Zookeeper is a distributed coordination service that provides naming, configuration, synchronization, and group services. It allows distributed processes to coordinate with each other through a shared hierarchical namespace of data registers called znodes. Zookeeper follows a leader-elected consensus protocol to guarantee atomic broadcast of state updates from the leader to followers. It uses a hierarchical namespace of znodes similar to a file system to store configuration data and other application-defined metadata. Zookeeper provides services like leader election, group membership, synchronization, and configuration management that are essential for distributed systems.

1. Apache Zookeeper
Anshul Patel

2. What is distributed system ?
• A distributed systems provides single coherent view of collection
of independent computing machines.
• Multiple software components on multiple computers, which run
as a single system.
• It can be locally or geographically connected.

3. Characteristics of Distributed System
• Concurrency
• Fault Tolerance
• Scalability
• Transparency
• Heterogeneity

4. Delusion of Distributed System
• Reliable Network
• Zero Latency
• Infinite Bandwidth
• Secure Network
• Topology Doesn’t change
• Homogeneous Network
• There is one administrator

5. Introducing Zookeeper
• Zookeeper provides coordination between distributed process via
shared hierarchical namespace.

6. Why Zookeeper ?
• Naming Service
• Locking
• Synchronization
• Configuration Management
• Leader Election

7. Zookeeper Architecture

8. Zookeeper Architecture
• Zookeeper follows client server architecture
• Collection of Zookeeper servers is called an ensemble
• At given time, a Zookeeper client is connected to one Zookeeper
server
• Client read requests are catered by connected Zookeeper server
• Client write requests are forwarded to Zookeeper leader.
• Write is considered successful, if quorum is achieved.

9. Zookeeper Architecture
• Client updates are sequentially consistent
• Updates are atomic
• Updates are reliable
• Timeliness

10. ZAB Protocol
• Zookeeper Atomic Broadcast
• Propagates state changes from leader to followers
• Guarantees the state change

11. Znodes
• Organized similar to standard Unix and Linux file system
• Similar to files and directories
• Both directory and leaf znode holds data
• Stores data <= 1MB

12. Znode - Hierarchical Namespace

13. Time in Zookeeper
• Zxid : Every change to ZK state receives a stamp in form of zxid
• Version Number : Change to Znode will cause change in Version
• Ticks : Ticks define timing of events
• Real Time : To put timestamp of stat structure and znode
modification

14. Znode Stat Structure
• Czxid : Change that caused Znode to be created
• Mzxid : Change that last modified Znode
• Pzxid : Change that last modified children of the Znode
• Ctime : Millisecond from epoch, when this Znode was created
• Mtime : Millisecond from epoch, when this Znode was last modified
• Version : Number of changes to data of Znode
• Cversion : Number of changes to children of Znode
• Aversion : Number of changes to ACL of Znode
• EphemeralOwner : SessionId of owner of this Znode
• DataLength : Length of data field
• Numchildren : Number of children

15. Zookeeper Operations
create Creates Znode
delete Deletes Znode
exists Does Znode exist?
getACL Get ACL permissions
setACL Set ACL permissions

16. Zookeeper Operations
getChildren Gets the childrens of Znode
getData Gets Data of Znode
setData Sets Data of Znode
Sync Syncs the client View

17. Types of Znodes
• Ephemeral Znode : Deleted with termination of client session,
also can’t have any child znode.
• Persistent Znode : Can have child znode, It is explicitly required
to delete it.
• Sequential Znode : Append a monotonicly increasing counter to
the end of path. It is unique to parent znode.

18. Znode Watchers
• One time triggers
• Sent to client which sets the watch
• Maintained locally at zookeeper
• Client won’t receive watches after getting disconnected
• Two lists of watches
– Data watches : create(),setData(), delete()
– Child watches : create(), delete()

19. Zookeeper ACL
• ACL Permission
– Create
– Read
– Write
– Delete
– Admin

20. Zookeeper ACL
• ACL Scheme
– World
– Auth
– Digest
– Host
– IP

21. Zookeeper Recipes/Higher Order
Functions/Apache Curator
• Naming Service
• Service Discovery
• Configuration Management
• Locks
• 2-phase commit
• Leader Election
• Barriers
• Queues

22. Netflix/Soabase Exhibitor
• Zookeeper server is fail fast
• Zookeeper is self-healing, (i.e. crashed/failed server if brought
online, will automatically join the cluster)
• Instance Monitoring
• Log Cleanup
• Backup/Restore
• Cluster-wide configuration
• Rolling Ensemble Change
• Rich REST API

23. Backup/Exhibitor
• Online Backup - Use Netflix Exhibitor
• Offline Backup - Bring down the old ensemble cluster and copy
the zookeeper data directory to the new ensemble cluster

24. Zookeeper Monitoring
• 4 Letter Words
• JMX

25. Zookeeper Monitoring - 4 Letter Words
conf srst
cons srvr
dump stat
envi dirs (3.5.1)
ruok mntr

26. Zookeeper Monitoring - JMX
• Mbeans
Quorum ServerCnxn
Leader LocalPeer
Follower LeaderElection
DataTree

27. Data Directory Cleanup
• autopurge.snapRetainCount : Maintains n number of recent
snapshots along with transaction log
• autopurge.purgeInterval : Time Interval at which purge event
will trigger

28. Zookeeper Future Releases - 3.5.X
• Dynamic Reconfiguration w/o Rolling restarts
• Separate Dynamic configuration file
• Adding/Removing Servers
• Modifying server parameters
• Rebalance Client Connections
• Backward Compatible
• Modifying Server Roles[Obersver/Participants]

29. Zookeeper in Real World
• Apache Kafka
• Apache Storm
• Apache HBase
• Apache Solr
• Apache Mesos
• Apache Hive

30. Questions ?
Thank You