_nodelay</tt>.
# Please check its documentation for more information: https://bogomips.org/unicorn/Unicorn/Configurator.html#method-i-listen
#
# If you are using Unicorn with NGINX, you may need to tweak NGINX.
# Streaming should work out of the box on Rainbows.
#
# ==== Passenger
#
# To be described.
#
module Streaming
extend ActiveSupport::Concern
private
# Set proper cache control and transfer encoding when streaming
def _process_options(options)
super
if options[:stream]
if request.version == "HTTP/1.0"
options.delete(:stream)
else
headers["Cache-Control"] ||= "no-cache"
headers["Transfer-Encoding"] = "chunked"
headers.delete("Content-Length")
end
end
end
# Call render_body if we are streaming instead of usual +render+.
def _render_template(options)
if options.delete(:stream)
Rack::Chunked::Body.new view_renderer.render_body(view_context, options)
else
super
end
end
end
end
