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Subject [GitHub] [zookeeper] maoling commented on a change in pull request #1073: ZOOKEEPER-3529: add a new doc: zookeeperUseCases.md
Date Sat, 14 Sep 2019 07:01:58 GMT
maoling commented on a change in pull request #1073: ZOOKEEPER-3529: add a new doc: zookeeperUseCases.md
URL: https://github.com/apache/zookeeper/pull/1073#discussion_r324415176

 File path: zookeeper-docs/src/main/resources/markdown/zookeeperUseCases.md
 @@ -0,0 +1,377 @@
+Copyright 2002-2019 The Apache Software Foundation
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+See the License for the specific language governing permissions and
+limitations under the License.
+# ZooKeeper Use Cases
+- Applications and organizations using ZooKeeper include (alphabetically)[1].
+- If your use case wants to be listed here. Please do not hesitate, submit a pull request
or write an email to **dev@zookeeper.apache.org**,
+  and then, your use case will be included.
+## Free Software Projects
+### [AdroitLogic UltraESB](http://adroitlogic.org/)
+  - Uses ZooKeeper to implement node coordination, in clustering support. This allows the
management of the complete cluster,
+  or any specific node - from any other node connected via JMX. A Cluster wide command framework
developed on top of the
+  ZooKeeper coordination allows commands that fail on some nodes to be retried etc. We also
support the automated graceful
+  round-robin-restart of a complete cluster of nodes using the same framework.
+### [Akka](http://akka.io/)
+  - Akka is the platform for the next generation event-driven, scalable and fault-tolerant
architectures on the JVM.
+  Or: Akka is a toolkit and runtime for building highly concurrent, distributed, and fault
tolerant event-driven applications on the JVM.
+### [Eclipse Communication Framework](http://www.eclipse.org/ecf) 
+  - The Eclipse ECF project provides an implementation of its Abstract Discovery services
using Zookeeper. ECF itself
+  is used in many projects providing base functionallity for communication, all based on
+### [Eclipse Gyrex](http://www.eclipse.org/gyrex)
+  - The Eclipse Gyrex project provides a platform for building your own Java OSGi based clouds.

+  - ZooKeeper is used as the core cloud component for node membership and management, coordination
of jobs executing among workers,
+  a lock service and a simple queue service and a lot more.
+### [GoldenOrb](http://www.goldenorbos.org/)
+  - massive-scale Graph analysis
+### [Juju](https://juju.ubuntu.com/)
+  - Service deployment and orchestration framework, formerly called Ensemble.
+### [Katta](http://katta.sourceforge.net/)
+  - Katta serves distributed Lucene indexes in a grid environment.
+  - Zookeeper is used for node, master and index management in the grid.
+### [KeptCollections](https://github.com/anthonyu/KeptCollections)
+  - KeptCollections is a library of drop-in replacements for the data structures in the Java
Collections framework.
+  - KeptCollections uses Apache ZooKeeper as a backing store, thus making its data structures
distributed and scalable.
+### [Neo4j](https://neo4j.com/)
+  - Neo4j is a Graph Database. It's a disk based, ACID compliant transactional storage engine
for big graphs and fast graph traversals,
+    using external indicies like Lucene/Solr for global searches.
+  - We use ZooKeeper in the Neo4j High Availability components for write-master election,
+    read slave coordination and other cool stuff. ZooKeeper is a great and focused project
- we like!
+### [Norbert](http://sna-projects.com/norbert)
+  - Partitioned routing and cluster management.
+### [OpenStack Nova](http://www.openstack.org/)
+  - OpenStack  is an open source software stack for the creation and management of private
and public clouds. It is designed to manage pools of compute,
+    storage, and networking resources in data centers, allowing the management of these resources
through a consolidated dashboard and flexible APIs.
+  - Nova is the software component in OpenStack, which is responsible for managing the compute
resources, where virtual machines (VMs) are hosted in a cloud computing environment. It is
also known as the OpenStack Compute Service. OpenStack
+    Nova provides a cloud computing fabric controller, supporting a wide variety of virtualization
technologies such as KVM, Xen, VMware, and many more. In addition to its native API, it also
includes compatibility with Amazon EC2 and S3 APIs.
