Add `pkg/discovery` for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

hack/vendor.sh

@@ -16,6 +16,7 @@ clone git github.com/kr/pty 5cf931ef8f
 clone git github.com/mattn/go-sqlite3 v1.1.0
 clone git github.com/microsoft/hcsshim 7f646aa6b26bcf90caee91e93cde4a80d0d8a83e
 clone git github.com/mistifyio/go-zfs v2.1.1
+clone git github.com/stretchr/testify 7c2b1e5640dcf2631213ca962d892bffa1e08860
 clone git github.com/tchap/go-patricia v2.1.0
 clone git golang.org/x/net 3cffabab72adf04f8e3b01c5baf775361837b5fe https://github.com/golang/net.git
 

pkg/discovery/README.md

@@ -0,0 +1,41 @@
+---
+page_title: Docker discovery
+page_description: discovery
+page_keywords: docker, clustering, discovery
+---
+
+# Discovery
+
+Docker comes with multiple Discovery backends.
+
+## Backends
+
+### Using etcd
+
+Point your Docker Engine instances to a common etcd instance. You can specify
+the address Docker uses to advertise the node using the `--discovery-address`
+flag.
+
+```bash
+$ docker daemon -H=<node_ip:2376> --discovery-address=<node_ip:2376> --discovery-backend etcd://<etcd_ip>/<path>
+```
+
+### Using consul
+
+Point your Docker Engine instances to a common Consul instance. You can specify
+the address Docker uses to advertise the node using the `--discovery-address`
+flag.
+
+```bash
+$ docker daemon -H=<node_ip:2376> --discovery-address=<node_ip:2376> --discovery-backend consul://<consul_ip>/<path>
+```
+
+### Using zookeeper
+
+Point your Docker Engine instances to a common Zookeeper instance. You can specify
+the address Docker uses to advertise the node using the `--discovery-address`
+flag.
+
+```bash
+$ docker daemon -H=<node_ip:2376> --discovery-address=<node_ip:2376> --discovery-backend zk://<zk_addr1>,<zk_addr2>>/<path>
+```

pkg/discovery/backends.go

@@ -0,0 +1,53 @@
+package discovery
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+var (
+	// Backends is a global map of discovery backends indexed by their
+	// associated scheme.
+	backends map[string]Backend
+)
+
+func init() {
+	backends = make(map[string]Backend)
+}
+
+// Register makes a discovery backend available by the provided scheme.
+// If Register is called twice with the same scheme an error is returned.
+func Register(scheme string, d Backend) error {
+	if _, exists := backends[scheme]; exists {
+		return fmt.Errorf("scheme already registered %s", scheme)
+	}
+	log.WithField("name", scheme).Debug("Registering discovery service")
+	backends[scheme] = d
+	return nil
+}
+
+func parse(rawurl string) (string, string) {
+	parts := strings.SplitN(rawurl, "://", 2)
+
+	// nodes:port,node2:port => nodes://node1:port,node2:port
+	if len(parts) == 1 {
+		return "nodes", parts[0]
+	}
+	return parts[0], parts[1]
+}
+
+// New returns a new Discovery given a URL, heartbeat and ttl settings.
+// Returns an error if the URL scheme is not supported.
+func New(rawurl string, heartbeat time.Duration, ttl time.Duration) (Backend, error) {
+	scheme, uri := parse(rawurl)
+	if backend, exists := backends[scheme]; exists {
+		log.WithFields(log.Fields{"name": scheme, "uri": uri}).Debug("Initializing discovery service")
+		err := backend.Initialize(uri, heartbeat, ttl)
+		return backend, err
+	}
+
+	return nil, ErrNotSupported
+}

pkg/discovery/discovery.go

@@ -0,0 +1,35 @@
+package discovery
+
+import (
+	"errors"
+	"time"
+)
+
+var (
+	// ErrNotSupported is returned when a discovery service is not supported.
+	ErrNotSupported = errors.New("discovery service not supported")
+
+	// ErrNotImplemented is returned when discovery feature is not implemented
+	// by discovery backend.
+	ErrNotImplemented = errors.New("not implemented in this discovery service")
+)
+
+// Watcher provides watching over a cluster for nodes joining and leaving.
+type Watcher interface {
+	// Watch the discovery for entry changes.
+	// Returns a channel that will receive changes or an error.
+	// Providing a non-nil stopCh can be used to stop watching.
+	Watch(stopCh <-chan struct{}) (<-chan Entries, <-chan error)
+}
+
+// Backend is implemented by discovery backends which manage cluster entries.
+type Backend interface {
+	// Watcher must be provided by every backend.
+	Watcher
+
+	// Initialize the discovery with URIs, a heartbeat and a ttl.
+	Initialize(string, time.Duration, time.Duration) error
+
+	// Register to the discovery.
+	Register(string) error
+}

