Zookeeper In Simple Words

ZOOKEEPER IN MY WORDS

[email protected]

Wednesday, January 12, 2011

WHAT’S ZK?

“ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services. All of these kinds of services are used in some form or another by distributed applications. Each time they are implemented there is a lot of work that goes into fixing the bugs and race conditions that are inevitable. Because of the difficulty of implementing these kinds of services, applications initially usually skimp on them ,which make them brittle in the presence of change and difficult to manage. Even when done correctly, different implementations of these services lead to management complexity when the applications are deployed.”


WHAT THE FUCK DOES THAT MEAN?

Simply Put: Coordination Service For Distributed Systems


WHAT ZK CAN DO?

Name Service

Configuration

Group Membership

Distributed Synchronization


WHAT ZK LOOKS LIKE?

File System ModelHierarchical Namespace

each node can have both data and children


HOW ZK WORKS?


HOW ZK WORKS?

Zab


HOW TO DEPLOY ZK?

single server quorum clustermajority algorithm(2n + 1)


ZK DEPLOYMENT IN DETAILS

configuration

data location

transaction log location (optional but necessary)

cluster members

myid file (under data location directory)


CONFIGURATION FILE SAMPLE

# The number of milliseconds of each ticktickTime=2000# The number of ticks that the initial # synchronization phase can takeinitLimit=10# The number of ticks that can pass between # sending a request and getting an acknowledgementsyncLimit=5# the directory where the snapshot is stored.dataDir=/Users/fujohnwang/Downloads/zookeeper-3.3.2/data# the port at which the clients will connectclientPort=2181

server.1=10.16.200.14:2888:3888server.2=10.20.135.206:2888:3888server.3=10.20.130.233:2888:3888


HOW TO INTERACTIVE WITH ZK?

Command Line Tool

Zookeeper API Bindings

Java Bindings

C Bindings


ZOOKEEPER CLI

bin/zkCli.sh -server 127.0.0.1:2181[zk: 10.20.130.233:2181(CONNECTED) 0] help ZooKeeper -server host:port cmd args printwatches on|off ls2 path [watch] listquota path close get path [watch] sync path delete path [version] quit addauth scheme auth getAcl path setAcl path acl setquota -n|-b val path redo cmdno create [-s] [-e] path data acl set path data [version] stat path [watch] connect host:port ls path [watch] history delquota [-n|-b] path


ZK API = SIMPLE API

create

delete

exists

set data

get data

get children

sync


WHAT THE FEATURES OF ZK?

Notification

Ordering

Atomicity

Versioned Write

Ephemeral Nodes

Sequential Nodes

High Availability


WATCH - NOTIFICATION

Concept

Usecase

dynamic configuration push

other (semi)real-time information notification


EPHEMERAL NODES

Concept

lifecycle-dependent

no children

Usecase

group membership

leader election


SEQUENTIAL NODES

Concept

Usecase

queue


GUARANTEES OF ZK

Sequential Consistency - Updates from a client will be applied in the order that they were sent.

Atomicity - Updates either succeed or fail. No partial results.

Single System Image - A client will see the same view of the service regardless of the server that it connects to.

Reliability - Once an update has been applied, it will persist from that time forward until a client overwrites the update.

Timeliness - The clients view of the system is guaranteed to be up-to-date within a certain time bound.


CAUTIONS TO TAKE

prefer small data size for each node(less than 1M)

one time trigger (add again and again if u need)

dataset in all must fit in memory


nicholsong

QUESTIONS?


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