keys are owned by successor

Despite being certain I was doing it wrong, the behavior I had
implemented was to assign ownership of a key to the peer that knows the
key's successor, not to the successor of the key.

.gitignore

@@ -0,0 +1 @@
+/chord-node

chord/client.go

@@ -9,38 +9,48 @@ import (
 
 // Client represents the communications a Chord node performs.
 type Client interface {
-	// Find asks s to find a value and the peer that owns it.
-	// If the ID is not associated with a key, the result must be the empty
-	// string with a nil error.
-	Find(ctx context.Context, s Peer, id ID) (Peer, string, error)
-	// Set asks s to save a value for an ID.
-	Set(ctx context.Context, s Peer, id ID, v string) error
+	// FindSuccessor asks s to find the peer that most closely follows a key.
+	FindSuccessor(ctx context.Context, s Peer, id ID) (Peer, error)
 	// Notify tells s we believe n to be its predecessor.
 	Notify(ctx context.Context, n *Node, s Peer) error
 	// Neighbors requests a peer's beliefs about its own neighbors.
 	Neighbors(ctx context.Context, p Peer) (pred Peer, succ []Peer, err error)
+
+	// Get asks s for a saved value.
+	Get(ctx context.Context, s Peer, id ID) (string, error)
+	// Set asks s to save a value.
+	Set(ctx context.Context, s Peer, id ID, v string) error
 }
 
-// TODO(branden): Find should be plural; if we have multiple keys to
+// TODO(branden): FindSuccessor should be plural; if we have multiple keys to
 // search, we shouldn't have to do the whole query for all of them, especially
 // considering we can sort by increasing distance from the origin and then do
 // the query in linear time.
 
-// Find gets a value and the peer that owns it.
-func Find(ctx context.Context, cl Client, n *Node, id ID) (Peer, string, error) {
-	if n.IsLocal(id) {
-		return n.self, "", nil
+// FindSuccessor gets the peer that most closely follows a key.
+func Find(ctx context.Context, cl Client, n *Node, id ID) (Peer, error) {
+	p, ok := n.Closest(id)
+	if ok {
+		return p, nil
 	}
-	p, s, err := cl.Find(ctx, n.Closest(id), id)
-	return p, s, err
+	p, err := cl.FindSuccessor(ctx, p, id)
+	return p, err
 }
 
-// TODO(branden): Set should be plural for the same reasons. It should also
-// return an error if the key isn't local to the peer.
+// TODO(branden): Get and Set should be plural for the same reasons.
+
+// Get gets a value in the Chord network.
+func Get(ctx context.Context, cl Client, n *Node, key ID) (string, error) {
+	p, err := Find(ctx, cl, n, key)
+	if err != nil {
+		return "", err
+	}
+	return cl.Get(ctx, p, key)
+}
 
 // Set saves a value in the Chord network.
 func Set(ctx context.Context, cl Client, n *Node, key ID, val string) error {
-	p, _, err := Find(ctx, cl, n, key)
+	p, err := Find(ctx, cl, n, key)
 	if err != nil {
 		return fmt.Errorf("couldn't find peer to save key: %w", err)
 	}
@@ -57,7 +67,7 @@ func Join(ctx context.Context, cl Client, addr netip.AddrPort, np Peer) (*Node,
 		return nil, errors.New("chord: invalid peer")
 	}
 	self := Address(addr)
-	p, _, err := cl.Find(ctx, np, self.id)
+	p, err := cl.FindSuccessor(ctx, np, self.id)
 	if err != nil {
 		return nil, fmt.Errorf("couldn't query own successor: %w", err)
 	}

chord/httpnode/client.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"net/netip"
 	"net/url"
 	"path"
 	"strings"
@@ -21,12 +22,36 @@ type Client struct {
 	APIBase string
 }
 
