Protype Candidate

candidate.go

@@ -0,0 +1,236 @@
+package elect
+
+import (
+	"crypto/hmac"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/dchest/uniuri"
+)
+
+// TODO: Ensure promotion takes longer than demotion
+
+type Candidate struct {
+	C <-chan CandidateState
+
+	numVoters  int
+	signingKey []byte
+	stop       chan<- bool
+	done       <-chan bool
+	resp       voteResponse
+	c          chan<- CandidateState
+
+	votes map[string]*vote
+	state CandidateState
+	mu    sync.Mutex
+}
+
+type CandidateState string
+
+var (
+	StateLeader    CandidateState = "LEADER"
+	StateNotLeader CandidateState = "NOT_LEADER"
+)
+
+func NewCandidate(numVoters int, signingKey string) *Candidate {
+	stop := make(chan bool)
+	done := make(chan bool)
+	change := make(chan CandidateState, 100)
+
+	c := &Candidate{
+		C:          change,
+		numVoters:  numVoters,
+		signingKey: []byte(signingKey),
+		votes:      map[string]*vote{},
+		stop:       stop,
+		done:       done,
+		c:          change,
+		resp: voteResponse{
+			CandidateID: uniuri.New(),
+		},
+	}
+
+	go c.loop(stop, done)
+
+	return c
+}
+
+func (c *Candidate) Stop() {
+	close(c.stop)
+	<-c.done
+}
+
+func (c *Candidate) State() CandidateState {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	return c.state
+}
+
+func (c *Candidate) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(
+			w,
+			fmt.Sprintf("method %s not supported", r.Method),
+			http.StatusMethodNotAllowed,
+		)
+
+		return
+	}
+
+	sig := r.Header.Get("Signature")
+	if sig == "" {
+		http.Error(
+			w,
+			"missing Signature header",
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
+	if r.Header.Get("Content-Type") != "application/json" {
+		http.Error(
+			w,
+			fmt.Sprintf("Content-Type %s not supported", r.Header.Get("Content-Type")),
+			http.StatusUnsupportedMediaType,
+		)
+
+		return
+	}
+
+	js, err := io.ReadAll(r.Body)
+	if err != nil {
+		http.Error(
+			w,
+			fmt.Sprintf("can't read request body: %s", err),
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
+	if !hmac.Equal([]byte(sig), []byte(mac(js, c.signingKey))) {
+		http.Error(
+			w,
+			"Signature verification failed",
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
+	v := &vote{}
+
+	err = json.Unmarshal(js, v)
+	if err != nil {
+		http.Error(
+			w,
+			fmt.Sprintf("can't parse request body: %s", err),
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
+	enc := json.NewEncoder(w)
+
+	err = enc.Encode(c.resp)
+	if err != nil {
+		http.Error(
+			w,
+			fmt.Sprintf("can't write response: %s", err),
+			http.StatusInternalServerError,
+		)
+
+		return
+	}
+
+	c.vote(v)
+}
+
+func (c *Candidate) vote(v *vote) {
+	v.received = time.Now()
+
+	{
+		c.mu.Lock()
+		c.votes[v.VoterID] = v
+		c.mu.Unlock()
+	}
+
+	c.elect()
+}
+
+func (c *Candidate) voteIfNo(v *vote) {
+	if v.LastSeenCandidateID == c.resp.CandidateID {
+		return
+	}
+
+	c.vote(v)
+}
+
+func (c *Candidate) elect() {
+	no := 0
+	yes := 0
+
+	cutoff := time.Now().Add(-10 * time.Second)
+
+	c.mu.Lock() /////////////
+
+	for key, vote := range c.votes {
+		if vote.received.Before(cutoff) {
+			delete(c.votes, key)
+			continue
+		}
+
+		if vote.LastSeenCandidateID != c.resp.CandidateID {
+			no++
+		}
+
+		if vote.NumPollsSinceChange < 10 {
+			continue
+		}
+
+		yes++
+	}
+
+	c.mu.Unlock() ////////////
+
+	if no == 0 && yes > c.numVoters/2 {
+		c.update(StateLeader)
+	} else {
+		c.update(StateNotLeader)
+	}
+}
+
+func (c *Candidate) update(state CandidateState) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if c.state == state {
+		return
+	}
+
+	c.state = state
+	c.c <- state
+}
+
+func (c *Candidate) loop(stop <-chan bool, done chan<- bool) {
+	t := time.NewTicker(1 * time.Second)
+	defer t.Stop()
+	defer close(done)
+
+	for {
+		select {
+		case <-stop:
+			return
+
+		case <-t.C:
+			c.elect()
+		}
+	}
+}