Lesson 14 · Build a POST - Build an HTTP Client

Build a POST

The chapter comes together as you build a complete POST request with a custom header and a body, and assert its exact wire bytes end to end. This is the whole request side of the client working at once.

The goal

Build a POST with a header and a body and serialize it to the exact expected bytes.

Start here - the target

TO DO

Scenario: A complete POST on the wire

Givena POST for "http://example.com/submit" with header Content-Type "text/plain", body "hi", and Host derived

Whenthe request is serialized

Thenthe bytes are exactly "POST /submit HTTP/1.1\r\nContent-Length: 2\r\nContent-Type: text/plain\r\nHost: example.com\r\n\r\nhi"

Andthe headers appear in sorted order (content-length, content-type, host) regardless of the order they were set

Background

This is the request side of the client proving itself. Everything from the chapter runs at once: the request line with method and target, the Host header derived from the URL, a caller-set Content-Type, a body framed by Content-Length, headers in deterministic sorted order, and the whole thing terminated by the blank line before the body. The result is POST /submit HTTP/1.1, three headers in alphabetical order, a blank line, then hi.

Because the header order is fixed and every value is derived by a rule, the entire message is predictable down to the last byte - which is exactly what makes it testable without a network. You now have half of a client: it can build any request a server will accept. Chapter three builds the other half - reading the response back off a stream.

Make it work

// put it together with what you built:
//   req := &Request{Method: "POST", URL: mustParse(...)}
//   req.setHost()
//   req.Set("Content-Type", "text/plain")
//   req.SetBody([]byte("hi"))
//   wire := req.Bytes()
// then assert wire equals the expected string byte-for-byte

CheckpointDONE

You can build and serialize a complete POST request to exact bytes. The request half of the client is done - commit and stop here.