Add headers for each HTTP request using client

Question

I know that I can add headers to each HTTP request manually using

cli := &http.Client{}
req, err := http.NewRequest("GET", "https://myhost", nil)
req.Header.Add("X-Test", "true")
if err != nil {
    panic(err)
}
rsp, err := cli.Do(req)

but I want to add this header automatically for each HTTP request in my app.

What is the best way to do it?

Why not simply wrap this into a function for making requests? — AKstat
Wrapping could work as long as you're the one using it directly. But once you have to hand over a client to another library that uses it in its own way, wrapping isn't sufficient anymore. — icza
Sure, I can wrap client.Do with custom function to append headers, but client.Get and client.Post looks more laconic than client.NewRequest, error checking, then myDo func. — Kirill

Flimzy Flimzy · Accepted Answer · 2019-01-08T09:40:49

I'm aware of three possible solutions to this. In (my) order of preference:

Wrap http.NewRequest with custom code that adds desired headers:
```
func MyRequest(method, path url, body io.Reader) (*http.Request, error) {
    req, err := http.NewRequest(method, path, body)
    if err != nil {
        return nil, err
    }
    req.Header.Add("X-Test", "true")
    return req, nil
}
```
This approach has the advantage of being straight-forward, non-magical, and portable. It will work with any third-party software, that adds its own headers, or sets custom transports.

The only case where this won't work is if you depend on a third-party library to create your HTTP requests. I expect this is rare (I don't recall ever running into this in my own experience). And even in such a case, perhaps you can wrap that call instead.
Wrap calls to client.Do to add headers, and possibly any other shared logic.
```
func MyDo(client *http.Client, req *http.Request) (*http.Response, error) {
    req.Header.Add("X-Test", "true")
    // Any other common handling of the request
    res, err := client.Do(req)
    if err != nil {
        return nil, err
    }
    // Any common handling of response
    return res, nil
}
```
This approach is also straight-forward, and has the added advantage (over #1) of making it easy to reduce other boilerplate. This general method can also work very well in conjunction with #1. One possible draw-back is that you must always call your MyDo method directly, meaning you cannot rely on third party software which calls http.Do itself.
Use a custom http.Transport
```
type myTransport struct{}

func (t *myTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    req.Header.Add("X-Test", "true")
    return http.DefaultTransport.RoundTrip(req)
}
```
Then use it like this:
```
client := &Client{Transport: &myTransport{}}
req := http.NewRequest("GET", "/foo", nil)
res, err := client.Do(req)
```
This approach has the advantage of working "behind the scenes" with just about any other software, so if you rely on a third-party library to create your http.Request objects, and to call http.Do, this may be your only option.

However, this has the potential disadvantage of being non-obvious, and possibly breaking if you're using any third-party software which also sets a custom transport (without bothering to honor an existing custom transport).

Ultimately, which method you use will depend on what type of portability you need with third-party software. But if that's not a concern, I suggest using the most obvious solution, which, by my estimation, is the order provided above.

Add headers for each HTTP request using client

2 Answers