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 # HTTP/2 Server Push
 24 Mar 2017
 Tags: http, technical
 Summary: How to use HTTP/2 server push to reduce page load times.

 Jaana Burcu Dogan

 Tom Bergan

 ## Introduction

 HTTP/2 is designed to address many of the failings of HTTP/1.x.
 Modern web pages use many resources: HTML, stylesheets,
 scripts, images, and so on. In HTTP/1.x, each of these resources must
 be requested explicitly. This can be a slow process.
 The browser starts by fetching the HTML, then learns of more resources
 incrementally as it parses and evaluates the page. Since the server
 must wait for the browser to make each request, the network is often
 idle and underutilized.

 To improve latency, HTTP/2 introduced _server push_, which allows the
 server to push resources to the browser before they are explicitly
 requested. A server often knows many of the additional resources a
 page will need and can start pushing those resources as it responds
 to the initial request. This allows the server to fully utilize an
 otherwise idle network and improve page load times.

 .image h2push/serverpush.svg _ 600

 At the protocol level, HTTP/2 server push is driven by `PUSH_PROMISE`
 frames. A `PUSH_PROMISE` describes a request that the server predicts the
 browser will make in the near future. As soon as the browser receives
 a `PUSH_PROMISE`, it knows that the server will deliver the resource.
 If the browser later discovers that it needs this resource, it will
 wait for the push to complete rather than sending a new request.
 This reduces the time the browser spends waiting on the network.

 ## Server Push in net/http

 Go 1.8 introduced support for pushing responses from an [`http.Server`](https://golang.org/pkg/net/http/#Server).
 This feature is available if the running server is an HTTP/2 server
 and the incoming connection uses HTTP/2. In any HTTP handler,
 you can assert if the http.ResponseWriter supports server push by checking
 if it implements the new [`http.Pusher`](https://golang.org/pkg/net/http/#Pusher) interface.

 For example, if the server knows that `app.js` will be required to
 render the page, the handler can initiate a push if `http.Pusher`
 is available:

 .code h2push/pusher.go /START/,/END/

 The Push call creates a synthetic request for `/app.js`,
 synthesizes that request into a `PUSH_PROMISE` frame, then forwards
 the synthetic request to the server's request handler, which will
 generate the pushed response. The second argument to Push specifies
 additional headers to include in the `PUSH_PROMISE`. For example,
 if the response to `/app.js` varies on Accept-Encoding,
 then the `PUSH_PROMISE` should include an Accept-Encoding value:

 .code h2push/pusher.go /START1/,/END1/

 A fully working example is available at:

 	$ go get golang.org/x/blog/content/h2push/server

 If you run the server and load [https://localhost:8080](https://localhost:8080),
 your browser's developer tools should show that `app.js` and
 `style.css` were pushed by the server.

 .image h2push/networktimeline.png _ 605

 ## Start Your Pushes Before You Respond

 It's a good idea to call the Push method before sending any bytes
 of the response. Otherwise it is possible to accidentally generate
 duplicate responses. For example, suppose you write part of an HTML
 response:

 	<html>
 	<head>
 		<link rel="stylesheet" href="a.css">...

 Then you call Push("a.css", nil). The browser may parse this fragment
 of HTML before it receives your PUSH\_PROMISE, in which case the browser
 will send a request for `a.css` in addition to receiving your
 `PUSH_PROMISE`. Now the server will generate two responses for `a.css`.
 Calling Push before writing the response avoids this possibility entirely.

 ## When To Use Server Push

 Consider using server push any time your network link is idle.
 Just finished sending the HTML for your web app? Don't waste time waiting,
 start pushing the resources your client will need. Are you inlining
 resources into your HTML file to reduce latency? Instead of inlining,
 try pushing. Redirects are another good time to use push because there
 is almost always a wasted round trip while the client follows the redirect.
 There are many possible scenarios for using push -- we are only getting started.

 We would be remiss if we did not mention a few caveats. First, you can only
 push resources your server is authoritative for -- this means you cannot
 push resources that are hosted on third-party servers or CDNs. Second,
 don't push resources unless you are confident they are actually needed
 by the client, otherwise your push wastes bandwidth. A corollary is to
 avoid pushing resources when it's likely that the client already has
 those resources cached. Third, the naive approach of pushing all
 resources on your page often makes performance worse. When in doubt, measure.

 The following links make for good supplemental reading:

   - [HTTP/2 Push: The Details](https://calendar.perfplanet.com/2016/http2-push-the-details/)
   - [Innovating with HTTP/2 Server Push](https://www.igvita.com/2013/06/12/innovating-with-http-2.0-server-push/)
   - [Cache-Aware Server Push in H2O](https://github.com/h2o/h2o/issues/421)
   - [The PRPL Pattern](https://developers.google.com/web/fundamentals/performance/prpl-pattern/)
   - [Rules of Thumb for HTTP/2 Push](https://docs.google.com/document/d/1K0NykTXBbbbTlv60t5MyJvXjqKGsCVNYHyLEXIxYMv0)
   - [Server Push in the HTTP/2 spec](https://tools.ietf.org/html/rfc7540#section-8.2)

 ## Conclusion

 With Go 1.8, the standard library provides out-of-the-box support for HTTP/2
 Server Push, giving you more flexibility to optimize your web applications.