pkg/discovery/discovery_test.go

@@ -0,0 +1,120 @@
+package discovery
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewEntry(t *testing.T) {
+	entry, err := NewEntry("127.0.0.1:2375")
+	assert.NoError(t, err)
+	assert.True(t, entry.Equals(&Entry{Host: "127.0.0.1", Port: "2375"}))
+	assert.Equal(t, entry.String(), "127.0.0.1:2375")
+
+	_, err = NewEntry("127.0.0.1")
+	assert.Error(t, err)
+}
+
+func TestParse(t *testing.T) {
+	scheme, uri := parse("127.0.0.1:2375")
+	assert.Equal(t, scheme, "nodes")
+	assert.Equal(t, uri, "127.0.0.1:2375")
+
+	scheme, uri = parse("localhost:2375")
+	assert.Equal(t, scheme, "nodes")
+	assert.Equal(t, uri, "localhost:2375")
+
+	scheme, uri = parse("scheme://127.0.0.1:2375")
+	assert.Equal(t, scheme, "scheme")
+	assert.Equal(t, uri, "127.0.0.1:2375")
+
+	scheme, uri = parse("scheme://localhost:2375")
+	assert.Equal(t, scheme, "scheme")
+	assert.Equal(t, uri, "localhost:2375")
+
+	scheme, uri = parse("")
+	assert.Equal(t, scheme, "nodes")
+	assert.Equal(t, uri, "")
+}
+
+func TestCreateEntries(t *testing.T) {
+	entries, err := CreateEntries(nil)
+	assert.Equal(t, entries, Entries{})
+	assert.NoError(t, err)
+
+	entries, err = CreateEntries([]string{"127.0.0.1:2375", "127.0.0.2:2375", ""})
+	assert.NoError(t, err)
+	expected := Entries{
+		&Entry{Host: "127.0.0.1", Port: "2375"},
+		&Entry{Host: "127.0.0.2", Port: "2375"},
+	}
+	assert.True(t, entries.Equals(expected))
+
+	_, err = CreateEntries([]string{"127.0.0.1", "127.0.0.2"})
+	assert.Error(t, err)
+}
+
+func TestContainsEntry(t *testing.T) {
+	entries, err := CreateEntries([]string{"127.0.0.1:2375", "127.0.0.2:2375", ""})
+	assert.NoError(t, err)
+	assert.True(t, entries.Contains(&Entry{Host: "127.0.0.1", Port: "2375"}))
+	assert.False(t, entries.Contains(&Entry{Host: "127.0.0.3", Port: "2375"}))
+}
+
+func TestEntriesEquality(t *testing.T) {
+	entries := Entries{
+		&Entry{Host: "127.0.0.1", Port: "2375"},
+		&Entry{Host: "127.0.0.2", Port: "2375"},
+	}
+
+	// Same
+	assert.True(t, entries.Equals(Entries{
+		&Entry{Host: "127.0.0.1", Port: "2375"},
+		&Entry{Host: "127.0.0.2", Port: "2375"},
+	}))
+
+	// Different size
+	assert.False(t, entries.Equals(Entries{
+		&Entry{Host: "127.0.0.1", Port: "2375"},
+		&Entry{Host: "127.0.0.2", Port: "2375"},
+		&Entry{Host: "127.0.0.3", Port: "2375"},
+	}))
+
+	// Different content
+	assert.False(t, entries.Equals(Entries{
+		&Entry{Host: "127.0.0.1", Port: "2375"},
+		&Entry{Host: "127.0.0.42", Port: "2375"},
+	}))
+}
+
+func TestEntriesDiff(t *testing.T) {
+	entry1 := &Entry{Host: "1.1.1.1", Port: "1111"}
+	entry2 := &Entry{Host: "2.2.2.2", Port: "2222"}
+	entry3 := &Entry{Host: "3.3.3.3", Port: "3333"}
+	entries := Entries{entry1, entry2}
+
+	// No diff
+	added, removed := entries.Diff(Entries{entry2, entry1})
+	assert.Empty(t, added)
+	assert.Empty(t, removed)
+
+	// Add
+	added, removed = entries.Diff(Entries{entry2, entry3, entry1})
+	assert.Len(t, added, 1)
+	assert.True(t, added.Contains(entry3))
+	assert.Empty(t, removed)
+
+	// Remove
+	added, removed = entries.Diff(Entries{entry2})
+	assert.Empty(t, added)
+	assert.Len(t, removed, 1)
+	assert.True(t, removed.Contains(entry1))
+
+	// Add and remove
+	added, removed = entries.Diff(Entries{entry1, entry3})
+	assert.Len(t, added, 1)
+	assert.True(t, added.Contains(entry3))
+	assert.Len(t, removed, 1)
+	assert.True(t, removed.Contains(entry2))
+}

