Difference between Setting & Adding the Headers in HTTP API in Golang

While working with APIs, headers are an essential aspect. Sometimes we set them, or sometimes we consume them and make decisions.

When I was exploring headers in Golang, I came across two methods. Headers.Set() & Headers.Add() Initially, I thought they were the same. But then I wondered if they are the same, then why two methods?

Let’s understand.

I will be using a proxy which will use Add & Set on headers, as shown below,

func (p Proxy) handle(w http.ResponseWriter, r *http.Request) {
    r.Header.Add("Header1", "head1")
    r.Header.Set("Header2", "head2")
    p.proxy.ServeHTTP(w, r)
}

Let’s try to curl the server along with Headers,

$ curl -i http://localhost:8080 -H 'header1: foo' -H 'header2: bar'
HTTP/1.1 200 OK
Content-Length: 34
Content-Type: text/plain; charset=utf-8
Date: Tue, 17 Jan 2023 20:46:34 GMT

header1: [head1 foo], header2: [bar]

What happened here?

For header1, we were using Add method. When we tried passing value to the same header, it returned an array. So basically, it is appended to the existing value.

Official documentation reference: https://pkg.go.dev/net/http#Header.Add

For header2, we were using Set method. When we tried passing value to the same header, it didn’t return as Array with old and newly appended values. Instead, it replaced the old value.

Official documentation reference: https://pkg.go.dev/net/http#Header.Set

Reference

https://tachingchen.com/blog/pitfall-of-golang-header-operation/

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