+    Nova depends on up-to-date information about the availability of the various compute
nodes and services that run on them, for its proper operation. For example, the virtual machine
placement operation requires to know the currently available compute nodes and their current
+    Nova uses ZooKeeper to implement an efficient membership service, which monitors the
availability of registered services. This is done through the ZooKeeper ServiceGroup Driver,
which works by using ZooKeeper's ephemeral znodes. Each service registers by creating an ephemeral
znode on startup. Now, when the service dies, ZooKeeper will automatically delete the corresponding
ephemeral znode. The removal of this znode can be used to trigger the corresponding recovery
+    For example, when a compute node crashes, the nova-compute service that is running in
that node also dies. This causes the session with ZooKeeper service to expire, and as a result,
ZooKeeper deletes the ephemeral znode created by the nova-compute service. If the cloud controller
keeps a watch on this node-deletion event, it will come to know about the compute node crash
and can trigger a migration procedure to evacuate all the VMs that are running in the failed
compute node to other nodes. This way, high availability of the VMs can be ensured in real
+  - ZooKeeper is also being considered for the following use cases in OpenStack Nova:
+    - Storing the configuration metadata (nova.conf)
+    - Maintaining high availability of services and automatic failover using
+    leader election
+### [spring-cloud-zookeeper](https://spring.io/projects/spring-cloud-zookeeper)
+  - Spring Cloud Zookeeper provides Apache Zookeeper integrations for Spring Boot apps through
+    and binding to the Spring Environment and other Spring programming model idioms. With
a few simple annotations
+    you can quickly enable and configure the common patterns inside your application and
build large distributed systems with Zookeeper.
+    The patterns provided include Service Discovery and Distributed Configuration.
+### [Talend ESB](http://www.talend.com/products-application-integration/application-integration-esb-se.php)
+  - Talend ESB is a versatile and flexible, enterprise service bus.
+  - It uses ZooKeeper as endpoint repository of both REST and SOAP Web services.
+    By using ZooKeeper Talend ESB is able to provide failover and load balancing capabilities
in a very light-weight manner
+### [redis_failover](https://github.com/ryanlecompte/redis_failover)
+  - Redis Failover is a ZooKeeper-based automatic master/slave failover solution for Ruby.
+## Apache Projects
+### [Apache Accumulo](https://accumulo.apache.org/)
+  - Accumulo is a distributed key/value store that provides expressive, cell-level access
+  - Apache ZooKeeper plays a central role within the Accumulo architecture. Its quorum consistency
model supports an overall
+    Accumulo architecture with no single points of failure. Beyond that, Accumulo leverages
ZooKeeper to store and communication 
+    configuration information for users and tables, as well as operational states of processes
and tablets.[2]
+### [Apache BookKeeper](https://bookkeeper.apache.org/)
+  - A scalable, fault-tolerant, and low-latency storage service optimized for real-time workloads.
+  - BookKeeper requires a metadata storage service to store information related to ledgers
and available bookies. BookKeeper currently uses
+    ZooKeeper for this and other tasks[3].
+### [Apache CXF DOSGi](http://cxf.apache.org/distributed-osgi.html)
+  - Apache CXF is an open source services framework. CXF helps you build and develop services
using frontend programming
+    APIs, like JAX-WS and JAX-RS. These services can speak a variety of protocols such as
+    or CORBA and work over a variety of transports such as HTTP, JMS or JBI.
+  - The Distributed OSGi implementation at Apache CXF uses ZooKeeper for its Discovery functionality.[4]
+### [Apache Dubbo](http://dubbo.apache.org)
+  - Apache Dubbo is a high-performance, java based open source RPC framework.
+  - Zookeeper is used for service registration discovery and configuration management in
+### [Apache Flink](https://flink.apache.org/)
+  - Apache Flink is a framework and distributed processing engine for stateful computations
over unbounded and bounded data streams.
+    Flink has been designed to run in all common cluster environments, perform computations
at in-memory speed and at any scale.
