The hints map[string]string prevents me from generating for example these headers:

Link: <https://fonts.google.com>; rel=preconnect
Link: </main.css>; rel=preload; as=style
Link: </common.js>; rel=preload; as=script
Link: </experimental.3eab3290.css>; rel=preload; as=style

Can you also add a test case that shows the Early Hints response and makes sure the normal response after it also works fine.

ctx.EarlyHints(map[string]string{
    "https://fonts.google.com": "rel=preconnect",
    "/main.css": "rel=preload; as=style",
    "/common.js": "rel=preload; as=script",
    "/experimental.3eab3290.css": "rel=preload; as=style", // <===== You wouldn't normally early hint this one because it's not stable
}) 

I'm wondering if this is the best API. Another way might be that this function has no arguments and just sends the headers set in the response already and then resets them. Not sure if an Early Hints response can also have other headers?

I'm wondering if this is the best API. Another way might be that this function has no arguments and just sends the headers set in the response already and then resets them. Not sure if an Early Hints response can also have other headers?

The point of early hints is that the Link Headers are sent before the processing of the full response.
In some cases, the full response may take "considerable" time to determine what the headers are and then write them to Response.

Also the early hint links are not necessarily the same as the final response links. It is usually a smaller subset (i.e. link resources that are stable such as perhaps overall sites css c.f a particular page's css that developer's may change regularly).

If the function didn't take arguments as to what the early hints are, I would need to waste time iterating over the currently set headers to determine which ones are eligible as valid early hints.

This is also similar to the API of node.js: https://github.com/nodejs/node/pull/44180/files (except I made it into a map instead of a slice)

@erikdubbelboer Is there a sample test that I can use as a template that tests something similar?

The API I was thinking of is just:

ctx.Response.Header.Add("Link", "...")
ctx.EarlyHints()

Which then only send the Link header and not any other header that might already have been set.

This design is more in line with what we already have and makes it easier to have no memory allocations.

For an example of a test you can take the 100 continue test:

The API I was thinking of is just:

ctx.Response.Header.Add("Link", "...")
ctx.EarlyHints()

Which then only send the Link header and not any other header that might already have been set.

This design is more in line with what we already have and makes it easier to have no memory allocations.

For an example of a test you can take the 100 continue test:

fasthttp/server_test.go

Line 1881 in 48f3a2f

func TestServerExpect100Continue(t *testing.T) {

But I still have to iterate over the headers to determine if the header is a Link (I.e by doing a comparison). That is slower.

There is no guarantee the developer will only add Links to header before calling EarlyHints.

The amount of headers will be so low that its probably around the same speed as the map. A map makes it really hard not to generate garbage which would make that much slower.

@erikdubbelboer Done

@erikdubbelboer Done

Tests are flaky.

Thanks! I'll tag a release tomorrow.

What's the difference between c.Conn() and using that compared to acquire/release?

Conn() returns the raw connection. Writing directly to it could cause each Write() to be a separate TCP packet. The writer is buffered and will send everything in a single packet on Flush().