Docs / readme improvement: clarify behavior with reverse proxied backends

[binaryfire]

Hi guys

Would it be possible to add some information to the readme re: how this module deals with reverse proxied content? Specifically:

• If the upstream content is already compressed with Brotli, does this module decompress+recompress the content or does it pass it through unmodified? Hopefully the latter since I'm trying to avoid unnecessary costly compression steps.

• Is there anything we need to configure differently when dealing with proxied backends? There doesn't seem to be a brotli_proxied directive (similar to gzip_proxied).

I've spent quite a while searching for answers to this but there's very little info out there. Could someone help? I think covering it in the readme would help a lot of people.

[Eagle3386]

@eustas? @PiotrSikora? Anyone?

[PiotrSikora]

1. Brotli filter doesn't recompress responses, see:

   ngx_brotli/filter/ngx_http_brotli_filter_module.c

   Lines 281 to 285 in a71f931

   	/* Bypass already compressed responses. */
   	if (r->headers_out.content_encoding &&
   	r->headers_out.content_encoding->value.len) {
   	return ngx_http_next_header_filter(r);
   	}

2. gzip_proxied applies to proxied requests (clients), not proxied responses (backends).