+  - To enable JobManager High Availability you have to set the high-availability mode to
zookeeper, configure a ZooKeeper quorum and set up a masters file with all JobManagers hosts
and their web UI ports.
+    Flink leverages ZooKeeper for distributed coordination between all running JobManager
instances. ZooKeeper is a separate service from Flink,
+    which provides highly reliable distributed coordination via leader election and light-weight
consistent state storage[23].
+### [Apache Flume](https://flume.apache.org/)
+  - Flume is a distributed, reliable, and available service for efficiently collecting, aggregating,
and moving large amounts
+    of log data. It has a simple and flexible architecture based on streaming data flows.
It is robust and fault tolerant
+    with tunable reliability mechanisms and many failover and recovery mechanisms. It uses
a simple extensible data model
+    that allows for online analytic application.
+  - Flume supports Agent configurations via Zookeeper. This is an experimental feature.[5]
+### [Apache Hadoop](http://hadoop.apache.org/)
+  - The Apache Hadoop software library is a framework that allows for the distributed processing
of large data sets across
+    clusters of computers using simple programming models. It is designed to scale up from
single servers to thousands of machines,
+    each offering local computation and storage. Rather than rely on hardware to deliver
+    the library itself is designed to detect and handle failures at the application layer,
so delivering a highly-available service on top of a cluster of computers, each of which may
be prone to failures.
+  - The implementation of automatic HDFS failover relies on ZooKeeper for the following things:
+    - **Failure detection** - each of the NameNode machines in the cluster maintains a persistent
session in ZooKeeper.
+      If the machine crashes, the ZooKeeper session will expire, notifying the other NameNode
that a failover should be triggered.
+    - **Active NameNode election** - ZooKeeper provides a simple mechanism to exclusively
elect a node as active. If the current active NameNode crashes,
+      another node may take a special exclusive lock in ZooKeeper indicating that it should
become the next active.
+  - The ZKFailoverController (ZKFC) is a new component which is a ZooKeeper client which
also monitors and manages the state of the NameNode.
+    Each of the machines which runs a NameNode also runs a ZKFC, and that ZKFC is responsible
+    - **Health monitoring** - the ZKFC pings its local NameNode on a periodic basis with
a health-check command.
+      So long as the NameNode responds in a timely fashion with a healthy status, the ZKFC
considers the node healthy.
+      If the node has crashed, frozen, or otherwise entered an unhealthy state, the health
monitor will mark it as unhealthy.
+    - **ZooKeeper session management** - when the local NameNode is healthy, the ZKFC holds
a session open in ZooKeeper.
+      If the local NameNode is active, it also holds a special “lock” znode. This lock
uses ZooKeeper’s support for “ephemeral” nodes;
+      if the session expires, the lock node will be automatically deleted.
+    - **ZooKeeper-based election** - if the local NameNode is healthy, and the ZKFC sees
that no other node currently holds the lock znode,
+      it will itself try to acquire the lock. If it succeeds, then it has “won the election”,
and is responsible for running a failover to make its local NameNode active.
+      The failover process is similar to the manual failover described above: first, the
previous active is fenced if necessary,
+      and then the local NameNode transitions to active state.[7]
+### [Apache HBase](https://hbase.apache.org/)
+  - HBase is the Hadoop database. It's an open-source, distributed, column-oriented store
+  - HBase uses ZooKeeper for master election, server lease management, bootstrapping, and
coordination between servers.
+    A distributed Apache HBase installation depends on a running ZooKeeper cluster. All participating
nodes and clients
+    need to be able to access the running ZooKeeper ensemble.[8]
+  - As you can see, ZooKeeper is a fundamental part of HBase. All operations that require
coordination, such as Regions
+    assignment, Master-Failover, replication, and snapshots, are built on ZooKeeper[20].

+### [Apache Hedwig](https://bookkeeper.apache.org/docs/r4.2.3/hedwigConsole.html)
+  - Hedwig is a publish-subscribe system designed to carry large amounts of data across the
internet in a guaranteed-delivery
+    fashion from those who produce it (publishers) to those who are interested in it (subscribers).
