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Implement H2 Early Hints for Rails #30744

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merged 1 commit into from Oct 4, 2017
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eileencodes commented Sep 28, 2017

When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the
client.

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
stylesheet_link_tag and the javascript_include_tag.

Once puma supports Early Hints the --early-hints argument can be
passed to the server to enable this. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass --early-hints to rails s.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:
https://tools.ietf.org/html/draft-ietf-httpbis-early-hints-04

cc/ @tenderlove who I worked with on this feature


In the screenshot below you can see the pushes from early hints.

screen shot 2017-09-26 at 1 26 57 pm

@eileencodes eileencodes force-pushed the eileencodes:early-hints branch 3 times, most recently Sep 28, 2017
@eileencodes eileencodes self-assigned this Sep 28, 2017
@eileencodes eileencodes added this to the 5.2.0 milestone Sep 28, 2017
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kaspth left a comment

iirc, sprockets-rails overrides these helpers, but that won't make it so that the early hints will be removed, right?

Looking great otherwise!

actionpack/lib/action_dispatch/http/request.rb Outdated
@@ -199,6 +199,23 @@ def headers
@headers ||= Http::Headers.new(self)
end

# Early Hints is an H2 status code that indicates hints to help a client start

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@kaspth

kaspth Sep 28, 2017 Member

Is H2 an accepted short hand for HTTP2? Otherwise we should probably stick to HTTP2 through out.

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@eileencodes

eileencodes Oct 3, 2017 Author Member

While it is an accepted shorthand I've replaced this with HTTP/2 because documentation should not assume knowledge of that.

actionpack/lib/action_dispatch/http/request.rb Outdated
#
# The +send_early_hints+ method accepts an array of links as follows:
#
# send_early_hints({ link: "path/to/stylesheet.css", as: "stylesheet}, { link: "path/to/javascript.js", as: "script"})

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@kaspth

kaspth Sep 28, 2017 Member

Missing a closing quote on as: "stylesheet. Should add a space before the two } as well.

actionpack/test/dispatch/request_test.rb Outdated
assert_equal early_hints.first[:link], "a_path.css"
assert_equal early_hints.first[:as], "stylesheet"
assert_equal early_hints.last[:link], "a_path.js"
assert_equal early_hints.last[:as], "script"

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@kaspth

kaspth Sep 28, 2017 Member

The argument order here should be reversed. It's expected, then actual.

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@eileencodes

eileencodes Oct 3, 2017 Author Member

Fixed when the API was updated.

actionview/lib/action_view/helpers/asset_tag_helper.rb Outdated
@@ -37,6 +37,9 @@ module AssetTagHelper
# When the Asset Pipeline is enabled, you can pass the name of your manifest as
# source, and include other JavaScript or CoffeeScript files inside the manifest.
#
# If the server supports Early Hints header links for these assets will be

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@kaspth

kaspth Sep 28, 2017 Member

Double space after assets.

actionview/lib/action_view/helpers/asset_tag_helper.rb Outdated
}.merge!(options)
content_tag("script".freeze, "", tag_options)
}.join("\n").html_safe

request.send_early_hints(early_hints_links)

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@kaspth

kaspth Sep 28, 2017 Member

This reads to me as if users can only call javascript_include_tag once, is that right?

It's possible people have calls in their application layout and then in individual template files (I'm not saying that's a good idea, but I think that does exist).

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@eileencodes

eileencodes Oct 3, 2017 Author Member

I wrote it this way so that if you pass multiple sources into javascript_include_tag that it will only send early hints once, but you can call javascript_include_tag as many times as you want and it will send early hints.

tag_options = {
"src" => path_to_javascript(source, path_options)
"src" => href
}.merge!(options)
content_tag("script".freeze, "", tag_options)

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@kaspth

kaspth Sep 28, 2017 Member

Would it make sense to change this to tag.script(src: href, **options) while we're here? (Though that would probably necessitate shifting to symbolize_keys above).

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@matthewd

matthewd Sep 28, 2017 Member

Is that desirable in general in internal stuff? The improvement for user code is nice, but it feels like fairly pointless (and comparatively allocation-heavy) indirection for us.

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@kaspth

kaspth Sep 28, 2017 Member

Right, it's a bit of a back and forth. I was focusing on removing the freeze call. Let's defer to some other time, since it's out of scope for this particular thing 👍

def send_early_hints(links)
return unless env["rack.early_hints"]

env["rack.early_hints"].call(links)

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@grosser

grosser Sep 28, 2017 Contributor

could avoid calling doing the same lookup twice:

env["rack.early_hints"]&.call(links)

or

callback = env["rack.early_hints"]
callback.call(links) if callback

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@matthewd

matthewd Sep 29, 2017 Member

It's a hash lookup. I'm sure it'll be fine.

