Module 4 · Exercise · ~30 min · Real Go code

Streaming Proxy Server

Build a minimal HTTP streaming proxy. The shape: forward an inbound request to an upstream, stream the response body back without buffering, count bytes, respect context cancellation. Same architecture as a real Canton transaction-stream proxy.

cd module-04-systems-go/exercises/exercise-01-streaming-proxy
go test -v -race ./...

What you're building

client ─[GET /x]─▶  Proxy.ServeHTTP  ─[GET upstream/x]─▶  upstream
                          │                                  │
client ◀─[stream]── ServeHTTP ◀─[chunked stream]───────────┘
                          ▲
                  BytesIn / BytesOut counters
                  context cancellation propagates

Tests check

• Forwarding correctness — what comes in, comes out.
• Header propagation — at minimum Content-Type.
• Byte counting — atomic counter matches actual bytes streamed.
• Cancellation — when the inbound request's context times out mid-stream, the upstream call cancels too. Without context propagation, the proxy would happily buffer everything and the test would see a full payload.

Shapes you'll use

• http.NewRequestWithContext(r.Context(), "GET", url, nil) — propagates cancellation.
• http.DefaultClient.Do(req) — sends.
• defer resp.Body.Close() — non-negotiable.
• io.Copy(w, resp.Body) — streams; returns n bytes copied.