Apache Zookeeper Training @ BigDataTraining.IN

Apache ZooKeeper is an effort to develop and maintain an open-source server which enables highly reliable distributed coordination.
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. 

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ZooKeeper is a high-performance coordination service for distributed applications. It exposes common services - such as naming, configuration management, synchronization, and group services - in a simple interface so you don't have to write them from scratch. You can use it off-the-shelf to implement consensus, group management, leader election, and presence protocols. And you can build on it for your own, specific needs.

ZooKeeper: A Distributed Coordination Service for Distributed Applications

ZooKeeper is a distributed, open-source coordination service for distributed applications. It exposes a simple set of primitives that distributed applications can build upon to implement higher level services for synchronization, configuration maintenance, and groups and naming. It is designed to be easy to program to, and uses a data model styled after the familiar directory tree structure of file systems. It runs in Java and has bindings for both Java and C.

Design Goals

ZooKeeper is simple. ZooKeeper allows distributed processes to coordinate with each other through a shared hierarchal namespace which is organized similarly to a standard file system. The name space consists of data registers - called znodes, in ZooKeeper parlance - and these are similar to files and directories. Unlike a typical file system, which is designed for storage, ZooKeeper data is kept in-memory, which means ZooKeeper can achieve high throughput and low latency numbers.

The ZooKeeper implementation puts a premium on high performance, highly available, strictly ordered access. The performance aspects of ZooKeeper means it can be used in large, distributed systems. The reliability aspects keep it from being a single point of failure. The strict ordering means that sophisticated synchronization primitives can be implemented at the client.

ZooKeeper is replicated. Like the distributed processes it coordinates, ZooKeeper itself is intended to be replicated over a sets of hosts called an ensemble.

ZooKeeper is ordered. ZooKeeper stamps each update with a number that reflects the order of all ZooKeeper transactions. Subsequent operations can use the order to implement higher-level abstractions, such as synchronization primitives.

ZooKeeper is fast. It is especially fast in "read-dominant" workloads. ZooKeeper applications run on thousands of machines, and it performs best where reads are more common than writes, at ratios of around 10:1.

Data model and the hierarchical namespace

The name space provided by ZooKeeper is much like that of a standard file system. A name is a sequence of path elements separated by a slash (/). Every node in ZooKeeper's name space is identified by a path.

The ZooKeeper Data Model

ZooKeeper has a hierarchal name space, much like a distributed file system. The only difference is that each node in the namespace can have data associated with it as well as children. It is like having a file system that allows a file to also be a directory. Paths to nodes are always expressed as canonical, absolute, slash-separated paths; there are no relative reference. Any unicode character can be used in a path subject to the following constraints:

• The null character (\u0000) cannot be part of a path name. (This causes problems with the C binding.)
• The following characters can't be used because they don't display well, or render in confusing ways: \u0001 - \u001F and \u007F - \u009F.
• The following characters are not allowed: \ud800 - uF8FF, \uFFF0 - uFFFF, \uXFFFE - \uXFFFF (where X is a digit 1 - E), \uF0000 - \uFFFFF.
• The "." character can be used as part of another name, but "." and ".." cannot alone be used to indicate a node along a path, because ZooKeeper doesn't use relative paths. The following would be invalid: "/a/b/./c" or "/a/b/../c".
• The token "zookeeper" is reserved.
    
  
  
  Getting Started with ZooKeeper

  Standalone Operation

  Setting up a ZooKeeper server in standalone mode is straightforward. The server is contained in a single JAR file, so installation consists of creating a configuration.
  Once you've downloaded a stable ZooKeeper release unpack it and cd to the root
  To start ZooKeeper you need a configuration file. Here is a sample, create it in conf/zoo.cfg:

  tickTime=2000
  dataDir=/var/lib/zookeeper
  clientPort=2181
  

  This file can be called anything, but for the sake of this discussion call it conf/zoo.cfg. Change the value of dataDir to specify an existing (empty to start with) directory. Here are the meanings for each of the fields:

  tickTime

  the basic time unit in milliseconds used by ZooKeeper. It is used to do heartbeats and the minimum session timeout will be twice the tickTime.

  dataDir

  the location to store the in-memory database snapshots and, unless specified otherwise, the transaction log of updates to the database.

  clientPort

  the port to listen for client connections

  Now that you created the configuration file, you can start ZooKeeper:

  bin/zkServer.sh start

  ZooKeeper logs messages using log4j -- more detail available in the Logging section of the Programmer's Guide. You will see log messages coming to the console (default) and/or a log file depending on the log4j configuration.
  The steps outlined here run ZooKeeper in standalone mode. There is no replication, so if ZooKeeper process fails, the service will go down. This is fine for most development situations, but to run ZooKeeper in replicated mode,

  Managing ZooKeeper Storage

  For long running production systems ZooKeeper storage must be managed externally (dataDir and logs).
  
  
  
  
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