commit b53d4e4003ef0315ea5b0a2a31adcdb1433dd258
Author: Asim Aslam <asim@aslam.me>
Date:   Fri May 31 00:22:43 2019 +0100

    Add rcache => cache

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..d5a40b7
--- /dev/null
+++ b/README.md
@@ -0,0 +1,31 @@
+# Registry Cache
+
+Cache is a library that provides a caching layer for the go-micro [registry](https://godoc.org/github.com/micro/go-micro/registry#Registry).
+
+If you're looking for caching in your microservices use the [selector](https://micro.mu/docs/fault-tolerance.html#caching-discovery).
+
+## Interface
+
+```
+// Cache is the registry cache interface
+type Cache interface {
+	// embed the registry interface
+	registry.Registry
+	// stop the cache watcher
+	Stop()
+}
+```
+
+## Usage
+
+```
+import (
+	"github.com/micro/go-micro/registry"
+	"github.com/micro/go-micro/registry/cache"
+)
+
+r := registry.NewRegistry()
+cache := cache.New(r)
+
+services, _ := cache.GetService("my.service")
+```
diff --git a/options.go b/options.go
new file mode 100644
index 0000000..284721a
--- /dev/null
+++ b/options.go
@@ -0,0 +1,12 @@
+package cache
+
+import (
+	"time"
+)
+
+// WithTTL sets the cache TTL
+func WithTTL(t time.Duration) Option {
+	return func(o *Options) {
+		o.TTL = t
+	}
+}
diff --git a/rcache.go b/rcache.go
new file mode 100644
index 0000000..53b1157
--- /dev/null
+++ b/rcache.go
@@ -0,0 +1,429 @@
+// Package cache provides a registry cache
+package cache
+
+import (
+	"math"
+	"math/rand"
+	"sync"
+	"time"
+
+	log "github.com/micro/go-log"
+	"github.com/micro/go-micro/registry"
+)
+
+// Cache is the registry cache interface
+type Cache interface {
+	// embed the registry interface
+	registry.Registry
+	// stop the cache watcher
+	Stop()
+}
+
+type Options struct {
+	// TTL is the cache TTL
+	TTL time.Duration
+}
+
+type Option func(o *Options)
+
+type cache struct {
+	registry.Registry
+	opts Options
+
+	// registry cache
+	sync.RWMutex
+	cache   map[string][]*registry.Service
+	ttls    map[string]time.Time
+	watched map[string]bool
+
+	exit chan bool
+}
+
+var (
+	DefaultTTL = time.Minute
+)
+
+func backoff(attempts int) time.Duration {
+	if attempts == 0 {
+		return time.Duration(0)
+	}
+	return time.Duration(math.Pow(10, float64(attempts))) * time.Millisecond
+}
+
+// isValid checks if the service is valid
+func (c *cache) isValid(services []*registry.Service, ttl time.Time) bool {
+	// no services exist
+	if len(services) == 0 {
+		return false
+	}
+
+	// ttl is invalid
+	if ttl.IsZero() {
+		return false
+	}
+
+	// time since ttl is longer than timeout
+	if time.Since(ttl) > c.opts.TTL {
+		return false
+	}
+
+	// ok
+	return true
+}
+
+func (c *cache) quit() bool {
+	select {
+	case <-c.exit:
+		return true
+	default:
+		return false
+	}
+}
+
+// cp copies a service. Because we're caching handing back pointers would
+// create a race condition, so we do this instead its fast enough
+func (c *cache) cp(current []*registry.Service) []*registry.Service {
+	var services []*registry.Service
+
+	for _, service := range current {
+		// copy service
+		s := new(registry.Service)
+		*s = *service
+
+		// copy nodes
+		var nodes []*registry.Node
+		for _, node := range service.Nodes {
+			n := new(registry.Node)
+			*n = *node
+			nodes = append(nodes, n)
+		}
+		s.Nodes = nodes
+
+		// copy endpoints
+		var eps []*registry.Endpoint
+		for _, ep := range service.Endpoints {
+			e := new(registry.Endpoint)
+			*e = *ep
+			eps = append(eps, e)
+		}
+		s.Endpoints = eps
+
+		// append service
+		services = append(services, s)
+	}
+
+	return services
+}
+
+func (c *cache) del(service string) {
+	delete(c.cache, service)
+	delete(c.ttls, service)
+}
+
+func (c *cache) get(service string) ([]*registry.Service, error) {
+	// read lock
+	c.RLock()
+
+	// check the cache first
+	services := c.cache[service]
+	// get cache ttl
+	ttl := c.ttls[service]
+
+	// got services && within ttl so return cache
+	if c.isValid(services, ttl) {
+		// make a copy
+		cp := c.cp(services)
+		// unlock the read
+		c.RUnlock()
+		// return servics
+		return cp, nil
+	}
+
+	// get does the actual request for a service and cache it
+	get := func(service string) ([]*registry.Service, error) {
+		// ask the registry
+		services, err := c.Registry.GetService(service)
+		if err != nil {
+			return nil, err
+		}
+
+		// cache results
+		c.Lock()
+		c.set(service, c.cp(services))
+		c.Unlock()
+
+		return services, nil
+	}
+
+	// watch service if not watched
+	if _, ok := c.watched[service]; !ok {
+		go c.run(service)
+	}
+
+	// unlock the read lock
+	c.RUnlock()
+
+	// get and return services
+	return get(service)
