ai-for-dummies/vote-service/main.go

// Command vote-service is a tiny, dependency-free HTTP API that lets the
// skills-review page (a static site) collect "prefer original / prefer
// improved" votes per submitted skill.
//
// It is intentionally minimal: one Go binary, no external dependencies, one
// JSON file on disk as the store. That fits the workshop scale of this
// feature (dozens of voters, not thousands) and keeps the container image
// and the Kubernetes footprint small.
//
// Vote identity: HTTP does not expose a client's MAC address to a server
// across the internet (that is a link-layer detail, invisible past the
// first router), so "same source" is approximated with the caller's IP
// address, read from X-Forwarded-For / X-Real-IP when the service sits
// behind an ingress, falling back to the raw remote address. One IP may
// hold at most one active vote per skill; casting a new choice updates that
// vote instead of adding a second one. A client-supplied X-Voter-Id header
// (a random id the frontend keeps in localStorage) is layered on top only
// to let a browser recognize and display its own prior vote — it is not
// trusted as the sole anti-abuse signal, since it is trivially resettable.
package main

import (
	"encoding/json"
	"errors"
	"log"
	"net"
	"net/http"
	"os"
	"strings"
	"sync"
)

type voteChoice string

const (
	choiceOriginal voteChoice = "original"
	choiceImproved voteChoice = "improved"
)

func (c voteChoice) valid() bool { return c == choiceOriginal || c == choiceImproved }

// store is the on-disk vote ledger. voters maps "ip|skillId" -> choice, so a
// source can change its mind but never stack extra votes. counts is kept in
// sync for O(1) tally reads.
type store struct {
	mu     sync.Mutex
	path   string
	Voters map[string]voteChoice         `json:"voters"`
	Counts map[string]map[voteChoice]int `json:"counts"`
}

func loadStore(path string) (*store, error) {
	s := &store{path: path, Voters: map[string]voteChoice{}, Counts: map[string]map[voteChoice]int{}}
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return s, nil
	}
	if err != nil {
		return nil, err
	}
	if len(data) == 0 {
		return s, nil
	}
	decoded := struct {
		Voters map[string]voteChoice `json:"voters"`
	}{}
	if err := json.Unmarshal(data, &decoded); err != nil {
		return nil, err
	}
	s.Voters = decoded.Voters
	for key, choice := range s.Voters {
		skillID := key[strings.IndexByte(key, '|')+1:]
		s.bump(skillID, choice, 1)
	}
	return s, nil
}

func (s *store) bump(skillID string, choice voteChoice, delta int) {
	if s.Counts[skillID] == nil {
		s.Counts[skillID] = map[voteChoice]int{}
	}
	s.Counts[skillID][choice] += delta
}

// cast records one vote from voterKey ("ip|skillId") for skillID, replacing
// any prior choice from the same key. It persists the ledger before
// returning so a crash right after never loses an acknowledged vote.
func (s *store) cast(voterKey, skillID string, choice voteChoice) (tally map[voteChoice]int, err error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if prev, ok := s.Voters[voterKey]; ok {
		if prev == choice {
			return s.snapshot(skillID), nil
		}
		s.bump(skillID, prev, -1)
	}
	s.Voters[voterKey] = choice
	s.bump(skillID, choice, 1)
	if err := s.persist(); err != nil {
		return nil, err
	}
	return s.snapshot(skillID), nil
}

func (s *store) snapshot(skillID string) map[voteChoice]int {
	tally := map[voteChoice]int{choiceOriginal: 0, choiceImproved: 0}
	for choice, count := range s.Counts[skillID] {
		tally[choice] = count
	}
	return tally
}

func (s *store) all() map[string]map[voteChoice]int {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make(map[string]map[voteChoice]int, len(s.Counts))
	for skillID := range s.Counts {
		out[skillID] = s.snapshot(skillID)
	}
	return out
}

func (s *store) persist() error {
	data, err := json.Marshal(struct {
		Voters map[string]voteChoice `json:"voters"`
	}{s.Voters})
	if err != nil {
		return err
	}
	tmp := s.path + ".tmp"
	if err := os.WriteFile(tmp, data, 0o600); err != nil {
		return err
	}
	return os.Rename(tmp, s.path)
}

