Purpose of Zookeeper
Distributed applications, require coordination among the systems. As explained - this required a lot of work and was complicated. Zookeeper's coordination service is robust is ready to use. You can store configuration data in Zookeeper and share that configuration information across all nodes in the cluster of the distributed system. Zookeeper is also used for its Naming service - which makes it easier to find a single machine on 1000s of clusters. Zookeeper is used for synchronization and is the building blocks behind queues for jobs in the cluster. In group services, Zookeeper allows you to choose a leader among tasks. It is reliable, efficient and makes the jobs in the cluster fast and is simple!
How Zookeeper works?
In a distributed Zookeeper implementation there are several modes - which is known as Zookeeper's replicated modes. Here one server is elected as a leader and all others are followers.
If the leader fails - then another one is elected. All the servers are aware of each others existence. Each server maintains and in-memory image and transaction logs in the persistent storage. The clients connect to just a single server. But once it is connected - it is provided with a list of servers. If the client or the server fails - then the client has the ability to connect to any server. Since the all servers have the same information, the client can perform its tasks without interruption. Zookeeper can also be used in standalone mode - but then the benefits of the replicated modes are lost. Standalone mode works for testing and learning purposes. Zookeeper provides consistency guarantees - sequential consistency, complete update/ or fail guarantee, same system view for client regardless of servers, reliability, client's view of the system is updated generally within tens of seconds.
- Zookeeper and Oozie: Hadoop Workflow and Cluster Managers
- Apache Zookeeper is an application library, which primarily focuses on coordination between the distributed applications. It exposes simple services like naming, synchronization, configuration management and grouping services, in a very simple manner, relieving the developers to program them from start. It provides off the shelf support for queuing, and leader election. Click to read more.
- Hadoop Components and Architecture:Big Data and Hadoop Training
- Zookeeper is the king of coordination and provides simple, fast, reliable and ordered operational services for a Hadoop cluster. Zookeeper is responsible for synchronization service, distributed configuration service and for providing a naming registry for distributed systems. Click to read more.
- Apache Zookeeper Tutorial
- Apache Zookeeper is a coordination service for distributed application that enables synchronization across a cluster. Zookeeper in Hadoop can be viewed as centralized repository where distributed applications can put data and get data out of it. It is used to keep the distributed system functioning together as a single unit, using its synchronization, serialization and coordination goals. Click to read more.
- Apache Zookepeer Tutorial: Watch Notification on change of Znode
- Having effective configuration management system is important and so is to keep track of changes happening in znode. One of the way to track changes is by getting notification for every changes made to znode. A watch can be set on znode. Client can get notification upon changes in znode if client has set watch on znode. Any change to the znode triggers the watch and notifies the client. ZooKeeper's definition of a watch says that "a watch event is one-time trigger, sent to the client that set the watch, which occurs when the data for which the watch was set changes". Click to read more.
Zookeeper Interview Questions
Can Apache Kafka be used without Zookeeper?
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It is not possible to use Apache Kafka without Zookeeper because if the Zookeeper is down Kafka cannot serve client request.
What is the role of Zookeeper in HBase architecture?
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In HBase architecture, ZooKeeper is the monitoring server that provides different services like - tracking server failure and network partitions, maintaining the configuration information, establishing communication between the clients and region servers, usability of ephemeral nodes to identify the available servers in the cluster.
Explain about ZooKeeper in Kafka.
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Apache Kafka uses ZooKeeper to be a highly distributed and scalable system. Zookeeper is used by Kafka to store various configurations and use them across the hadoop cluster in a distributed manner. To achieve distributed-ness, configurations are distributed and replicated throughout the leader and follower nodes in the ZooKeeper ensemble. We cannot directly connect to Kafka by bye-passing ZooKeeper because if the ZooKeeper is down it will not be able to serve the client request.
Zookeeper - writes to nodes
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If the leader writes to majority and confirms success, a subsequent reader on that node (from a minority!) might send a write request that (most) likely fails (dirty read). What is the reason for this design strategy?
This is some code example we have developed for the Apache ZooKeeper book. This code constitutes complementary material to the book and it has been written to illustrate how to implement an application with ZooKeeper. It hasn't been heavily tested or debugged, and it misses features, so don't take it as production-ready code. In fact, if you're able to fix bugs and extend this implementation, it probably means that you have learned how to program with ZooKeeper!