From 0738c47887921f5ac225899994050a14c372ecb4 Mon Sep 17 00:00:00 2001
From: Ivan Babrou <ibobrik@gmail.com>
Date: Sun, 3 Apr 2016 00:03:14 +0100
Subject: [PATCH] Implement lazy state updates

---
 cmd/zoidberg/main.go |  7 ++++---
 explorer.go          | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/cmd/zoidberg/main.go b/cmd/zoidberg/main.go
index 8c99eb7..04edcac 100644
--- a/cmd/zoidberg/main.go
+++ b/cmd/zoidberg/main.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"log"
@@ -9,8 +10,6 @@ import (
 	"strings"
 	"time"
 
-	"errors"
-
 	"github.com/bobrik/zoidberg"
 	"github.com/bobrik/zoidberg/application"
 	"github.com/bobrik/zoidberg/balancer"
@@ -24,6 +23,8 @@ func main() {
 	bff := flag.String("balancer-finder", os.Getenv("BALANCER_FINDER"), "balancer finder")
 	aff := flag.String("application-finder", os.Getenv("APPLICATION_FINDER"), "application finder")
 	z := flag.String("zk", os.Getenv("ZK"), "zk connection in host:port,host:port/path format")
+	i := flag.Duration("interval", time.Second, "discovery interval")
+	l := flag.Duration("laziness", time.Minute, "time to skip balancer updates if there are no changes")
 
 	application.RegisterFlags()
 	balancer.RegisterFlags()
@@ -50,7 +51,7 @@ func main() {
 		log.Fatal(err)
 	}
 
-	e, err := zoidberg.NewExplorer(*n, af, bf, zc, zp)
+	e, err := zoidberg.NewExplorer(*n, af, bf, zc, zp, *i, *l)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/explorer.go b/explorer.go
index bd3fadf..590ea46 100644
--- a/explorer.go
+++ b/explorer.go
@@ -6,6 +6,7 @@ import (
 	"log"
 	"net/http"
 	"path"
+	"reflect"
 	"strings"
 	"sync"
 	"time"
@@ -24,13 +25,16 @@ type Explorer struct {
 	zookeeper *zk.Conn
 	zp        string
 	state     state.State
+	updated   map[string]update
+	interval  time.Duration
+	laziness  time.Duration
 	mutex     sync.Mutex
 }
 
 // NewExplorer creates a new Explorer instance with a name,
 // application and balancer finders and zookeeper connection
 // to persist versioning information
-func NewExplorer(name string, af application.Finder, bf balancer.Finder, zc *zk.Conn, zp string) (*Explorer, error) {
+func NewExplorer(name string, af application.Finder, bf balancer.Finder, zc *zk.Conn, zp string, interval, laziness time.Duration) (*Explorer, error) {
 	s := state.State{}
 
 	ss, _, err := zc.Get(zp)
@@ -54,6 +58,9 @@ func NewExplorer(name string, af application.Finder, bf balancer.Finder, zc *zk.
 		zookeeper: zc,
 		zp:        zp,
 		state:     s,
+		updated:   map[string]update{},
+		interval:  interval,
+		laziness:  laziness,
 		mutex:     sync.Mutex{},
 	}, nil
 }
@@ -62,7 +69,7 @@ func NewExplorer(name string, af application.Finder, bf balancer.Finder, zc *zk.
 // and updates load balancers' state
 func (e *Explorer) Run() error {
 	for {
-		time.Sleep(time.Second)
+		time.Sleep(e.interval)
 
 		d, err := e.discover()
 		if err != nil {
@@ -98,9 +105,19 @@ func (e *Explorer) updateBalancers(discovery *Discovery) {
 	state := e.getState()
 
 	wg := sync.WaitGroup{}
-	wg.Add(len(discovery.Balancers))
+
+	now := time.Now()
 
 	for _, b := range discovery.Balancers {
+		bs := b.String()
+		if reflect.DeepEqual(e.updated[bs].apps, discovery.Apps) {
+			if now.Sub(e.updated[bs].time) < e.laziness {
+				continue
+			}
+		}
+
+		wg.Add(1)
+
 		go func(b balancer.Balancer) {
 			defer wg.Done()
 
@@ -109,6 +126,11 @@ func (e *Explorer) updateBalancers(discovery *Discovery) {
 				log.Printf("error updating state on %s: %s\n", b, err)
 				return
 			}
+
+			e.updated[b.String()] = update{
+				time: now,
+				apps: discovery.Apps,
+			}
 		}(b)
 	}
 
@@ -238,3 +260,8 @@ func (e *Explorer) ServeMux() *http.ServeMux {
 
 	return mux
 }
+
+type update struct {
+	time time.Time
+	apps application.Apps
+}