The document provides an overview of Apache Zookeeper as a distributed coordination service, detailing its architecture, data model, and consistency guarantees. It distinguishes between embedded and external Zookeeper and outlines best practices for deployment and management in production environments. Important disclaimers regarding the reliability of the information and potential future product plans are also included.

1. © 2015 IBM Corporation
ZooKeeper And Embedded ZooKeeper
IBM InfoSphere Streams Version 4.0
Yip-Hing Ng
Senior Software Engineer
Streams Platform Team
yipng@us.ibm.com

2. 2 © 2015 IBM Corporation
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Agenda
 Apache ZooKeeper Overview
 ZooKeeper Architecture
 ZooKeeper Data Model
 ZooKeeper Consistency Guarantees
 Embedded ZooKeeper
 External ZooKeeper
 ZooKeeper Guidelines/Best Practices
 Questions

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Apache ZooKeeper Overview
 A highly scalable, open source, distributed coordination service for
distributed applications
 Key component and Prerequisite of Streams Version 4.0
– Requires v3.4.6 or above
 Apache Software Foundation
– Used in Apache Hadoop and HBase projects
 Provides a set of primitives to implement higher level constructs in a
distributed system such as:
– Configuration maintenance
– Synchronization
– Leader Election
– Groups and Naming services
– Work Queues
 High Availability
– Replication

5. 5 © 2015 IBM Corporation
ZooKeeper Architecture
ZooKeeper
(Follower)
Host A
Client
(Read)
Client
(Write)
Client
(Read)
ZooKeeper
(Leader)
Host B
Client
(Read)
Client
(Read)
Client
(Read)
ZooKeeper
(Follower)
Host C
ZooKeeper Ensemble

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ZooKeeper Data Model
 Hierarchical namespace (e.g.: similar to distributed file system)
 Each node called Znode can have its own data and child nodes
 Path is represented as canonical absolute path (no relative path)
 e.g.: /app1/p1
 Znode maintains a stat structure
 Version (conditional update)
 ACL
 Watcher for data change notification, single trigger
 Znode Types
 Persistent
 Exists until they are explicitly deleted
 Ephemeral
 gets deleted when session expires
 Not allowed to have children
 Sequential
 Can be persistent or ephemeral
 Monotonic sequence counter, helpful for synchronization, e.g.: /app2/p1-0000000001
/
/app1 /app2
/app1/p3/app1/p2/app1/p1 /app2/p1

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ZooKeeper Consistency Guarantees
 Sequential Consistency
 Updates are applied in the order they are received by ZooKeeper
 Atomicity
 All or nothing, no partial results
 Reliability
 Once an update has been applied, it will persist from that time forward until
overwritten by another update
 Timeliness
 Client view is guaranteed to be up-to-date within certain time-bound
 Single System Image
 Client sees the same view of the service regardless of the ZooKeeper server it
connects to

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Embedded ZooKeeper
 Managed by Streams (start, stop, etc.) to simplify Streams prerequiste
 Basic Domain creation by Domain Manager or via streamtool
 e.g.: streamtool mkdomain -d streamsdomain1 --embeddedzk
 Primarily use for a single node developer environment. It is not
recommended for a production environment.
 A Supervisor process/watchdog runs side by side with Embedded
ZooKeeper
 Can be manually started or stopped via streamtool (when no active domain)
 e.g.: To start it: streamtool embeddedzk --start
 e.g.: To stop it: streamtool embeddedzk --stop
 e.g.: To get its status: streamtool embeddedzk --status

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Embedded ZooKeeper (cont.)
 Embedded ZooKeeper configuration can be set via streamtool
 ZooKeeper server related config parameters are prepended with:
 streams.zookeeper.property.
 e.g.: To update its server port to 21810:
 streamtool setbootproperty streams.zookeeper.property.clientPort=21810
 Default Embedded ZooKeeper dataDir location
 $HOME/.streams/var/embeddedzk/datadir
 Default Embedded ZooKeeper and ZKMonitor log/trace file location
 $HOME/.streams/var/embeddedzk

