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It provides a shared hierarchical namespace that is organized like a standard file system.

Zookeeper - Quick Guide

The namespace consists of data registers called znodes, for ZooKeeper data nodes, which are similar to files and directories. The znode hierarchy is kept in-memory within each ZooKeeper server in order to minimize latency and to provide high throughput of workloads.

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The ZooKeeper service is replicated across a set of hosts called an ensemble. One of the hosts is designated as the leader, while the other hosts are followers. ZooKeeper uses a leader election process to determine which ZooKeeper server acts as the leader, or master. If the ZooKeeper leader fails, a new leader is automatically chosen to take its place. As long as a majority a quorum of the ZooKeeper servers are available, the Zookeeper service is available. For example, if the ZooKeeper service is configured to run on five nodes, three of them form a quorum.

If two nodes fail or one is taken off-line for maintenance and another one fails , a quorum can still be maintained by the remaining three nodes. An ensemble of five ZooKeeper nodes can tolerate two failures. An ensemble of three ZooKeeper nodes can tolerate only one failure. Because a quorum requires a majority, an ensemble of four ZooKeeper nodes can only tolerate one failure, and therefore offers no advantages over an ensemble of three ZooKeeper nodes.

In most cases, you should run three or five ZooKeeper nodes on a cluster. Larger quorum sizes result in slower write operations. Each ZooKeeper server maintains a record of all znode write requests in a transaction log on the disk. Apache Zookeeper is used to manage and coordinate large cluster of machines. Distributed applications are difficult to coordinate and work with as they are much more error prone due to huge number of machines attached to network.

As many machines are involved, race condition and deadlocks are common problems when implementing distributed applications. Race condition occurs when a machine tries to perform two or more operations at a time and this can be taken care by serialization property of ZooKeeper.

Deadlocks are when two or more machines try to access same shared resource at the same time. Synchronization in Zookeeper helps to solve the deadlock. Another major issue with distributed application can be partial failure of process, which can lead to inconsistency of data.

Zookeeper handles this through atomicity, which means either whole of the process will finish or nothing will persist after failure. Thus Zookeeper is an important part of Hadoop that take care of these small but important issues so that developer can focus more on functionality of the application.

Distributed Application

Hadoop ZooKeeper, is a distributed application that follows a simple client-server model where clients are nodes that make use of the service, and servers are nodes that provide the service. Multiple server nodes are collectively called ZooKeeper ensemble.

On-Host Integrations

At any given time, one ZooKeeper client is connected to at least one ZooKeeper server. If the master node fails, another master is chosen in no time and it takes over the previous master. Other than master and slaves there are also observers in Zookeeper. Observers were brought in to address the issue of scaling.

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  • With the addition of slaves the write performance is going to be affected as voting process is expensive. So observers are slaves that do not take part into voting process but have similar duties as other slaves. All the writes in Zookeeper go through the Master node, thus it is guaranteed that all writes will be sequential.

    Adding a ZooKeeper Role

    On performing write operation to the Zookeeper, each server attached to that client persists the data along with master. Thus, this makes all the servers updated about the data. However this also means that concurrent writes cannot be made. Linear writes guarantee can be problematic if Zookeeper is used for write dominant workload. Zookeeper is helpful till the time the data is shared but if application has concurrent data writing then Zookeeper can come in way of the application and impose strict ordering of operations.

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    Zookeeper is best at reads as reads can be concurrent. There can be cases where client may have an outdated view, which gets updated with a little delay. All the details mentioned above are done by the Zookeeper and the user does not have to do anything. The master is elected, the observers are set and the stage is made ready for the user to use the Zookeeper. As compared earlier user can use Zookeeper like a file system where directories can be created and data can be stored inside it.

    The directories made above can also have children and grandchildren like any other file system. This file system is stored centrally thus giving access from any spot. Example of Apache Zookeeper can be a data model.

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    • Step 1 — Creating a User for ZooKeeper.
    • Sign up for product updates!.
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    • Managing ZooKeeper.
    • Move the ZooKeeper server.
    • They are containers for data and other nodes. It stores statistical data like version details and user data up to 1Mb. This tiny space available to store information makes it clear that Zookeeper is not used for data storage like database but instead it is used for storing small amount of data like configuration data that needs to be shared.

      Zookeeper Hadoop can be kept as a watch guard and on change of data in one node other nodes can be informed about it through notifications.

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      Learn how you can build Big Data Projects. Introduction to Apache Zookeeper The formal definition of Apache Zookeeper says that it is a distributed, open-source configuration, synchronization service along with naming registry for distributed applications.