+### [Apache Helix](http://helix.apache.org/)
+  - Apache Helix is a cluster management framework for partitioned and replicated distributed
+  - It provides a generic way of automatically managing the resources in a cluster. Helix
acts as a decision subsystem for the cluster, and is responsible for the following tasks and
many more:
+    - Automating the reassignment of resources to the nodes
+    - Handling node failure detection and recovery
+    - Dynamic cluster reconfiguration (node and resource addition/deletion)
+    - Scheduling of maintenance tasks (backups, index rebuilds, and so on)
+    - Maintaining load balancing and flow control in the cluster
+    In order to store the current cluster state, Helix needs a distributed and highly available
configuration or cluster metadata store, for which it uses ZooKeeper.
+  - ZooKeeper provides Helix with the following capabilities:
+    - This framework represents the PERSISTENT state, which remains until it's removed
+    - This framework also represents the TRANSIENT/EPHEMERAL state, which goes away when
the process that created the state leaves the cluster
+    - This framework notifies the subsystem when there is a change in the PERSISTENT and
EPHEMERAL state of the cluster
+  - Helix also allows simple lookups of task assignments through the configuration store
built on top of ZooKeeper. Through this, clients can look up where the tasks are currently
+    This way, Helix can also provide a service discovery registry.[21]
+### [Apache Kafka](https://kafka.apache.org/)
+  - Kafka is a distributed publish/subscribe messaging system
+  - Apache Kafka relies on ZooKeeper for the following things:
+    - **Controller election**
+    The controller is one of the most important broking entity in a Kafka ecosystem, and
it also has the responsibility
+    to maintain the leader-follower relationship across all the partitions. If a node by
some reason is shutting down,
+    it’s the controller’s responsibility to tell all the replicas to act as partition
leaders in order to fulfill the
+    duties of the partition leaders on the node that is about to fail. So, whenever a node
shuts down, a new controller
+    can be elected and it can also be made sure that at any given time, there is only one
controller and all the follower nodes have agreed on that.
+    - **Configuration Of Topics**
+    The configuration regarding all the topics including the list of existing topics, the
number of partitions for each topic,
+    the location of all the replicas, list of configuration overrides for all topics and
which node is the preferred leader, etc.
+    - **Access control lists**
+    Access control lists or ACLs for all the topics are also maintained within Zookeeper.
+    - **Membership of the cluster**
+    Zookeeper also maintains a list of all the brokers that are functioning at any given
moment and are a part of the cluster.[9]
+### [Apache Mesos](http://mesos.apache.org/)
+  - Apache Mesos abstracts CPU, memory, storage, and other compute resources away from machines
(physical or virtual),
+    enabling fault-tolerant and elastic distributed systems to easily be built and run effectively.
+  - Mesos has a high-availability mode that uses multiple Mesos masters: one active master
(called the leader or leading master)
+    and several backups in case it fails. The masters elect the leader, with Apache ZooKeeper
both coordinating the election
+    and handling leader detection by masters, agents, and scheduler drivers.[10]
+### [Apache S4](https://github.com/apache/incubator-retired-s4)
+  - S4(**retired** on 2014-06-19[11]) is a general-purpose, distributed, scalable, partially
fault-tolerant, pluggable platform that allows programmers
+    to easily develop applications for processing continuous unbounded streams of data.
+### [Apache Solr](https://lucene.apache.org/solr/)
+  - Solr is the popular, blazing-fast, open source enterprise search platform built on Apache
+  - In the "Cloud" edition (v4.x and up) of enterprise search engine Apache Solr, ZooKeeper
is used for configuration,
+    leader election and more[12,13]
+### [Apache Spark](https://spark.apache.org/)
+  - Apache Spark is a unified analytics engine for large-scale data processing.
+  - Utilizing ZooKeeper to provide leader election and some state storage, you can launch
multiple Masters in your cluster connected to the same ZooKeeper instance.