 Go to our [HTTP/2 Server Push demo](https://http2.golang.org/serverpush)
 page to see it in action.
	# HTTP/2 Server Push
	24 Mar 2017
	Tags: http, technical
	Summary: How to use HTTP/2 server push to reduce page load times.

	Jaana Burcu Dogan

	Tom Bergan

	## Introduction

	HTTP/2 is designed to address many of the failings of HTTP/1.x.
	Modern web pages use many resources: HTML, stylesheets,
	scripts, images, and so on. In HTTP/1.x, each of these resources must
	be requested explicitly. This can be a slow process.
	The browser starts by fetching the HTML, then learns of more resources
	incrementally as it parses and evaluates the page. Since the server
	must wait for the browser to make each request, the network is often
	idle and underutilized.

	To improve latency, HTTP/2 introduced _server push_, which allows the
	server to push resources to the browser before they are explicitly
	requested. A server often knows many of the additional resources a
	page will need and can start pushing those resources as it responds
	to the initial request. This allows the server to fully utilize an
	otherwise idle network and improve page load times.

	.image h2push/serverpush.svg _ 600

	At the protocol level, HTTP/2 server push is driven by `PUSH_PROMISE`
	frames. A `PUSH_PROMISE` describes a request that the server predicts the
	browser will make in the near future. As soon as the browser receives
	a `PUSH_PROMISE`, it knows that the server will deliver the resource.
	If the browser later discovers that it needs this resource, it will
	wait for the push to complete rather than sending a new request.
	This reduces the time the browser spends waiting on the network.

	## Server Push in net/http

	Go 1.8 introduced support for pushing responses from an [`http.Server`](https://golang.org/pkg/net/http/#Server).
	This feature is available if the running server is an HTTP/2 server
	and the incoming connection uses HTTP/2. In any HTTP handler,
	you can assert if the http.ResponseWriter supports server push by checking
	if it implements the new [`http.Pusher`](https://golang.org/pkg/net/http/#Pusher) interface.

	For example, if the server knows that `app.js` will be required to
	render the page, the handler can initiate a push if `http.Pusher`
	is available:

	.code h2push/pusher.go /START/,/END/

	The Push call creates a synthetic request for `/app.js`,
	synthesizes that request into a `PUSH_PROMISE` frame, then forwards
	the synthetic request to the server's request handler, which will
	generate the pushed response. The second argument to Push specifies
	additional headers to include in the `PUSH_PROMISE`. For example,
	if the response to `/app.js` varies on Accept-Encoding,
	then the `PUSH_PROMISE` should include an Accept-Encoding value:

	.code h2push/pusher.go /START1/,/END1/

	A fully working example is available at:

	$ go get golang.org/x/blog/content/h2push/server

	If you run the server and load [https://localhost:8080](https://localhost:8080),
	your browser's developer tools should show that `app.js` and
	`style.css` were pushed by the server.

	.image h2push/networktimeline.png _ 605

	## Start Your Pushes Before You Respond

	It's a good idea to call the Push method before sending any bytes
	of the response. Otherwise it is possible to accidentally generate
	duplicate responses. For example, suppose you write part of an HTML
	response:

	<html>
	<head>
	<link rel="stylesheet" href="a.css">...

	Then you call Push("a.css", nil). The browser may parse this fragment
	of HTML before it receives your PUSH\_PROMISE, in which case the browser
	will send a request for `a.css` in addition to receiving your
	`PUSH_PROMISE`. Now the server will generate two responses for `a.css`.
	Calling Push before writing the response avoids this possibility entirely.

	## When To Use Server Push

	Consider using server push any time your network link is idle.
	Just finished sending the HTML for your web app? Don't waste time waiting,
	start pushing the resources your client will need. Are you inlining
	resources into your HTML file to reduce latency? Instead of inlining,
	try pushing. Redirects are another good time to use push because there
	is almost always a wasted round trip while the client follows the redirect.
	There are many possible scenarios for using push -- we are only getting started.

	We would be remiss if we did not mention a few caveats. First, you can only
	push resources your server is authoritative for -- this means you cannot
	push resources that are hosted on third-party servers or CDNs. Second,
	don't push resources unless you are confident they are actually needed
	by the client, otherwise your push wastes bandwidth. A corollary is to
	avoid pushing resources when it's likely that the client already has
	those resources cached. Third, the naive approach of pushing all
	resources on your page often makes performance worse. When in doubt, measure.

	The following links make for good supplemental reading:

	- [HTTP/2 Push: The Details](https://calendar.perfplanet.com/2016/http2-push-the-details/)
	- [Innovating with HTTP/2 Server Push](https://www.igvita.com/2013/06/12/innovating-with-http-2.0-server-push/)
	- [Cache-Aware Server Push in H2O](https://github.com/h2o/h2o/issues/421)
	- [The PRPL Pattern](https://developers.google.com/web/fundamentals/performance/prpl-pattern/)
	- [Rules of Thumb for HTTP/2 Push](https://docs.google.com/document/d/1K0NykTXBbbbTlv60t5MyJvXjqKGsCVNYHyLEXIxYMv0)
	- [Server Push in the HTTP/2 spec](https://tools.ietf.org/html/rfc7540#section-8.2)

	## Conclusion

	With Go 1.8, the standard library provides out-of-the-box support for HTTP/2
	Server Push, giving you more flexibility to optimize your web applications.

	Go to our [HTTP/2 Server Push demo](https://http2.golang.org/serverpush)
	page to see it in action.