Lesson 34 · A urlencoded POST - Build an HTTP Client

A urlencoded POST

The chapter's payoff is building a POST whose body is a form and whose headers say so. Today you wire the form encoder into a request with the right Content-Type and length.

The goal

Build a form POST whose body is encoded pairs, with the correct Content-Type and Content-Length.

Start here - the target

TO DO

Scenario: Posting a form

Givena POST for "http://example.com/submit" with form pairs (name, "Ada") and (city, "NYC"), Host derived

Whenthe request is built and serialized

Thenthe body is "name=Ada&city=NYC", the Content-Type header is "application/x-www-form-urlencoded", and Content-Length is "17"

Andthe headers serialize in sorted order (content-length, content-type, host) before the blank line and body

Background

This is the everyday form submission a browser makes, built from your own pieces. The pairs (name, "Ada") and (city, "NYC") encode to the body name=Ada&city=NYC; the Content-Type header declares it application/x-www-form-urlencoded so the server knows how to parse it; and attaching the body sets Content-Length to its seventeen bytes automatically, using the body-framing from chapter two.

Everything composes: the form encoder from this chapter produces the body, the request builder from chapter two frames it, and the header block serializes in sorted order. The client can now authenticate and submit forms - the two encodings that carry real data to a server. The final chapter turns to how a client follows a conversation across responses: redirects and cookies.

Make it work

// combine the pieces: encode the pairs into a body, set the
// form Content-Type, and attach the body (which sets Content-Length).
//   body := encodeForm(pairs)
//   req.Set("Content-Type", "application/x-www-form-urlencoded")
//   req.SetBody([]byte(body))

CheckpointDONE

You can post a form with a correctly encoded body and matching headers. The encodings chapter is done - commit and stop here.