+    One will be elected “leader” and the others will remain in standby mode. If the current
leader dies, another Master will be elected,
+    recover the old Master’s state, and then resume scheduling[14]
+## Companies
+### [101tec](http://101tec.com/)
+ - we do consulting in the area of enterprise distributed systems.
+ - We use zookeeper to manage a system build out of hadoop, katta, oracle batch jobs and
a web component.
+### [AGETO](http://www.ageto.de/)
+ - The AGETO RnD team uses ZooKeeper in a variety of internal as well as external consulting
+### [Benipal Technologies](http://www.benipaltechnologies.com/)
+ - ZooKeeper is used for internal application development with Solr and Hadoop with Hbase.
+### [Box](http://box.net/)
+ - Box uses ZooKeeper for service discovery, service coordination, Solr and Hadoop support,
+### [Deepdyve](http://www.deepdyve.com/)
+ - We do search for research and provide access to high quality content using advanced search
technologies Zookeeper is used to
+   manage server state, control index deployment and a myriad other tasks.
+### [eBay](https://www.ebay.com/)
+ - eBay uses ZooKeeper to develop a job type limiter. The job limiter limits the execution
of the same type of jobs from running simultaneously beyond a specified number in the grid.
+   For each job type, the type limiter keeps track of the running count and the limit. When
the running count hits the limit, spawning of a new job
+   is not allowed until a job of that type finishes or terminates. This job type limiter
is used for jobs that use third-party services using APIs to update the maximum bid for keyword
+   Usually, the API call capacity is limited. Even if the grid has enough capacity to run
hundreds of jobs, the job limiter system built with ZooKeeper allows
+   only a predetermined number (for example, N) of concurrent jobs against a partner API.
The type limiter ensures that the (N+1)th job waits until one of the N running jobs has completed[21,22].
+### [Facebook](https://www.facebook.com/)
+ - Facebook uses ZooKeeper in its messaging platform and many other use cases. Facebook's
messaging system is powered by a system called cell system.
+   The entire messaging system (e-mail, SMS, Facebook Chat, and the Facebook Inbox) is divided
into cells. Cells are composed of a cluster of servers and services,
+   and each cell serves only a fraction of Facebook users. The use of cells has the following
+   - Incremental scaling with minuscule failures
+   - Easy upgrading of hardware and software
+   - Failures affect only the cell boundary, which affects limited users
+   - Hosting cells in distributed data centers avoids disasters
+   - Each cell comprises a cluster of servers and is coordinated by a ZooKeeper ensemble.
The servers that host a particular application register themselves with ZooKeeper.
+     The mapping between users and servers is then done using the consistent hashing algorithm.
Basically, the nodes are arranged in a ring,
+     and each node is responsible for a specified range of the users depending on the hash
+   - If a node fails, another node in the neighboring position in the ring takes over the
load for those users affected by the failure.
+     This allows for an even distribution of the load and also enables easy addition and
removal of nodes into and from the cluster in the cell[21].
+ - Facebook uses the Zeus([17,18]) for configuration management which is a forked version
of ZooKeeper, with many scalability
+   and performance en- hancements in order to work at the Facebook scale.
+   It runs a consensus protocol among servers distributed across mul- tiple regions for resilience.
If the leader fails,
+   a follower is converted into a new leader.
+### [Idium Portal](http://www.idium.no/no/idium_portal/)
+ - Idium Portal is a hosted web-publishing system delivered by Norwegian company, Idium AS.
+ - ZooKeeper is used for cluster messaging, service bootstrapping, and service coordination.
+### [Makara](http://www.makara.com/)
+ - Using ZooKeeper on 2-node cluster on VMware workstation, Amazon EC2, Zen
+ - Using zkpython
+ - Looking into expanding into 100 node cluster
+### [Midokura](http://www.midokura.com/)
+ - We do virtualized networking for the cloud computing era. We use ZooKeeper for various
aspects of our distributed control plane.
+### [Netflix](https://www.netflix.com)
+ - Netflix([21]) is an avid ZooKeeper user in its distributed platform, which led them to
develop Curator and Exhibitor to enhance the functionalities of ZooKeeper.
