There is one more thing I managed to get into the upcoming Apache httpd 2.4.24: earlier PUSHes.
The table below shows how long it took for a HTTP/2 client (
nghttp) to see response
and associated PUSHed resources. Time is given in seconds since connection start, so this includes
all TLS handshakes and such. Client and server running on the same machine.
|Request||Directive||Promises||103 Status||Promised Response||Main Response|
|Local File||Header add||0.013||-||0.013||0.013|
|Proxy File||Header add||0.024||-||0.025||0.024|
|Local CGI||Header add||0.091||-||0.091||0.091|
The red numbers are from Apache 2.4.23 and older, the green ones are from 2.4.24. As you can see, PUSHes can now happen much earlier and reach the client faster than before.
There are three variations of a resource setup listed, all of which are configured to PUSH 2 local file resources. The main, requested resource is a:
mod_proxy>to server a file from another local server. This example is from a warmed-up proxy connction. But still the new PUSHes happen 11 ms earlier.
mod_cgid. This is a simple CGI setup without any optimizations. Here you can see improvements of 78 ms.
What causes this difference?
There are now two different methods to PUSH resources now in Apache httpd 2.4.24:
Link:headers to a response which then triggers
mod_http2to perform PUSHes.
Link:headers as needed.
The idea for this comes from Tatsuhiro Tsujikawa, maker of nghttp2 and is proposed by Kazohu Oku, the person behind he h2o server, as a new standard. They both thought about how to improve on the Link header response and Tatsuhiro came up with the idea of using an interim response. The draft describing this has been submitted to the HTTP working group.
103, being in the 1xx status range, is an interim response, meaning it precedes
the real response which is yet to come. The HTTP standard talks about it
here. You might be familiar
with the 100 status code that is often used when uploading data to a server. But a client
may receive many 1xx resposnes before the real one and is expected to parse them and ignore
them if they mean nothing to it. That is what the standard says.
But implementations do not always follow the standards and also mod_http2 was guilty of not
respecting this aspect correctly. No more! 1xx responses, generated by the application, are
forwarded to the client on HTTP/2 connections. And not only forwarded, but if such a response
Link headers, the can trigger PUSHes just as the final response does.
For using Apache as a reverse proxy, this currently means:
103 Early Hintsat any time and trigger resource PUSHing this way. This does not need extra configuration on the frontend.
As usual, you can also throw this into your 2.4.23 Apache with v1.8.0 from github.
Stefan Eissing, greenbytes GmbH
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