问题
I'm a bit rusty on nuances of the HTTP protocol and I'm wondering if it can support publish/subscribe directly?
HTTP is a request reponse protocol. So client sends a request and the server sends back a response. In HTTP 1.0 a new connection was made for each request. Now HTTP 1.1 improved on HTTP 1.0 by allowing the client to keep the connection open and make multiple requests.
I realise you can upgrade an HTTP connection to a websocket for fast 2 way communications. What I'm curious about is whether this is strictly necessary?
For example if I request a resource "http://somewhere.com/fetch/me/slowly"
Is the server free to reply directly twice? Such as first with a 202 accepted and then shortly later with the content when it is ready, but without the client sending an additional request first?
i.e.
Client: GET http://somewhere.com/fetch/me/slowly
Server: 202 "please wait..."
Server: 200 "here's your document"
Would it be correct to implement a publish/subscribe service this way? For example:
Client: http://somewhere.com/subscribe
Server: item 1
...
Server: item 2
I get the impression that this 'might' work because clients will typically have an event loop watching the connection but is technically wrong (because a client following the protocol need not be implemented that way).
However, if you use chunked transfer encoding this would work.
HTTP/2 seems to allow this as well but I'm not clear whether something changed to make it possible.
I haven't seen much discussion of this in relation to pub/sub so what if anything is wrong with using plain HTTP/1.1 with or without chunked encoding?
If this works why do you need things like RSS or ATOM?
回答1:
A HTTP request can have multiple 'responses', but the responses all have statuscodes in the 1xx range, such as 102 Processing.
However, these responses are only headers, never bodies.
HTTP/1.1 (like 1.0 before it) is a request/response protocol. Sending a response unsolicited is not allowed. HTTP/2 is a frames protocol which adds server push which allows the server to suggest offer extra data and handle multiple requests in parallel but doesn't change its request/response nature.
It is possible to keep a HTTP connection open and keep sending more data though. Many (audio, video) streaming services will use this.
However, this just looks like a continuous body that keeps on streaming, rather than many multiple HTTP responses.
If this works why do you need things like RSS or ATOM
Because keeping a TCP connection open is not free.
来源：https://stackoverflow.com/questions/53231492/are-multiple-responses-legal-in-http