diff --git a/scheduler/scheduler.go b/scheduler/scheduler.go
index 1f88f4b..47ad57b 100644
--- a/scheduler/scheduler.go
+++ b/scheduler/scheduler.go
@@ -4,6 +4,8 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/play-with-docker/play-with-docker/event"
@@ -30,6 +32,7 @@ type scheduledSession struct {
 	session *types.Session
 	cancel  context.CancelFunc
 	ticker  *time.Ticker
+	busy    int32
 }
 
 type scheduler struct {
@@ -119,7 +122,7 @@ func (s *scheduler) Start() {
 }
 
 func (s *scheduler) register(session *types.Session) *scheduledSession {
-	ss := &scheduledSession{session: session}
+	ss := &scheduledSession{session: session, busy: 0}
 	s.scheduledSessions[session.Id] = ss
 	return ss
 }
@@ -128,12 +131,17 @@ func (s *scheduler) cron(ctx context.Context, session *scheduledSession) {
 	for {
 		select {
 		case <-session.ticker.C:
-			if time.Now().After(session.session.ExpiresAt) {
-				// Session has expired. Need to close the session.
-				s.pwd.SessionClose(session.session)
-				return
+			if atomic.CompareAndSwapInt32(&session.busy, 0, 1) {
+				if time.Now().After(session.session.ExpiresAt) {
+					// Session has expired. Need to close the session.
+					s.pwd.SessionClose(session.session)
+					return
+				} else {
+					s.processSession(ctx, session.session)
+				}
+				atomic.StoreInt32(&session.busy, 0)
 			} else {
-				s.processSession(ctx, session.session)
+				log.Printf("Session [%s] is currently busy. Will try next time.\n", session.session.Id)
 			}
 		case <-ctx.Done():
 			return
@@ -153,15 +161,27 @@ func (s *scheduler) processSession(ctx context.Context, session *types.Session)
 		return
 	}
 
+	wg := sync.WaitGroup{}
+	wg.Add(len(updatedSession.Instances))
 	for _, ins := range updatedSession.Instances {
-		go s.processInstance(ctx, ins)
+		go func(ins *types.Instance) {
+			s.processInstance(ctx, ins)
+			wg.Done()
+		}(ins)
 	}
+	wg.Wait()
 }
 
 func (s *scheduler) processInstance(ctx context.Context, instance *types.Instance) {
+	wg := sync.WaitGroup{}
+	wg.Add(len(s.tasks))
 	for _, task := range s.tasks {
-		go task.Run(ctx, instance)
+		go func(task Task) {
+			task.Run(ctx, instance)
+			wg.Done()
+		}(task)
 	}
+	wg.Wait()
 }
 
 func (s *scheduler) Schedule(session *types.Session) error {