[niftich]

In 2016, the Chromium team at Google produced a document [1] that examines usecases for HTTP/2 Push, talks about deployment models, and analyzes whether it's worth it. In this particular case, you'd push static content because you know it will be needed later, and this way the information arrives in the HTTP header instead of in the payload's content body, so by the time 'main.css' is needed, the UA's HTTP cache may already be populated with the file.

That being said, I fail to see how in the general case, setting static headers in the server software's config for Push is useful [2][3], and wish that more implementations converged on a common way of describing what to push [4], so that tools could be built around discovering dependencies, and around interpreting that manifest to execute push.

[1] https://docs.google.com/document/d/1K0NykTXBbbbTlv60t5MyJvXj... [2] https://news.ycombinator.com/item?id=14077955#14081237 [3] https://news.ycombinator.com/item?id=12719563#12722383 [4] https://github.com/GoogleChromeLabs/http2-push-manifest

[zzzcpan]

Pushes are probably best implemented in a caching layer, not manually describing what to push. A web server should not just cache resources, but also learn what kind of resources are often requested with each page and just push those next time someone makes a request. And some sort of push prediction policy should be configurable.

[niftich]

It's not sensible for pushes to be implemented in a caching layer, because pushes are effectively the manual overrides to the User-Agent's own caching; conversely, the User-Agent's cache is perfectly appropriate as a cache, and doesn't need HTTP/2 Push to work. HTTP/2 Push is effectively the server declaring they know better, so they prime the UA's cache to avoid additional roundtrips.

Nginx does have a module [1] and a corresponding configuration option to scan outgoing headers for Link header preload directives, and once it has learned of a preload being declared by a resource, it will push that resource thereafter. Nginx talks about the justification for this feature, where they too admit that statically configuring pushes in the server config is not terribly useful -- it's quite often the wrong place to specify relationships between resources.

[1] https://www.nginx.com/blog/nginx-1-13-9-http2-server-push/

[zzzcpan]

If you can predict with high enough accuracy what resource is going to be requested by the client next, I don't see why pushing it would be a bad idea. Speculation is how we hide latency after all.

And if you think about it, static pushes in general have very limited usefulness, almost non existent. Imagine when some url becomes popular and almost all of the requests to that url come from people who never visited the website before. It would make sense for a web server to learn what kind of resources clients request with that url and start pushing those resources to people ahead of time.

[pas]

For that it's easier to parse the pushed content. If it's HTML, then catch stlyesheets, JS, and some other static <img src=.../> things. It doesn't have to be flawless, after all it's just a speed-up. (And if you want a speed up write nice markup.)

Similarly, it should be the backend behind the reverse-proxy that knows what's the page that has been just rendered, and knows about the user's session (is it brand new, or maybe it's not new, but still needs to push things because it's too old, and since then that particular page's background changed, etc.).

And in case of an Angular/React/SPA thing, then the "bundler/compiler" should create a list of things to push for various URLs. Or the Angular/React team should talk with the Nginx team to figure out how to speed up things. (In case of SSR - server side rendering - the NodeJS server can emit the necessary Link headers, for example.)

[zzzcpan]

Common, how parsing things is easier, than gathering some very basic stats?

[pas]

Gathering stats requires keeping them somewhere. Making inferences. Documenting the inference engine. Explaining the magic to users. Sounds a lot more complicated than explaining that what HTML tags will be parsed.

Proxies are already complicated as is. Caching proxies more so. (Think of how Varnish has a - probably Turing complete - DSL to decide what to serve and/or cache and when, and how.)

[sly010]

> It's not sensible for pushes to be implemented in a caching layer, because pushes are effectively the manual overrides to the User-Agent's own caching; conversely, the User-Agent's cache is perfectly appropriate as a cache, and doesn't need HTTP/2 Push to work.

By "caching layer" he probably meant a proxy or load balancer level cache not the user agent cache. It would make total sense for a load balancer to statistically discover relationships.

[wolfgang42]

Once you have a config setting, you've done all the work to actually get Push support, which is the hard part. Support for reading a manifest can be added later, or other people can write tools to read manifests and generate config files for the server.