pkg/discovery/entry.go

@@ -0,0 +1,97 @@
+package discovery
+
+import (
+	"fmt"
+	"net"
+)
+
+// NewEntry creates a new entry.
+func NewEntry(url string) (*Entry, error) {
+	host, port, err := net.SplitHostPort(url)
+	if err != nil {
+		return nil, err
+	}
+	return &Entry{host, port}, nil
+}
+
+// An Entry represents a host.
+type Entry struct {
+	Host string
+	Port string
+}
+
+// Equals returns true if cmp contains the same data.
+func (e *Entry) Equals(cmp *Entry) bool {
+	return e.Host == cmp.Host && e.Port == cmp.Port
+}
+
+// String returns the string form of an entry.
+func (e *Entry) String() string {
+	return fmt.Sprintf("%s:%s", e.Host, e.Port)
+}
+
+// Entries is a list of *Entry with some helpers.
+type Entries []*Entry
+
+// Equals returns true if cmp contains the same data.
+func (e Entries) Equals(cmp Entries) bool {
+	// Check if the file has really changed.
+	if len(e) != len(cmp) {
+		return false
+	}
+	for i := range e {
+		if !e[i].Equals(cmp[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// Contains returns true if the Entries contain a given Entry.
+func (e Entries) Contains(entry *Entry) bool {
+	for _, curr := range e {
+		if curr.Equals(entry) {
+			return true
+		}
+	}
+	return false
+}
+
+// Diff compares two entries and returns the added and removed entries.
+func (e Entries) Diff(cmp Entries) (Entries, Entries) {
+	added := Entries{}
+	for _, entry := range cmp {
+		if !e.Contains(entry) {
+			added = append(added, entry)
+		}
+	}
+
+	removed := Entries{}
+	for _, entry := range e {
+		if !cmp.Contains(entry) {
+			removed = append(removed, entry)
+		}
+	}
+
+	return added, removed
+}
+
+// CreateEntries returns an array of entries based on the given addresses.
+func CreateEntries(addrs []string) (Entries, error) {
+	entries := Entries{}
+	if addrs == nil {
+		return entries, nil
+	}
+
+	for _, addr := range addrs {
+		if len(addr) == 0 {
+			continue
+		}
+		entry, err := NewEntry(addr)
+		if err != nil {
+			return nil, err
+		}
+		entries = append(entries, entry)
+	}
+	return entries, nil
+}