+   A few of the use cases of ZooKeeper/Curator at Netflix are as follows:
+    - Ensuring the generation of unique values in various sequence ID generators
+    - Cassandra backups
+    - Implementing the TrackID service
+    - Performing leader election with ZooKeeper for various distributed tasks
+    - Implementing a distributed semaphore for concurrent jobs
+    - Distributed caching
+### [Nutanix](http://www.nutanix.com/)
+  - Nutanix develops a hyper-converged storage and compute solution, which leverages local
components of a host machine, such as storage capacity (disks) and compute (CPUs),
+    to create a distributed platform for virtualization. This solution is known as the Nutanix
Virtual Computing Platform. It supports industry-standard hypervisor ESXi, KVM, and Hyper-V.
+    The platform is bundled in the appliance form factor with two nodes or four nodes. A
VM in the platform known as the Nutanix Controller VM works as the decision subsystem, which
manages the platform.
+  - The Nutanix platform uses Apache ZooKeeper as a Cluster configuration manager. The configuration
data that pertains to the platform, such as hostnames, IP addresses,
+    and the cluster state, is stored in a ZooKeeper ensemble. ZooKeeper is also used to query
the state of the various services that run in the platform[21].
+### [Pinterest](https://www.pinterest.com/)
+ - Pinterest uses the ZooKeeper for Service discovery and dynamic configuration.Like many
large scale web sites, Pinterest’s infrastructure consists of servers that communicate with
+   backend services composed of a number of individual servers for managing load and fault
tolerance. Ideally, we’d like the configuration to reflect only the active hosts,
+   so clients don’t need to deal with bad hosts as often. ZooKeeper provides a well known
pattern to solve this problem[19].
+### [Rackspace](http://www.rackspace.com/email_hosting)
+ - The Email & Apps team uses ZooKeeper to coordinate sharding and responsibility changes
in a distributed e-mail client
+   that pulls and indexes data for search. ZooKeeper also provides distributed locking for
connections to prevent a cluster from overwhelming servers.
+### [Sematext](http://sematext.com/)
+ - Uses ZooKeeper in SPM (which includes ZooKeeper monitoring component, too!), Search Analytics,
and Logsene.
+### [Tubemogul](http://tubemogul.com/)
+ - Uses ZooKeeper for leader election, configuration management, locking, group membership
+### [Twitter](https://twitter.com/)
+ - ZooKeeper is used at Twitter as the source of truth for storing critical metadata. It
serves as a coordination kernel to
+   provide distributed coordination services, such as leader election and distributed locking.
+   Some concrete examples of ZooKeeper in action include[15,16]:
+   - ZooKeeper is used to store service registry, which is used by Twitter’s naming service
for service discovery.
+   - Manhattan, Twitter’s in-house key-value database, stores its cluster topology information
in ZooKeeper.
+   - EventBus, Twitter’s pub-sub messaging system, stores critical metadata in ZooKeeper
and uses ZooKeeper for leader election.
+   - Mesos, Twitter’s compute platform, uses ZooKeeper for leader election.
+### [Vast.com](http://www.vast.com/)
+ - Used internally as a part of sharding services, distributed synchronization of data/index
updates, configuration management and failover support
+### [VMware vSphere Storage Appliance](https://www.vmware.com/products/vsphere-storage-appliance.html)
+ - VMware vSphere Storage Appliance (VSA) is a software-storage appliance. The VMware VSA
comes in a cluster configuration of two or three nodes,
+   known as the VSA Storage Cluster. A virtual machine instance inside each of the VMware
ESXiTM host in the VSA Storage
+   Cluster claims all the available local directed attached storage space and presents it
as one mirrored volume of all the ESXi hosts in the VMware vCenter Server datacenter.
+   It uses the NFS protocol to export the volume.
+ - VSA uses ZooKeeper as the base clustering library for the following primitives:
+    - As a cluster membership model to detect VSA failures in the cluster
+    - As a distributed metadata storage system to store the cluster states
+    - As a leader elector to select a master VSA that performs metadata operations
+### [Wealthfront](http://wealthfront.com/)
+ - Wealthfront uses ZooKeeper for service discovery, leader election and distributed locking
among its many backend services.