func clientIP(r *http.Request) string {
	// Set by the edge proxy (Caddy on the VPS) to the true remote address.
	// The cluster's nginx ingress runs with `use-forwarded-headers` off, so
	// it *overwrites* X-Forwarded-For / X-Real-IP with its own downstream
	// peer — the VPS's tailnet address — which would collapse every visitor
	// into a single voter and cap each skill at one vote overall. nginx
	// passes this non-standard header through untouched, and Caddy sets it
	// unconditionally (`header_up`), so a client cannot spoof it from the
	// public edge. Trust here is exactly the trust already placed in
	// X-Forwarded-For below.
	if edge := r.Header.Get("X-Client-IP"); edge != "" {
		return strings.TrimSpace(strings.Split(edge, ",")[0])
	}
	if forwarded := r.Header.Get("X-Forwarded-For"); forwarded != "" {
		return strings.TrimSpace(strings.Split(forwarded, ",")[0])
	}
	if real := r.Header.Get("X-Real-IP"); real != "" {
		return real
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

func withCORS(allowedOrigin string, next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Access-Control-Allow-Origin", allowedOrigin)
		w.Header().Set("Vary", "Origin")
		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
		w.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-Voter-Id")
		if r.Method == http.MethodOptions {
			w.WriteHeader(http.StatusNoContent)
			return
		}
		next(w, r)
	}
}

func main() {
	dbPath := envOr("VOTE_DB_PATH", "/data/votes.json")
	allowedOrigin := envOr("ALLOWED_ORIGIN", "https://netcracker.pages.marcospaulo.dev.br")
	addr := ":" + envOr("PORT", "8080")

	s, err := loadStore(dbPath)
	if err != nil {
		log.Fatalf("load vote store %s: %v", dbPath, err)
	}

	mux := http.NewServeMux()
	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) })

	mux.HandleFunc("/api/votes", withCORS(allowedOrigin, func(w http.ResponseWriter, r *http.Request) {
		switch r.Method {
		case http.MethodGet:
			handleResults(w, r, s)
		case http.MethodPost:
			handleVote(w, r, s)
		default:
			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
		}
	}))

	log.Printf("vote-service listening on %s (db=%s, origin=%s)", addr, dbPath, allowedOrigin)
	log.Fatal(http.ListenAndServe(addr, mux))
}

// handleResults returns the tally for every skill. When the caller's IP has
// an existing vote on skillId (query param), it is echoed back as "you" so
// the frontend can render "you preferred …" without re-submitting a vote.
func handleResults(w http.ResponseWriter, r *http.Request, s *store) {
	tallies := s.all()
	skillID := strings.TrimSpace(r.URL.Query().Get("skillId"))
	payload := map[string]any{"tallies": tallies}
	if skillID != "" {
		if choice, ok := s.mine(clientIP(r), skillID); ok {
			payload["you"] = choice
		}
	}
	writeJSON(w, http.StatusOK, payload)
}

func (s *store) mine(ip, skillID string) (voteChoice, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	choice, ok := s.Voters[ip+"|"+skillID]
	return choice, ok
}

func handleVote(w http.ResponseWriter, r *http.Request, s *store) {
	var body struct {
		SkillID string     `json:"skillId"`
		Choice  voteChoice `json:"choice"`
	}
	if err := json.NewDecoder(http.MaxBytesReader(w, r.Body, 1<<12)).Decode(&body); err != nil {
		http.Error(w, "invalid request body", http.StatusBadRequest)
		return
	}
	body.SkillID = strings.TrimSpace(body.SkillID)
	if body.SkillID == "" || strings.ContainsAny(body.SkillID, "|") || !body.Choice.valid() {
		http.Error(w, "skillId and a valid choice are required", http.StatusBadRequest)
		return
	}

	ip := clientIP(r)
	voterKey := ip + "|" + body.SkillID

	tally, err := s.cast(voterKey, body.SkillID, body.Choice)
	if err != nil {
		log.Printf("persist vote: %v", err)
		http.Error(w, "could not save vote", http.StatusInternalServerError)
		return
	}
	writeJSON(w, http.StatusOK, map[string]any{
		"skillId":  body.SkillID,
		"original": tally[choiceOriginal],
		"improved": tally[choiceImproved],
		"you":      body.Choice,
	})
}

func writeJSON(w http.ResponseWriter, status int, payload any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(payload)
}

func envOr(key, fallback string) string {
	if value := os.Getenv(key); value != "" {
		return value
	}
	return fallback
}