pkg/discovery/file/file.go

@@ -0,0 +1,109 @@
+package file
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"time"
+
+	"github.com/docker/docker/pkg/discovery"
+)
+
+// Discovery is exported
+type Discovery struct {
+	heartbeat time.Duration
+	path      string
+}
+
+func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
+	discovery.Register("file", &Discovery{})
+}
+
+// Initialize is exported
+func (s *Discovery) Initialize(path string, heartbeat time.Duration, ttl time.Duration) error {
+	s.path = path
+	s.heartbeat = heartbeat
+	return nil
+}
+
+func parseFileContent(content []byte) []string {
+	var result []string
+	for _, line := range strings.Split(strings.TrimSpace(string(content)), "\n") {
+		line = strings.TrimSpace(line)
+		// Ignoring line starts with #
+		if strings.HasPrefix(line, "#") {
+			continue
+		}
+		// Inlined # comment also ignored.
+		if strings.Contains(line, "#") {
+			line = line[0:strings.Index(line, "#")]
+			// Trim additional spaces caused by above stripping.
+			line = strings.TrimSpace(line)
+		}
+		for _, ip := range discovery.Generate(line) {
+			result = append(result, ip)
+		}
+	}
+	return result
+}
+
+func (s *Discovery) fetch() (discovery.Entries, error) {
+	fileContent, err := ioutil.ReadFile(s.path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read '%s': %v", s.path, err)
+	}
+	return discovery.CreateEntries(parseFileContent(fileContent))
+}
+
+// Watch is exported
+func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-chan error) {
+	ch := make(chan discovery.Entries)
+	errCh := make(chan error)
+	ticker := time.NewTicker(s.heartbeat)
+
+	go func() {
+		defer close(errCh)
+		defer close(ch)
+
+		// Send the initial entries if available.
+		currentEntries, err := s.fetch()
+		if err != nil {
+			errCh <- err
+		} else {
+			ch <- currentEntries
+		}
+
+		// Periodically send updates.
+		for {
+			select {
+			case <-ticker.C:
+				newEntries, err := s.fetch()
+				if err != nil {
+					errCh <- err
+					continue
+				}
+
+				// Check if the file has really changed.
+				if !newEntries.Equals(currentEntries) {
+					ch <- newEntries
+				}
+				currentEntries = newEntries
+			case <-stopCh:
+				ticker.Stop()
+				return
+			}
+		}
+	}()
+
+	return ch, errCh
+}
+
+// Register is exported
+func (s *Discovery) Register(addr string) error {
+	return discovery.ErrNotImplemented
+}

pkg/discovery/file/file_test.go

@@ -0,0 +1,106 @@
+package file
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/docker/docker/pkg/discovery"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInitialize(t *testing.T) {
+	d := &Discovery{}
+	d.Initialize("/path/to/file", 1000, 0)
+	assert.Equal(t, d.path, "/path/to/file")
+}
+
+func TestNew(t *testing.T) {
+	d, err := discovery.New("file:///path/to/file", 0, 0)
+	assert.NoError(t, err)
+	assert.Equal(t, d.(*Discovery).path, "/path/to/file")
+}
+
+func TestContent(t *testing.T) {
+	data := `
+1.1.1.[1:2]:1111
+2.2.2.[2:4]:2222
+`
+	ips := parseFileContent([]byte(data))
+	assert.Len(t, ips, 5)
+	assert.Equal(t, ips[0], "1.1.1.1:1111")
+	assert.Equal(t, ips[1], "1.1.1.2:1111")
+	assert.Equal(t, ips[2], "2.2.2.2:2222")
+	assert.Equal(t, ips[3], "2.2.2.3:2222")
+	assert.Equal(t, ips[4], "2.2.2.4:2222")
+}
+
+func TestRegister(t *testing.T) {
+	discovery := &Discovery{path: "/path/to/file"}
+	assert.Error(t, discovery.Register("0.0.0.0"))
+}
+
+func TestParsingContentsWithComments(t *testing.T) {
+	data := `
+### test ###
+1.1.1.1:1111 # inline comment
+# 2.2.2.2:2222
+      ### empty line with comment
+    3.3.3.3:3333
+### test ###
+`
+	ips := parseFileContent([]byte(data))
+	assert.Len(t, ips, 2)
+	assert.Equal(t, "1.1.1.1:1111", ips[0])
+	assert.Equal(t, "3.3.3.3:3333", ips[1])
+}
+
+func TestWatch(t *testing.T) {
+	data := `
+1.1.1.1:1111
+2.2.2.2:2222
+`
+	expected := discovery.Entries{
+		&discovery.Entry{Host: "1.1.1.1", Port: "1111"},
+		&discovery.Entry{Host: "2.2.2.2", Port: "2222"},
+	}
+
+	// Create a temporary file and remove it.
+	tmp, err := ioutil.TempFile(os.TempDir(), "discovery-file-test")
+	assert.NoError(t, err)
+	assert.NoError(t, tmp.Close())
+	assert.NoError(t, os.Remove(tmp.Name()))
+
+	// Set up file discovery.
+	d := &Discovery{}
+	d.Initialize(tmp.Name(), 1000, 0)
+	stopCh := make(chan struct{})
+	ch, errCh := d.Watch(stopCh)
+
+	// Make sure it fires errors since the file doesn't exist.
+	assert.Error(t, <-errCh)
+	// We have to drain the error channel otherwise Watch will get stuck.
+	go func() {
+		for range errCh {
+		}
+	}()
+
+	// Write the file and make sure we get the expected value back.
+	assert.NoError(t, ioutil.WriteFile(tmp.Name(), []byte(data), 0600))
+	assert.Equal(t, expected, <-ch)
+
+	// Add a new entry and look it up.
+	expected = append(expected, &discovery.Entry{Host: "3.3.3.3", Port: "3333"})
+	f, err := os.OpenFile(tmp.Name(), os.O_APPEND|os.O_WRONLY, 0600)
+	assert.NoError(t, err)
+	assert.NotNil(t, f)
+	_, err = f.WriteString("\n3.3.3.3:3333\n")
+	assert.NoError(t, err)
+	f.Close()
+	assert.Equal(t, expected, <-ch)
+
+	// Stop and make sure it closes all channels.
+	close(stopCh)
+	assert.Nil(t, <-ch)
+	assert.Nil(t, <-errCh)
+}