-// Find asks s to find a value and the peer that owns it.
-func (cl *Client) Find(ctx context.Context, s chord.Peer, id chord.ID) (chord.Peer, string, error) {
+// FindSuccessor asks s to find a value and the peer that owns it.
+func (cl *Client) FindSuccessor(ctx context.Context, s chord.Peer, id chord.ID) (chord.Peer, error) {
 	_, addr := s.Values()
 	if !addr.IsValid() {
-		return chord.Peer{}, "", errors.New("Find with invalid peer")
+		return chord.Peer{}, errors.New("FindSuccessor with invalid peer")
 	}
+	url := url.URL{
+		Scheme: "http",
+		Host:   addr.String(),
+		Path:   path.Join("/", cl.APIBase, "find", id.String()),
+	}
+	req, err := http.NewRequestWithContext(ctx, "GET", url.String(), nil)
+	if err != nil {
+		return chord.Peer{}, err
+	}
+	slog.InfoContext(ctx, "find", slog.String("url", url.String()))
+	resp, err := cl.HTTP.Do(req)
+	if err != nil {
+		return chord.Peer{}, err
+	}
+	p, err := readResponse[netip.AddrPort](resp)
+	if err != nil {
+		return chord.Peer{}, fmt.Errorf("%w (%s)", err, resp.Status)
+	}
+	slog.InfoContext(ctx, "found", slog.String("peer", p.String()))
+	return chord.Address(p), nil
+}
+
+func (cl *Client) Get(ctx context.Context, s chord.Peer, id chord.ID) (string, error) {
+	_, addr := s.Values()
 	url := url.URL{
 		Scheme: "http",
 		Host:   addr.String(),
@@ -34,19 +59,21 @@ func (cl *Client) Find(ctx context.Context, s chord.Peer, id chord.ID) (chord.Pe
 	}
 	req, err := http.NewRequestWithContext(ctx, "GET", url.String(), nil)
 	if err != nil {
-		return chord.Peer{}, "", err
+		return "", nil
 	}
-	slog.InfoContext(ctx, "find", slog.String("url", url.String()))
 	resp, err := cl.HTTP.Do(req)
 	if err != nil {
-		return chord.Peer{}, "", err
+		return "", err
 	}
-	p, err := readResponse[peervalue](resp)
-	if err != nil {
-		return chord.Peer{}, "", fmt.Errorf("%w (%s)", err, resp.Status)
+	switch resp.StatusCode {
+	case http.StatusOK:
+		v, err := readResponse[string](resp)
+		return v, err
+	case http.StatusNotFound:
+		return "", nil
+	default:
+		return "", errors.New(resp.Status)
 	}
-	slog.InfoContext(ctx, "found", slog.String("peer", p.Peer.String()), slog.String("value", p.Value))
-	return chord.Address(p.Peer), p.Value, nil
 }
 
 func (cl *Client) Set(ctx context.Context, s chord.Peer, id chord.ID, v string) error {

chord/httpnode/httpnode.go

@@ -48,8 +48,3 @@ type neighbors struct {
 	Succ []netip.AddrPort `json:"succ"`
 	Pred netip.AddrPort   `json:"pred,omitzero"`
 }
-
-type peervalue struct {
-	Peer  netip.AddrPort `json:"peer"`
-	Value string         `json:"value,omitzero"`
-}

chord/httpnode/server.go

@@ -45,7 +45,8 @@ func New(l net.Listener, cl chord.Client, self *chord.Node) (*Node, error) {
 // Router creates a handler for the Chord HTTP endpoints.
 func (n *Node) Router() http.Handler {
 	m := http.NewServeMux()
-	m.HandleFunc("GET /key/{id}", n.key)
+	m.HandleFunc("GET /find/{id}", n.find)
+	m.HandleFunc("GET /key/{id}", n.get)
 	m.HandleFunc("POST /key/{id}", n.set)
 	m.HandleFunc("POST /pred", n.notify)
 	m.HandleFunc("GET /neighbors", n.neighbors)
@@ -66,7 +67,7 @@ func (n *Node) Check(ctx context.Context) error {
 	return nil
 }
 
-func (n *Node) key(w http.ResponseWriter, r *http.Request) {
+func (n *Node) find(w http.ResponseWriter, r *http.Request) {
 	s := r.PathValue("id")
 	slog.InfoContext(r.Context(), "received find", slog.String("id", s), slog.String("from", r.RemoteAddr))
 	id, err := chord.ParseID(s)
@@ -74,15 +75,29 @@ func (n *Node) key(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, err.Error())
 		return
 	}
-	p, v, err := chord.Find(r.Context(), n.client, n.self, id)
+	p, err := chord.Find(r.Context(), n.client, n.self, id)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
 	_, addr := p.Values()
-	slog.InfoContext(r.Context(), "tell found", slog.String("id", s), slog.String("addr", addr.String()), slog.String("value", v))
-	pv := peervalue{addr, v}
-	writeOk(w, pv)
+	slog.InfoContext(r.Context(), "tell found", slog.String("id", s), slog.String("addr", addr.String()))
+	writeOk(w, addr)
+}
+
+func (n *Node) get(w http.ResponseWriter, r *http.Request) {
+	s := r.PathValue("id")
+	id, err := chord.ParseID(s)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+	v, ok := n.self.GetLocal(id)
+	if !ok {
+		writeError(w, http.StatusNotFound, s+" not found")
+		return
+	}
+	writeOk(w, v)
 }
 