+}
+
+func (c *cache) set(service string, services []*registry.Service) {
+	c.cache[service] = services
+	c.ttls[service] = time.Now().Add(c.opts.TTL)
+}
+
+func (c *cache) update(res *registry.Result) {
+	if res == nil || res.Service == nil {
+		return
+	}
+
+	c.Lock()
+	defer c.Unlock()
+
+	services, ok := c.cache[res.Service.Name]
+	if !ok {
+		// we're not going to cache anything
+		// unless there was already a lookup
+		return
+	}
+
+	if len(res.Service.Nodes) == 0 {
+		switch res.Action {
+		case "delete":
+			c.del(res.Service.Name)
+		}
+		return
+	}
+
+	// existing service found
+	var service *registry.Service
+	var index int
+	for i, s := range services {
+		if s.Version == res.Service.Version {
+			service = s
+			index = i
+		}
+	}
+
+	switch res.Action {
+	case "create", "update":
+		if service == nil {
+			c.set(res.Service.Name, append(services, res.Service))
+			return
+		}
+
+		// append old nodes to new service
+		for _, cur := range service.Nodes {
+			var seen bool
+			for _, node := range res.Service.Nodes {
+				if cur.Id == node.Id {
+					seen = true
+					break
+				}
+			}
+			if !seen {
+				res.Service.Nodes = append(res.Service.Nodes, cur)
+			}
+		}
+
+		services[index] = res.Service
+		c.set(res.Service.Name, services)
+	case "delete":
+		if service == nil {
+			return
+		}
+
+		var nodes []*registry.Node
+
+		// filter cur nodes to remove the dead one
+		for _, cur := range service.Nodes {
+			var seen bool
+			for _, del := range res.Service.Nodes {
+				if del.Id == cur.Id {
+					seen = true
+					break
+				}
+			}
+			if !seen {
+				nodes = append(nodes, cur)
+			}
+		}
+
+		// still got nodes, save and return
+		if len(nodes) > 0 {
+			service.Nodes = nodes
+			services[index] = service
+			c.set(service.Name, services)
+			return
+		}
+
+		// zero nodes left
+
+		// only have one thing to delete
+		// nuke the thing
+		if len(services) == 1 {
+			c.del(service.Name)
+			return
+		}
+
+		// still have more than 1 service
+		// check the version and keep what we know
+		var srvs []*registry.Service
+		for _, s := range services {
+			if s.Version != service.Version {
+				srvs = append(srvs, s)
+			}
+		}
+
+		// save
+		c.set(service.Name, srvs)
+	}
+}
+
+// run starts the cache watcher loop
+// it creates a new watcher if there's a problem
+func (c *cache) run(service string) {
+	// set watcher
+	c.Lock()
+	c.watched[service] = true
+	c.Unlock()
+
+	// delete watcher on exit
+	defer func() {
+		c.Lock()
+		delete(c.watched, service)
+		c.Unlock()
+	}()
+
+	var a, b int
+
+	for {
+		// exit early if already dead
+		if c.quit() {
+			return
+		}
+
+		// jitter before starting
+		j := rand.Int63n(100)
+		time.Sleep(time.Duration(j) * time.Millisecond)
+
+		// create new watcher
+		w, err := c.Registry.Watch(
+			registry.WatchService(service),
+		)
+
+		if err != nil {
+			if c.quit() {
+				return
+			}
+
+			d := backoff(a)
+
+			if a > 3 {
+				log.Log("rcache: ", err, " backing off ", d)
+				a = 0
+			}
+
+			time.Sleep(d)
+			a++
+
+			continue
+		}
+
+		// reset a
+		a = 0
+
+		// watch for events
+		if err := c.watch(w); err != nil {
+			if c.quit() {
+				return
+			}
+
+			d := backoff(b)
+
+			if b > 3 {
+				log.Log("rcache: ", err, " backing off ", d)
+				b = 0
+			}
+
+			time.Sleep(d)
+			b++
+
+			continue
+		}
+
+		// reset b
+		b = 0
+	}
+}
+
+// watch loops the next event and calls update
+// it returns if there's an error
+func (c *cache) watch(w registry.Watcher) error {
+	defer w.Stop()
+
+	// manage this loop
+	go func() {
+		// wait for exit
+		<-c.exit
+		w.Stop()
+	}()
+
+	for {
+		res, err := w.Next()
+		if err != nil {
+			return err
+		}
+		c.update(res)
+	}
+}
+
+func (c *cache) GetService(service string) ([]*registry.Service, error) {
+	// get the service
+	services, err := c.get(service)
+	if err != nil {
+		return nil, err
+	}
+
+	// if there's nothing return err
+	if len(services) == 0 {
+		return nil, registry.ErrNotFound
+	}
+
+	// return services
+	return services, nil
+}
+
+func (c *cache) Stop() {
+	select {
+	case <-c.exit:
+		return
+	default:
+		close(c.exit)
+	}
+}
+
+func (c *cache) String() string {
+	return "rcache"
+}
+
+// New returns a new cache
+func New(r registry.Registry, opts ...Option) Cache {
+	rand.Seed(time.Now().UnixNano())
+	options := Options{
+		TTL: DefaultTTL,
+	}
+
+	for _, o := range opts {
+		o(&options)
+	}
+
+	return &cache{
+		Registry: r,
+		opts:     options,
+		watched:  make(map[string]bool),
+		cache:    make(map[string][]*registry.Service),
+		ttls:     make(map[string]time.Time),
+		exit:     make(chan bool),
+	}
+}