pkg/discovery/generator.go

@@ -0,0 +1,35 @@
+package discovery
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+)
+
+// Generate takes care of IP generation
+func Generate(pattern string) []string {
+	re, _ := regexp.Compile(`\[(.+):(.+)\]`)
+	submatch := re.FindStringSubmatch(pattern)
+	if submatch == nil {
+		return []string{pattern}
+	}
+
+	from, err := strconv.Atoi(submatch[1])
+	if err != nil {
+		return []string{pattern}
+	}
+	to, err := strconv.Atoi(submatch[2])
+	if err != nil {
+		return []string{pattern}
+	}
+
+	template := re.ReplaceAllString(pattern, "%d")
+
+	var result []string
+	for val := from; val <= to; val++ {
+		entry := fmt.Sprintf(template, val)
+		result = append(result, entry)
+	}
+
+	return result
+}

pkg/discovery/generator_test.go

@@ -0,0 +1,55 @@
+package discovery
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGeneratorNotGenerate(t *testing.T) {
+	ips := Generate("127.0.0.1")
+	assert.Equal(t, len(ips), 1)
+	assert.Equal(t, ips[0], "127.0.0.1")
+}
+
+func TestGeneratorWithPortNotGenerate(t *testing.T) {
+	ips := Generate("127.0.0.1:8080")
+	assert.Equal(t, len(ips), 1)
+	assert.Equal(t, ips[0], "127.0.0.1:8080")
+}
+
+func TestGeneratorMatchFailedNotGenerate(t *testing.T) {
+	ips := Generate("127.0.0.[1]")
+	assert.Equal(t, len(ips), 1)
+	assert.Equal(t, ips[0], "127.0.0.[1]")
+}
+
+func TestGeneratorWithPort(t *testing.T) {
+	ips := Generate("127.0.0.[1:11]:2375")
+	assert.Equal(t, len(ips), 11)
+	assert.Equal(t, ips[0], "127.0.0.1:2375")
+	assert.Equal(t, ips[1], "127.0.0.2:2375")
+	assert.Equal(t, ips[2], "127.0.0.3:2375")
+	assert.Equal(t, ips[3], "127.0.0.4:2375")
+	assert.Equal(t, ips[4], "127.0.0.5:2375")
+	assert.Equal(t, ips[5], "127.0.0.6:2375")
+	assert.Equal(t, ips[6], "127.0.0.7:2375")
+	assert.Equal(t, ips[7], "127.0.0.8:2375")
+	assert.Equal(t, ips[8], "127.0.0.9:2375")
+	assert.Equal(t, ips[9], "127.0.0.10:2375")
+	assert.Equal(t, ips[10], "127.0.0.11:2375")
+}
+
+func TestGenerateWithMalformedInputAtRangeStart(t *testing.T) {
+	malformedInput := "127.0.0.[x:11]:2375"
+	ips := Generate(malformedInput)
+	assert.Equal(t, len(ips), 1)
+	assert.Equal(t, ips[0], malformedInput)
+}
+
+func TestGenerateWithMalformedInputAtRangeEnd(t *testing.T) {
+	malformedInput := "127.0.0.[1:x]:2375"
+	ips := Generate(malformedInput)
+	assert.Equal(t, len(ips), 1)
+	assert.Equal(t, ips[0], malformedInput)
+}

