Vary: Accept-Encoding missing in zlib examples

[nigoroll]

Ref: https://github.com/nodejs/node/blob/master/doc/api/zlib.md#compressing-http-requests-and-responses

It appears to me that the examples lack setting the Vary: Accept-Encoding Response header.

[DamianRivas]

@Fishrock123 Hi! I'd like to help out here. I'm afraid I can't tell what the problem is exactly. Would the change be something like this?

http.createServer((request, response) => {
  ...
  response.setHeader('Vary: Accept-Encoding');
  ...

EDIT: Oops, that's the client.

[addaleax]

@Fishrock123 @nigoroll Are you sure that this should happen? It’s not central to the point of the documentation, as it introduces the much bigger topic of HTTP caching into our compression docs.

@DamianRivas I think this only makes sense for the server example, since Vary is a HTTP server header.

[nigoroll]

@addaleax It is plain wrong to use the value of Accept-Encoding to determine the Content-Encoding and not set Vary, unless any other downstream caching is prevented, and even then, for good interop with caches, I'd advise to still set it correctly, because cache admins may override cache directives.

I am not sure what exactly you mean by It’s not central to the point of the documentation, in my mind documentation examples should be as simple as possible, but at any rate they should be correct.

Ref: https://tools.ietf.org/html/rfc7231#section-7.1.4

An origin server SHOULD send a Vary header field when its algorithm
for selecting a representation varies based on aspects of the request
message other than the method and request target, unless the variance
cannot be crossed or the origin server has been deliberately
configured to prevent cache transparency.

[nigoroll]

@DamianRivas your example looks good to me, but I am not a node dev

[addaleax]

This has been fixed by #26308.