1. ZooKeeper: A Distributed Coordination Service for Distributed Applications

2. Motivation
Large-scale distributed application require different forms of coordination:
Configuration
Group membership and leader election
Synchronization
Configuration: a list of operational parameters for the system processes
Group membership: often processes need to know which other processes are alive and what those processes are in charge of

3. Related Works Amazon Simple Queue Service-queuing [25]-leader election
[27]-configuration
Chubby[6]-locking service with strong synchronization

4. Zookeeper
Zookeeper is a distributed service for distributed applications. It support:
Synchronization
Configuration management
Naming service

5. Why Do We Need Zookeeper
Zookeeper is simple
Zookeeper is replicated
Zookeeper is ordered
Zookeeper is fast

6. Data Model
Regular Znode
Ephemeral Znode
Sequential flag

7. Watches Zookeeper Client1 Client4 create/exist/(WATCH) NOTIFICATION
setData
Client1
Client4

8. Client API create(path, data, flags) delete(path, version)
exists(path, watch)
getData(path, watch)
setData(path, data, version)
getChildren(path, watch)
sync(path)

9. Zookeeper Service Architecture
Read request is handled by local server
Write request is sent to the leader, the leader broadcasts the change to the Zookeeper through Zab an atomic broadcast protocol.

10. Setup Zookeeper
Download:
Configure Zookeeper:
Standalone Mode
Replicated Mode
tickTime: the basic time unit in milliseconds used by ZooKeeper. It is used to do heartbeats and the minimum session timeout will be twice the tickTime.
dataDir: the location to store the in-memory database snapshots and, unless specified otherwise, the transaction log of updates to the database.
clientPort: the port to listen for client connections

11. Standalone Mode create file zoo.cfg with the content:
Start server: bin/zkServer.sh start
Test with zookeeper client: bin/zkCli.sh -server :2181
tickTime=2000 dataDir=/var/lib/zookeeper clientPort=2181

12. Standalone Mode (2)
ls /
get
Set …

13. Setup Zookeeper: Replicated mode
Every server has the same configuration file.
Create file named myid In the datadir directory. The content of myid file is an unique number.
tickTime=2000
dataDir=/home/sdn/zookeeper
clientPort=2181
initLimit=5
syncLimit=2
server.1= :2888:3888
server.2= :2888:3888 …
server.n= :2888:3888

14. Use cases Naming service Configuration management Synchronization
Message Queue
Notification system

15. Synchronization: Simple Lock
Client has smallest number have permission to access locked object
When the client finishes work with object, child node is deleted. Another client has smallest number have permission to access locked object
AppRoot
P(n)
P(n+1)
P(n+2)
P(n+3)
Zookeeper
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
Client1
Client4
Client2
Client3

16. Synchronization: Simple Lock
Check existing of approot and create it.
Create child node with Sequential and Ephemeral flag And receive a number.
When receive the notification, check whether the number is smallest of child nodes number

17. Synchronization: Barrier
public class SimpleLock implements Watcher
ZooKeeper zooKeeper = new ZooKeeper(" :2181", 3000, this);
Stat res = zooKeeper.exists(root, true);
if(res==null) String abc = zooKeeper.create(root, null, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
String createRes = zooKeeper.create(root+"/", null, Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
int number = Integer.parseInt(createRes.substring(root.length()+1));
while (true) {
synchronized (root) {
//TODO check whether the number is smallest
if(smallest) {dost();}else{root.wait();} }}
public void process(WatchedEvent event) { //Watcher event
synchronized (root) {root.notify();} }
private void dost(){
System.out.println("Access System.out at "+System.currentTimeMillis());
Thread.sleep(5000); }

18. Synchronization: Simple Lock
public static void main(String[] args){
new SimpleLock(); }
/simpleLock/ :9
Access System.out at
/simpleLock/ :10
Access System.out at
/simpleLock/ :11
Access System.out at
/simpleLock/ :12
Access System.out at

19. Synchronization: Barrier
Every client creates child node of approot.
Whenever the number of child nodes is enough, client will start work.
AppRoot
P(n)
P(n+1)
P(n+2)
P(n+3)
Zookeeper
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
N=create(P, EPHEMERAL|SEQUENTIAL)
Client1
Client4
Client2
Client3

20. Synchronization: Barrier
Check existing of approot and create it.
Create child node with Sequential and Ephemeral flag And receive a number.
When receive the notification, check whether the number is smallest of child nodes number

21. Synchronization: Barrier
public static void main(String[] args){
new Barrier(3).run();}
Run 3 instances:
/barrier/
Starting at
/barrier/
Starting at
/barrier/
Starting at

22. N=create(P, SEQUENTIAL) N=create(P, SEQUENTIAL)
Message Queue
Every client creates child node of approot.
Whenever the number of child nodes is enough, client will start work.
AppRoot
P(n)
P(n+1)
P(n+2)
P(n+3)
N=create(P, SEQUENTIAL)
sender1
get(smallestP)
Zookeeper
receiver
N=create(P, SEQUENTIAL)
sender2

23. Message Queue public class MessageQueue implements Watcher
public static class Sender extends MessageQueue implements Runnable
public static class Receiver extends MessageQueue implements Runnable
public void sendMessage() { //Watcher event
String sendMessage = "sendMessage at "+System.currentTimeMillis();
zooKeeper.create(root+"/", sendMessage.getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT_SEQUENTIAL); }
while (true) {
synchronized (root) {
List<String> childs = zooKeeper.getChildren(root, true);
if(childs.size()==0){root.wait();}else{
//TODO sort the list
for (String child : childs) {
byte[] data = zooKeeper.getData(root+"/"+child, false, new Stat());
System.out.println("readMessage:"+new String(data));
zooKeeper.delete(root+"/"+child, 0);
} } }}
public void process(WatchedEvent event) { //Watcher event
synchronized (root) {root.notify();} }

24. Message Queue: Run 2 sender instances and 1 receiver instance:
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at
readMessage:sendMessage at