pkg/discovery/kv/kv.go

@@ -0,0 +1,148 @@
+package kv
+
+import (
+	"fmt"
+	"path"
+	"strings"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/docker/docker/pkg/discovery"
+	"github.com/docker/libkv"
+	"github.com/docker/libkv/store"
+	"github.com/docker/libkv/store/consul"
+	"github.com/docker/libkv/store/etcd"
+	"github.com/docker/libkv/store/zookeeper"
+)
+
+const (
+	discoveryPath = "docker/nodes"
+)
+
+// Discovery is exported
+type Discovery struct {
+	backend   store.Backend
+	store     store.Store
+	heartbeat time.Duration
+	ttl       time.Duration
+	prefix    string
+	path      string
+}
+
+func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
+	// Register to libkv
+	zookeeper.Register()
+	consul.Register()
+	etcd.Register()
+
+	// Register to internal discovery service
+	discovery.Register("zk", &Discovery{backend: store.ZK})
+	discovery.Register("consul", &Discovery{backend: store.CONSUL})
+	discovery.Register("etcd", &Discovery{backend: store.ETCD})
+}
+
+// Initialize is exported
+func (s *Discovery) Initialize(uris string, heartbeat time.Duration, ttl time.Duration) error {
+	var (
+		parts = strings.SplitN(uris, "/", 2)
+		addrs = strings.Split(parts[0], ",")
+		err   error
+	)
+
+	// A custom prefix to the path can be optionally used.
+	if len(parts) == 2 {
+		s.prefix = parts[1]
+	}
+
+	s.heartbeat = heartbeat
+	s.ttl = ttl
+	s.path = path.Join(s.prefix, discoveryPath)
+
+	// Creates a new store, will ignore options given
+	// if not supported by the chosen store
+	s.store, err = libkv.NewStore(s.backend, addrs, nil)
+	return err
+}
+
+// Watch the store until either there's a store error or we receive a stop request.
+// Returns false if we shouldn't attempt watching the store anymore (stop request received).
+func (s *Discovery) watchOnce(stopCh <-chan struct{}, watchCh <-chan []*store.KVPair, discoveryCh chan discovery.Entries, errCh chan error) bool {
+	for {
+		select {
+		case pairs := <-watchCh:
+			if pairs == nil {
+				return true
+			}
+
+			log.WithField("discovery", s.backend).Debugf("Watch triggered with %d nodes", len(pairs))
+
+			// Convert `KVPair` into `discovery.Entry`.
+			addrs := make([]string, len(pairs))
+			for _, pair := range pairs {
+				addrs = append(addrs, string(pair.Value))
+			}
+
+			entries, err := discovery.CreateEntries(addrs)
+			if err != nil {
+				errCh <- err
+			} else {
+				discoveryCh <- entries
+			}
+		case <-stopCh:
+			// We were requested to stop watching.
+			return false
+		}
+	}
+}
+
+// Watch is exported
+func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-chan error) {
+	ch := make(chan discovery.Entries)
+	errCh := make(chan error)
+
+	go func() {
+		defer close(ch)
+		defer close(errCh)
+
+		// Forever: Create a store watch, watch until we get an error and then try again.
+		// Will only stop if we receive a stopCh request.
+		for {
+			// Set up a watch.
+			watchCh, err := s.store.WatchTree(s.path, stopCh)
+			if err != nil {
+				errCh <- err
+			} else {
+				if !s.watchOnce(stopCh, watchCh, ch, errCh) {
+					return
+				}
+			}
+
+			// If we get here it means the store watch channel was closed. This
+			// is unexpected so let's retry later.
+			errCh <- fmt.Errorf("Unexpected watch error")
+			time.Sleep(s.heartbeat)
+		}
+	}()
+	return ch, errCh
+}
+
+// Register is exported
+func (s *Discovery) Register(addr string) error {
+	opts := &store.WriteOptions{TTL: s.ttl}
+	return s.store.Put(path.Join(s.path, addr), []byte(addr), opts)
+}
+
+// Store returns the underlying store used by KV discovery.
+func (s *Discovery) Store() store.Store {
+	return s.store
+}
+
+// Prefix returns the store prefix
+func (s *Discovery) Prefix() string {
+	return s.prefix
+}