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@eileencodes

eileencodes Oct 3, 2017 Author Member

I've considered this but agree with @matthewd.

Drying things up isn't a benefit if the code is harder to read and understand and the performance impact of calling a hash lookup twice will be negligible.

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@Deradon

Deradon Dec 6, 2017 Contributor

I was just curious about the performance impact, so I did a quick benchmarking.

TL;DR

... the performance impact of calling a hash lookup twice will be negligible.

Details

# 1.18x slower than v2 if env["rack.early_hints"]
def v1
  return unless env["rack.early_hints"]
  env["rack.early_hints"].call(links)
end

# 1.09x slower than v1 IF NOT env["rack.early_hints"]
def v2
  callback = env["rack.early_hints"]
  callback.call(links) if callback
end
Click to show code
#!/usr/bin/env ruby

require 'benchmark/ips'
require 'date'

LOOKUP = { "foo" => ->() {} }

def happy_case
  return unless LOOKUP["foo"]

  LOOKUP["foo"].call()
end

def happy_case_with_assignment
  callback = LOOKUP["foo"]

  callback.call() if callback
end

def alternase_case
  return unless LOOKUP["bar"]

  LOOKUP["bar"].call()
end

def alternase_case_with_assignment
  callback = LOOKUP["bar"]

  callback.call() if callback
end


# == Result
#
# happy_case: 1.18x slower
Benchmark.ips do |x|
  # Configure the number of seconds used during
  # the warmup phase (default 2) and calculation phase (default 5)
  x.config(time: 5, warmup: 2)

  x.report("happy_case") { happy_case }
  x.report("happy_case_with_assignment") { happy_case_with_assignment }

  # Compare the iterations per second of the various reports!
  x.compare!
end

# == Result
# 
# alternase_case_with_assignment: 1.09x slower
Benchmark.ips do |x|
  # Configure the number of seconds used during
  # the warmup phase (default 2) and calculation phase (default 5)
  x.config(time: 5, warmup: 2)

  x.report("alternase_case") { alternase_case }
  x.report("alternase_case_with_assignment") { alternase_case_with_assignment }

  # Compare the iterations per second of the various reports!
  x.compare!
end

__END__

Calculating -------------------------------------
          happy_case   111.305k i/100ms
happy_case_with_assignment
                       127.680k i/100ms
-------------------------------------------------
          happy_case      3.704M (± 2.1%) i/s -     18.588M
happy_case_with_assignment
                          4.353M (± 1.2%) i/s -     21.833M

Comparison:
happy_case_with_assignment:  4353466.9 i/s
          happy_case:  3704451.9 i/s - 1.18x slower

Calculating -------------------------------------
      alternase_case   146.388k i/100ms
alternase_case_with_assignment
                       154.871k i/100ms
-------------------------------------------------
      alternase_case      7.631M (± 2.9%) i/s -     38.207M
alternase_case_with_assignment
                          6.977M (± 6.8%) i/s -     34.846M

Comparison:
      alternase_case:  7630584.3 i/s
alternase_case_with_assignment:  6977239.4 i/s - 1.09x slower

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@mathieujobin

mathieujobin Apr 20, 2018

interesting, would have been nice to see how the &. syntax compares.

@@ -161,7 +162,8 @@ def server_options
daemonize: options[:daemon],
pid: pid,
caching: options["dev-caching"],
restart_cmd: restart_command
restart_cmd: restart_command,
early_hints: early_hints

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@grosser

grosser Sep 28, 2017 Contributor

use it directly like dev-caching / daemon ?

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@eileencodes

eileencodes Oct 3, 2017 Author Member

I put the documentation in the early hints option. It's passed in like --early-hints or can be set in the puma.config with early_hints(true).

@grosser
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grosser commented Sep 29, 2017

@eileencodes eileencodes force-pushed the eileencodes:early-hints branch Oct 3, 2017
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eileencodes commented Oct 3, 2017

@kaspth I've fixed the typos and other recommendations you made. Thanks for the review!

FYI, this is on hold until the Puma branch is merged.

@eileencodes eileencodes force-pushed the eileencodes:early-hints branch Oct 4, 2017
@eileencodes eileencodes changed the title WIP: Implement H2 Early Hints for Rails Implement H2 Early Hints for Rails Oct 4, 2017
When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the
client.

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
`stylesheet_link_tag` and the `javascript_include_tag`.