+   ZK is an essential part of Wealthfront's continuous [deployment infrastructure](http://eng.wealthfront.com/2010/05/02/deployment-infrastructure-for-continuous-deployment/).
+### [Yahoo!](http://www.yahoo.com/)
+ - ZooKeeper is used for a myriad of services inside Yahoo! for doing leader election, configuration
management, sharding, locking, group membership etc[1].
+ - ZooKeeper was originally developed by Yahoo! before it became an Apache project. It is
used for a multitude of services inside Yahoo! to perform leader election, configuration management,

+   naming system, sharding, distributed locking, group membership, and so on. Yahoo! is a
big Hadoop shop that performs analysis in its massive datasets,
+   and ZooKeeper is used for various distributed coordination and synchronization tasks[21].
+### [Zynga](http://www.zynga.com/)
+ - ZooKeeper at Zynga is used for a variety of services including configuration management,
leader election, sharding and more...[1]
+ - Zynga uses ZooKeeper for configuration management of their hosted games. ZooKeeper allows
Zynga to update the configuration files for a plethora of online games,
+   which are used across the world by millions of users in a very short span of time. The
games are served from Zynga's multiple data centers.
+   With ZooKeeper, the configuration's system updates thousands of configuration files in
a very small span of time.
+   The configurations are validated by validation systems against the business logic to ensure
that configurations are updated correctly and services are properly configured to the updated
+   In the absence of ZooKeeper, the configuration update at the same time in a short time
interval would be a real nightmare.
+   Again, failure to sync these configuration files within the available time span would
have caused severe service disruption[21].
+#### References
+- [1] https://cwiki.apache.org/confluence/display/ZOOKEEPER/PoweredBy
+- [2] https://www.youtube.com/watch?v=Ew53T6h9oRw
+- [3] https://bookkeeper.apache.org/docs/4.7.3/getting-started/concepts/#ledgers
+- [4] http://cxf.apache.org/dosgi-discovery-demo-page.html
+- [5] https://flume.apache.org/FlumeUserGuide.html
+- [6] http://dubbo.apache.org/en-us/blog/dubbo-zk.html
+- [7] https://hadoop.apache.org/docs/r2.7.1/hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html
+- [8] https://hbase.apache.org/book.html#zookeeper
+- [9] https://www.cloudkarafka.com/blog/2018-07-04-cloudkarafka_what_is_zookeeper.html
+- [10] http://mesos.apache.org/documentation/latest/high-availability/
+- [11] http://incubator.apache.org/projects/s4.html
+- [12] https://lucene.apache.org/solr/guide/6_6/using-zookeeper-to-manage-configuration-files.html#UsingZooKeepertoManageConfigurationFiles-StartupBootstrap
+- [13] https://lucene.apache.org/solr/guide/6_6/setting-up-an-external-zookeeper-ensemble.html
+- [14] https://spark.apache.org/docs/latest/spark-standalone.html#standby-masters-with-zookeeper
+- [15] https://blog.twitter.com/engineering/en_us/topics/infrastructure/2018/zookeeper-at-twitter.html
+- [16] https://blog.twitter.com/engineering/en_us/topics/infrastructure/2018/dynamic-configuration-at-twitter.html
AND KARL, R. Holistic Configuration Management
+       at Facebook. In Proceedings of the 25th Symposium on Operating System Principles (SOSP’15)
(Monterey, CA,USA, Oct. 2015).
+- [18] https://www.youtube.com/watch?v=SeZV373gUZc
+- [19] https://medium.com/@Pinterest_Engineering/zookeeper-resilience-at-pinterest-adfd8acf2a6b
+- [20] https://blog.cloudera.com/what-are-hbase-znodes/
+- [21] Saurav Haloi(2015). *Apache ZooKeeper essentials*. Packt Publishing.
 Review comment:
   Oops, I have written an email to the author of this book. If he/she doesn't agree or no
reply within ten days,I will revert/delete all the references from that book

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