pkg/discovery/kv/kv_test.go

@@ -0,0 +1,119 @@
+package kv
+
+import (
+	"errors"
+	"path"
+	"testing"
+	"time"
+
+	"github.com/docker/docker/pkg/discovery"
+	"github.com/docker/libkv/store"
+	libkvmock "github.com/docker/libkv/store/mock"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+)
+
+func TestInitialize(t *testing.T) {
+	storeMock, err := libkvmock.New([]string{"127.0.0.1"}, nil)
+	assert.NotNil(t, storeMock)
+	assert.NoError(t, err)
+
+	d := &Discovery{backend: store.CONSUL}
+	d.Initialize("127.0.0.1", 0, 0)
+	d.store = storeMock
+
+	s := d.store.(*libkvmock.Mock)
+	assert.Len(t, s.Endpoints, 1)
+	assert.Equal(t, s.Endpoints[0], "127.0.0.1")
+	assert.Equal(t, d.path, discoveryPath)
+
+	storeMock, err = libkvmock.New([]string{"127.0.0.1:1234"}, nil)
+	assert.NotNil(t, storeMock)
+	assert.NoError(t, err)
+
+	d = &Discovery{backend: store.CONSUL}
+	d.Initialize("127.0.0.1:1234/path", 0, 0)
+	d.store = storeMock
+
+	s = d.store.(*libkvmock.Mock)
+	assert.Len(t, s.Endpoints, 1)
+	assert.Equal(t, s.Endpoints[0], "127.0.0.1:1234")
+	assert.Equal(t, d.path, "path/"+discoveryPath)
+
+	storeMock, err = libkvmock.New([]string{"127.0.0.1:1234", "127.0.0.2:1234", "127.0.0.3:1234"}, nil)
+	assert.NotNil(t, storeMock)
+	assert.NoError(t, err)
+
+	d = &Discovery{backend: store.CONSUL}
+	d.Initialize("127.0.0.1:1234,127.0.0.2:1234,127.0.0.3:1234/path", 0, 0)
+	d.store = storeMock
+
+	s = d.store.(*libkvmock.Mock)
+	if assert.Len(t, s.Endpoints, 3) {
+		assert.Equal(t, s.Endpoints[0], "127.0.0.1:1234")
+		assert.Equal(t, s.Endpoints[1], "127.0.0.2:1234")
+		assert.Equal(t, s.Endpoints[2], "127.0.0.3:1234")
+	}
+	assert.Equal(t, d.path, "path/"+discoveryPath)
+}
+
+func TestWatch(t *testing.T) {
+	storeMock, err := libkvmock.New([]string{"127.0.0.1:1234"}, nil)
+	assert.NotNil(t, storeMock)
+	assert.NoError(t, err)
+
+	d := &Discovery{backend: store.CONSUL}
+	d.Initialize("127.0.0.1:1234/path", 0, 0)
+	d.store = storeMock
+
+	s := d.store.(*libkvmock.Mock)
+	mockCh := make(chan []*store.KVPair)
+
+	// The first watch will fail.
+	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, errors.New("test error")).Once()
+	// The second one will succeed.
+	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, nil).Once()
+	expected := discovery.Entries{
+		&discovery.Entry{Host: "1.1.1.1", Port: "1111"},
+		&discovery.Entry{Host: "2.2.2.2", Port: "2222"},
+	}
+	kvs := []*store.KVPair{
+		{Key: path.Join("path", discoveryPath, "1.1.1.1"), Value: []byte("1.1.1.1:1111")},
+		{Key: path.Join("path", discoveryPath, "2.2.2.2"), Value: []byte("2.2.2.2:2222")},
+	}
+
+	stopCh := make(chan struct{})
+	ch, errCh := d.Watch(stopCh)
+
+	// It should fire an error since the first WatchTree call failed.
+	assert.EqualError(t, <-errCh, "test error")
+	// We have to drain the error channel otherwise Watch will get stuck.
+	go func() {
+		for range errCh {
+		}
+	}()
+
+	// Push the entries into the store channel and make sure discovery emits.
+	mockCh <- kvs
+	assert.Equal(t, <-ch, expected)
+
+	// Add a new entry.
+	expected = append(expected, &discovery.Entry{Host: "3.3.3.3", Port: "3333"})
+	kvs = append(kvs, &store.KVPair{Key: path.Join("path", discoveryPath, "3.3.3.3"), Value: []byte("3.3.3.3:3333")})
+	mockCh <- kvs
+	assert.Equal(t, <-ch, expected)
+
+	// Make sure that if an error occurs it retries.
+	// This third call to WatchTree will be checked later by AssertExpectations.
+	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, nil)
+	close(mockCh)
+	// Give it enough time to call WatchTree.
+	time.Sleep(3)
+
+	// Stop and make sure it closes all channels.
+	close(stopCh)
+	assert.Nil(t, <-ch)
+	assert.Nil(t, <-errCh)
+
+	s.AssertExpectations(t)
+}

