Go Web应用程序中用于处理功能的代码设计

I'm learning go and ran into some design issues while developing web app. The app has main route "/" where user can submit a simple form. With those form values I am calling external API and unmarshaling response into some struct. Now from here I want to make another call based on retrieved values to another external API and I'm not sure what's the proper way of doing this. Here is a snippet for better understandment:

func main() {
    http.HandleFunc("/", mainHandler)
    log.Fatal(http.ListenAndServe(":8080", nil))
}

func mainHandler(w http.ResponseWriter, r *http.Request) {
    //renders form template
    //makes post and retrieves data from api
    //here with retrieved data I want to make another call to different API,
    // but mainHandler would get too big and complex. I'm not sure how should I pass this data to
    // another handler or redirect to another handler with this data.
}

Hi golearner, in the mainHandler just render the form and make another handler kinda "/formaction" to handle the form, in that way you can easily organize your code.

The handlers' semantics should be designed to match the desired HTTP behavior, regardless of the code complexity. If you want to handle a single client request by doing a bunch of stuff, that should be a single handler. If the handler becomes too complex, break it up. Handlers are just functions and can be broken up exactly like any other function - by extracting some part of it into another function and calling that new function. To take you pseudocode:

func mainHandler(w http.ResponseWriter, r *http.Request) {
    err := renderTemplate(w)
    if err != nil { ... }
    err, data := postToApi()
    if err != nil { ... }
    err, data2 := postToApi2(data)
    if err != nil { ... }
}

There's no reason for those functions to be handlers themselves or to get the client involved with a redirect. Just break up your logic the way you normally break up logic - it doesn't matter that it's an HTTP handler.