Once puma supports Early Hints the `--early-hints` argument can be
passed to the server to enable this or set in the puma config with
`early_hints(true)`. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass `--early-hints` to
`rails s`.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:
https://tools.ietf.org/html/draft-ietf-httpbis-early-hints-04

[Eileen M. Uchitelle, Aaron Patterson]
@eileencodes eileencodes force-pushed the eileencodes:early-hints branch to 59a02fb Oct 4, 2017
@eileencodes eileencodes merged commit bd2542b into rails:master Oct 4, 2017
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@eileencodes eileencodes deleted the eileencodes:early-hints branch Oct 4, 2017
@madmax
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madmax commented Oct 8, 2017

@eileencodes What about javascript_pack_tag? It will also work with it?

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eileencodes commented Oct 9, 2017

@madmax Is that implemented by webpacker? I haven't used Rails webpacker and I don't see a javascript_pack_tag in Rails.

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madmax commented Oct 9, 2017

Yes it is part of webpacker gem. And there is also manifest.json can't we use it?

@eileencodes
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eileencodes commented Oct 9, 2017

At this moment I don't plan on implementing in webpacker but I don't see a reason to not implement it there if the tag has access to the request object and someone wants to work on that.

@guilleiguaran
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guilleiguaran commented Oct 9, 2017

IIRC javascript_pack_tag just calls Rails' default javascript_include_tag underneath.

@eileencodes
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eileencodes commented Oct 9, 2017

Ok then I think it will Just Work™️ 😄

@printercu
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printercu commented Oct 10, 2017

According to code, links for early hints are formatted even when server does not support early hints. Should this be better enabled with early_hints: true option?

Also this should be a better option to provide method to send hints from controller, this way browser can start loading scripts even before server starts business logic and page rendering. Layout rendering is the last stage (except streaming mode) of request processing, hints can be even earlier (in before_action?).

WDYT?


sources_tags = sources.uniq.map { |source|
href = path_to_stylesheet(source, path_options)
early_hints_links << "<#{href}>; rel=preload; as=stylesheet"

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@mengqing

mengqing Jan 31, 2018

According to the example described in https://w3c.github.io/preload/#as-attribute, it seems that the as target attribute for stylesheets should be style instead of stylesheet

@leastbad
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leastbad commented Aug 16, 2018

Hey @eileencodes! Thanks so much for your work getting early hints supported in Rails.

I'm seeing some intermittent socket timeout errors when I have --early-hints enabled on my app, which is hosted on Heroku. It's only showing up about once a day, and only in production.

Some lucky visitor's request is throwing one Puma:ConnectionError for each javascript_include_tag in my application layout. If one call fails, they all fail in proper sequence. I've never experienced the end-result personally, leading me to guess that the visitor probably won't notice as their script tags would just load normally.

Any hunches?

Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/puma-3.11.4/lib/puma/server.rb:972:in 'rescue in fast_write' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/puma-3.11.4/lib/puma/server.rb:963:in 'fast_write' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/puma-3.11.4/lib/puma/server.rb:609:in 'block in handle_request' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/actionpack-5.2.0/lib/action_dispatch/http/request.rb:217:in 'send_early_hints' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/actionview-5.2.0/lib/action_view/helpers/asset_tag_helper.rb:96:in 'javascript_include_tag' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/helper.rb:156:in 'block in javascript_include_tag' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/helper.rb:154:in 'map' 
Aug 16 12:34:37 /app/vendor/bundle/ruby/2.4.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/helper.rb:154:in 'javascript_include_tag' 
Aug 16 12:34:37 /app/app/views/layouts/application.html.erb:97:in 'block in _app_views_layouts_application_html_erb__4118899528439397404_70152019751760' 
Aug 16 12:34:37 /app/app/views/layouts/application.html.erb:95:in 'each' 
Aug 16 12:34:37 /app/app/views/layouts/application.html.erb:95:in '_app_views_layouts_application_html_erb__4118899528439397404_70152019751760'```
@eileencodes
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eileencodes commented Aug 25, 2018

Hey @pjforde1978 I'm not sure what's going on but I suspect that this is more on the puma side than the Rails side since Rails just adds the tags, but Puma makes the early hints actually work. Can you open an issue over there including what your application is using (Ruby version, Rails version, Puma version, proxy you're using), and anything else that can help us track this down. Unfortunately we're not able to use this in prod at GitHub yet so I'm not sure where to start looking.

@Jesus
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Jesus commented Jun 18, 2019

Message for Eileen:

Hi @eileencodes, thanks for adding this fantastic feature.

I've been investigating the error reported by @pjforde1978. This happens if the connection is lost before we're able to send any response headers. Rails will try to write the early hints on a socket which no longer exists so it'll fail. This happens very rarely and probably no end-users are being affected as I'm assuming the connection is lost when the user abandons the page load.

From Rails' perspective, there's nothing we can do if the socket has been closed. I think we can ignore this error safely.

But do you think Rails should bother to abort completing this request if the early hints report a lost connection somehow?

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