pkg/discovery/nodes/nodes.go

@@ -0,0 +1,54 @@
+package nodes
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/docker/docker/pkg/discovery"
+)
+
+// Discovery is exported
+type Discovery struct {
+	entries discovery.Entries
+}
+
+func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
+	discovery.Register("nodes", &Discovery{})
+}
+
+// Initialize is exported
+func (s *Discovery) Initialize(uris string, _ time.Duration, _ time.Duration) error {
+	for _, input := range strings.Split(uris, ",") {
+		for _, ip := range discovery.Generate(input) {
+			entry, err := discovery.NewEntry(ip)
+			if err != nil {
+				return fmt.Errorf("%s, please check you are using the correct discovery (missing token:// ?)", err.Error())
+			}
+			s.entries = append(s.entries, entry)
+		}
+	}
+
+	return nil
+}
+
+// Watch is exported
+func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-chan error) {
+	ch := make(chan discovery.Entries)
+	go func() {
+		defer close(ch)
+		ch <- s.entries
+		<-stopCh
+	}()
+	return ch, nil
+}
+
+// Register is exported
+func (s *Discovery) Register(addr string) error {
+	return discovery.ErrNotImplemented
+}

pkg/discovery/nodes/nodes_test.go

@@ -0,0 +1,43 @@
+package nodes
+
+import (
+	"testing"
+
+	"github.com/docker/docker/pkg/discovery"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInitialize(t *testing.T) {
+	d := &Discovery{}
+	d.Initialize("1.1.1.1:1111,2.2.2.2:2222", 0, 0)
+	assert.Equal(t, len(d.entries), 2)
+	assert.Equal(t, d.entries[0].String(), "1.1.1.1:1111")
+	assert.Equal(t, d.entries[1].String(), "2.2.2.2:2222")
+}
+
+func TestInitializeWithPattern(t *testing.T) {
+	d := &Discovery{}
+	d.Initialize("1.1.1.[1:2]:1111,2.2.2.[2:4]:2222", 0, 0)
+	assert.Equal(t, len(d.entries), 5)
+	assert.Equal(t, d.entries[0].String(), "1.1.1.1:1111")
+	assert.Equal(t, d.entries[1].String(), "1.1.1.2:1111")
+	assert.Equal(t, d.entries[2].String(), "2.2.2.2:2222")
+	assert.Equal(t, d.entries[3].String(), "2.2.2.3:2222")
+	assert.Equal(t, d.entries[4].String(), "2.2.2.4:2222")
+}
+
+func TestWatch(t *testing.T) {
+	d := &Discovery{}
+	d.Initialize("1.1.1.1:1111,2.2.2.2:2222", 0, 0)
+	expected := discovery.Entries{
+		&discovery.Entry{Host: "1.1.1.1", Port: "1111"},
+		&discovery.Entry{Host: "2.2.2.2", Port: "2222"},
+	}
+	ch, _ := d.Watch(nil)
+	assert.True(t, expected.Equals(<-ch))
+}
+
+func TestRegister(t *testing.T) {
+	d := &Discovery{}
+	assert.Error(t, d.Register("0.0.0.0"))
+}