 func (n *Node) set(w http.ResponseWriter, r *http.Request) {
@@ -97,10 +112,7 @@ func (n *Node) set(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
-	if !n.self.SetLocal(id, string(val)) {
-		writeError(w, http.StatusNotFound, "id does not belong to this peer")
-		return
-	}
+	n.self.SetLocal(id, string(val))
 	w.WriteHeader(http.StatusNoContent)
 }
 

chord/topology.go

@@ -47,22 +47,21 @@ func (n *Node) Neighbors(s []Peer) (Peer, []Peer) {
 	return n.pred, append(s, n.succ...)
 }
 
-func (n *Node) localLocked(id ID) bool {
+func (n *Node) succOwnsLocked(id ID) bool {
 	return contains(n.self.id, n.succ[0].id, id)
 }
 
-// IsLocal reports whether this node owns the given key.
-func (n *Node) IsLocal(id ID) bool {
-	n.mu.Lock()
-	defer n.mu.Unlock()
-	return n.localLocked(id)
-}
-
 // Closest finds the locally known peer which is the closest predecessor of key.
-func (n *Node) Closest(id ID) Peer {
+// If the boolean value is true, then this node believes that the peer is the
+// closest predecessor in the network.
+func (n *Node) Closest(id ID) (Peer, bool) {
 	self := n.self.id
 	n.mu.Lock()
 	defer n.mu.Unlock()
+	// Check the immediate successor first to satisfy the contract.
+	if n.succOwnsLocked(id) {
+		return n.succ[0], true
+	}
 	l := n.fingers
 	for i := len(l) - 1; i >= 0; i-- {
 		f := l[i]
@@ -74,7 +73,7 @@ func (n *Node) Closest(id ID) Peer {
 			if id == f.id {
 				continue
 			}
-			return f
+			return f, false
 		}
 	}
 	// Also try successors.
@@ -85,32 +84,26 @@ func (n *Node) Closest(id ID) Peer {
 			if id == f.id {
 				continue
 			}
-			return f
+			return f, false
 		}
 	}
-	return n.self
+	// No known node is closer than we are.
+	return n.self, true
 }
 
-// GetLocal obtains the value for a key if it is local to and owned by the node.
+// GetLocal obtains the value for a key.
 func (n *Node) GetLocal(k ID) (v string, found bool) {
 	n.mu.Lock()
 	defer n.mu.Unlock()
-	if n.localLocked(k) {
-		v, found = n.data[k]
-	}
+	v, found = n.data[k]
 	return v, found
 }
 
 // SetLocal sets the value for a key.
-// Returns false if the key is not owned by the node.
-func (n *Node) SetLocal(k ID, v string) bool {
+func (n *Node) SetLocal(k ID, v string) {
 	n.mu.Lock()
 	defer n.mu.Unlock()
-	if n.localLocked(k) {
-		n.data[k] = v
-		return true
-	}
-	return false
+	n.data[k] = v
 }
 
 // SuccessorFailed marks the node's current successor as having failed, e.g.
@@ -175,6 +168,11 @@ func (p Peer) Values() (ID, netip.AddrPort) {
 	return p.id, p.addr
 }
 
+// String formats the peer's address for debugging.
+func (p Peer) String() string {
+	return p.addr.String()
+}
+
 // Create creates a new Chord network using the given address as the initial node.
 func Create(addr netip.AddrPort) (*Node, error) {
 	if !addr.IsValid() {

main.go

@@ -183,6 +183,8 @@ func cliJoin(ctx context.Context, cmd *cli.Command) error {
 				slog.ErrorContext(ctx, "stabilize", slog.Any("err", err))
 				node.SuccessorFailed()
 			}
+			pred, succ := node.Neighbors(nil)
+			slog.InfoContext(ctx, "neighbors", slog.Any("predecessor", pred), slog.Any("successors", succ))
 		}
 	}()
 	<-ctx.Done()

test.bash

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -ex
+
+go build -o ./chord-node
+
+# Test create and join.
+./chord-node join -ip 127.0.0.1:3000 &
+FIRST=$!
+sleep 3
+
+./chord-node join -ip 127.0.0.1:3001 -c 127.0.0.1:3000 &
+SECOND=$!
+./chord-node join -ip 127.0.0.1:3002 -c 127.0.0.1:3000 &
+THIRD=$!
+
+sleep 5
+# Each node logs its predecessor and successors. At this point, we see the ring.
+kill $FIRST $SECOND $THIRD