10. 10 © 2015 IBM Corporation
Embedded ZooKeeper (cont.)
ZooKeeper
Controller
ZK Monitor
Single Host
Audit Log
SWS
JMX
AAS
SAM SRM
View
APP
SCH
HC

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External ZooKeeper
 Not Managed by Streams
 Standalone or Replicated Mode
 Specify STREAMS_ZKCONNECT env var or streamtool –zkconnect option
 e.g.: streamtool mkdomain -d streamsdomain2
--zkconnect zkserver1:2181,zkserver2:2181,zkserver3:2181
 Enterprise Domain, use for multi-users and hosts
 For reliability and high availability on a production environment, its
recommended to run as an ensemble of ZooKeeper servers.
 ZooKeeper Ensemble
 Writes
 All writes go through leader
 Global ordering (zxid)
 Reads
 In memory
 Follow-the-leader (can lag from leader – but eventual consistency)

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External ZooKeeper (Standalone Mode)
ZooKeeper
Controller
Single Host
Audit Log
SWS
JMX
AAS
SAM SRM
View
APP
SCH
HC

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External ZooKeeper (Replicated Mode)
ZooKeeper
(Follower)
Controller
Host A
AAS
SAM
SWS
Audit Log
JMX
SRM
SCH
View
ZooKeeper
(Leader)
Controller
Host B
AAS
SAM
Audit Log
JMX
SRM
SCH
View
ZooKeeper
(Follower)
Controller
Host C
AAS
SAM
Audit Log
JMX
SRM
SCH
View
Host D
Controller
HC
APP
Host E
Controller
HC
APP
Host F
Controller
HC
APP

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ZooKeeper Guidelines/Best Practices
 The ZooKeeper Admin Guide does not recommend standalone mode in a production
environment. ZooKeeper runs as an ensemble of ZooKeeper servers. For reliability and
availability, run ZooKeeper on at least 3 hosts. Running ZooKeeper on 5 hosts is preferred.
 For optimal performance and response time, run the ZooKeeper server on a dedicated
machine, and use a dedicated device for the transaction log.
 Having a supervisory process that manages each of the ZooKeeper server processes
ensures that if the ZooKeeper process exits abnormally, it is restarted automatically and
rejoins the cluster.
 If you use the default ZooKeeper configuration, ZooKeeper does not remove old snapshots
and log files that are stored in the data directory. To configure automatic purging of the old
files, you can use the autopurge.snapRetainCount and autopurge.purgeInterval
parameters.
 Ensure that the value of the maxClientCnxns configuration parameter is high enough to
avoid the loss of connections.

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ZooKeeper Guidelines/Best Practices (cont.)
 ZooKeeper keeps data in memory and in a persistent store. The amount of data that
InfoSphere Streams stores in ZooKeeper depends on the application runtime size. A typical
amount is three times the application description language (ADL) file size.
 The default Java™ heap size for ZooKeeper is the JVM default for the system. If the
maximum heap size is not sufficient for the ZooKeeper runtime system and data in memory,
increase the size by using the JVMFLAGS environment variable.
 Tune JVM GC flags to avoid long garbage collection pauses (Parallel/CMS/Incremental GC)
 To avoid disk swapping, ensure that the Java heap size is less than the unused physical
memory.
 The ZooKeeper Administrator’s Guide recommends having a dedicated disk for the
dataLogDir directory that is separate from the dataDir directory. Set the dataLogDir
parameter in the ZooKeeper-installation-directory/conf/zoo.cfg file.
 Periodically backing up the ZooKeeper data and data log directory is a good practice.
Recovering from backups might be necessary in case a catastrophic failure, such as a
corrupted disk, occurs.

16. 16 © 2015 IBM Corporation
ZooKeeper Guidelines/Best Practices (cont.)
 If ZooKeeper follower(s) throw exception that it has fail to follow leader, it may be caused by
 Network issues
 Disk IO contention
 ZK snapshot is too large
 This can be resolved by:
 Monitoring network
 Reduce IO contention
 Increase initLimit and syncLimit on all ZooKeeper servers and restart

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Questions?