[Matthias247]

Duplex streams are not really a HTTP/2-only feature. You can do the same bidirectional streaming with HTTP/1.1 too. The flow is always: 1. The client sends a header set. 2. It can then start to stream data in the form of an unlimited-length byte-stream to the server. 3. The server starts to send a header set back to the client. 4. The server can then start to stream data in the form of an unlimited-length byte-stream to the client.

There is not even a fixed order between 2) and 3). The server can start sending headers or body data before the client sent any body byte.

What is correct is that a lot of servers and clients (including javascript in browsers) don't support this and make stricter assumptions regarding how HTTP requests are used - e.g. that the request bytes are fully sent before the response happens. I think ReadableStream/WritableStream APIs on browsers were supposed to change that, but I haven't followed the progress in the last few years.

NGINX falls into the same category. It's HTTP/2 support (and gRPC support) had been built with a very limited use-case in mind. That's also why various CDNs and service meshes use different kinds of HTTP proxies - so that various streaming workloads don't break in case way the protocol is used is not strictly request->response.

[jchw]

No browser I'm aware of is planning on allowing the request and response bodies to be streamed simultaneously for the same request using ReadableStream and WriteableStream. When using streaming request bodies, you have to set the request explicitly to half-duplex.

Anyways, yes, this is technically true, but the streaming semantics are not really that well-defined for HTTP/1.1, probably because it was simply never envisioned. The HTTP/1.1 request and response were viewed as unary entities and the fact that their contents were streamed was mostly an implementation detail. Most HTTP/1.1 software, not just browsers, ultimately treat the requests and responses of HTTP as different and distinct phases. For most uses of HTTP, this makes sense. e.g. for a form post, the entire request entity is going to need to be read before the status can possibly be known.

Even if we do allow bidirectional full-duplex streaming over HTTP/1.1, it will block an entire TCP connection for a given hostname, since HTTP/1.1 is not multiplexed. This is true even if the connection isn't particularly busy. Obviously, this is still an issue even with long-polling, but that's all the more reason why HTTP/2 is simply nicer.

NGINX may always be stuck in an old school HTTP/1 mindset, but modern software like Envoy shows a lot of promise for how architecting around HTTP/2 can work and bring advantages while remaining fully backwards compatible with HTTP/1 software.

[nitely]

> I think ReadableStream/WritableStream APIs on browsers were supposed to change that, but I haven't followed the progress in the last few years.

There has been a lot of pushback against supporting full-duplex streams[0].

[0]: https://github.com/whatwg/fetch/issues/1254

[rixed]

> a lot of servers and clients (including javascript in browsers) don't support this

To say nothing about the many http proxies in between.